**2. Bundle Structure**

The introduction of a metric *g* and an orthogonality relation via a minkowskian metric *η* are two fundamental ingredients for building up a fiber bundle where we want the orthogonal group to act freely and transitively on the fibers. This will allow us to have a principal connection and to see the perfect analogy with an ordinary gauge theory ([1] chapter III).

Such a construction underlies the concept of *principal bundle*, and tetrads will be an isomorphism from the tangent bundle1 *TM* to an associated bundle V.


3.5 Consistency internal reliability

Consistency internal reliability was tested both for composite reliability, and Cronbach alpha. The consistency of internal reliability values in this study also more than 0.60 (>0.60), and are thus reliable (Table 5).


**8. Conclusions**

Direct surface modification techniques using lasers or electron beams are pretty popular due to their reasonable cost and easy adaptation for small and large surfaces with complex geometry. The new generation of low-power femtosecond lasers with unique properties has enhanced the possibility of their use in many new exciting applications. Notably, for the surface modification of polymers, excimer lasers offer great potential. In addition, the medical industry is another sector where laser ablation and electron beam ablation play a key role. The medical industry has an ever-growing demand for the surfaces of implants and stents to enhance their performance. The technological advancements in e-beam ablation have resulted in the development of a novel surface processing technique capable of addressing these needs by rapidly producing bespoke surface features in a clean environment. It is being investigated for a range of medical applications.


A. Appendix and nomenclature: Exemplary diet

#### Diet plan

#### The energy supply was set at 1500 kcal

#### Exemplary menu 1500 kcal—version 1

I breakfast—330 kcal

#### Italian sandwich "Caprese"

• 1 boiled egg

Lunch—440 kcal

Snack—160 kcal

Dinner—290 kcal

• Rucola salad

• 1/3 red pepper

• 100 g cocktail tomatoes

• Canned tuna in its own sauce (50 g—2 spoons)

• Natural yogurt 2% fat (75 g—3 spoons)

• A few radishes

Tomato juice—small bottle (300 ml)

Spaghetti should be boiled al dente.

Tangerines (2 medium size—150 g)

1 slice of wholemeal bread (40 g)

Tuna salad with parsley sauce (1 portion)

Natural yogurt—2% fat (180–200 g) with 1 spoon of wheat bran.

Ingredients for 3 potions: • 250 g wholemeal pasta

• 10 capers • 10 black olives • 1 small onion • 2 cloves of garlic • Olive oil (1 spoon) • Fresh parsley (2 spoons)

Angiogenesis in Adipose Tissue: How can Moderate Caloric Restriction Affects Obesity-Related Endothelial…

• Pepper, basil

• 1 chicken breast (200 g)

• 1 cane of tomatoes or fresh tomatoes

http://dx.doi.org/10.5772/intechopen.72624

275

Italian spaghetti—1 portion

#### II breakfast—310 kcal

#### Sandwich with egg and radishes

• 1 boiled egg

Pathophysiology, Poznań University of Medical Science, Poland, for their involvement in this medical project. This project was financially supported by Grant NCN-NN404 151340 from the

Meal Hours Share of energy supply (%) Caloric value (kcal)

I breakfast 7:00–8:00 20 300 II breakfast 10:00–11:00 20 300 Lunch 13:00–14:00 10 150 Snack 16:00–17:00 30 450 Dinner 18:30–19:30 20 300

Polish Ministry of Science and Higher Education—National Science Centre Poland.

Conflict of interest

Diet plan

The author do not declare the conflict of interest.

274 Endothelial Dysfunction - Old Concepts and New Challenges

The energy supply was set at 1500 kcal

Exemplary menu 1500 kcal—version 1

I breakfast—330 kcal

• 1 wholemeal roll (80 g) • 1 small tomato • Cottage cheese (50 g) • Fresh basil

• Black pepper

II breakfast—310 kcal

• Lettuce

Sandwich with egg and radishes

• 1 teaspoon of butter (5 g)

• 2 slice of wholemeal bread (80 g)

Italian sandwich "Caprese"

• 2 teaspoons of olive oil (10 g)

A. Appendix and nomenclature: Exemplary diet

• A few radishes

Tomato juice—small bottle (300 ml)

Lunch—440 kcal

Italian spaghetti—1 portion

Spaghetti should be boiled al dente.

Tangerines (2 medium size—150 g)

Dinner—290 kcal

Tuna salad with parsley sauce (1 portion)

1 slice of wholemeal bread (40 g)

#### Exemplary menu 1500 kcal—version 2

#### I breakfast—330 kcal.

#### Sandwich with ham and vegetables

#### Natural yogurt 2% fat (400 g)

II breakfast—280 kcal

#### Cottage cheese

#### 1 slice of ray bread (40 g)

Lunch—440 kcal.

#### Asparagus cream soup—1 portion

Ingredients for 4 portions

1 wholemeal roll (80 g) or 2 slices of rye bread with butter (1 teaspoon)

Angiogenesis in Adipose Tissue: How can Moderate Caloric Restriction Affects Obesity-Related Endothelial…

http://dx.doi.org/10.5772/intechopen.72624

277

1 orange (350 g) or ½ pomelo

Oatmeal with dried plums (49 g)

Salad with baked pepper and tomatoes

• 1 spoon of olive oil or sunflower oil

• 1 spoon of chopped celery leaves

• 1 spoon of chopped lettuce

• 1 slice of ray bread (40 g)

• 2 slices of chicken ham (40 g)

Address all correspondence to<EMAIL_ADDRESS>
Clinica Chimica Acta. 2005;360:9-26

Pathophysiology Department, Poznań University of Medical Science, Poznań, Poland

[1] Kaplan NM. The deadly quartet. Upper-body obesity, glucose intolerance, hypertriglyceridemia, and hypertension. Archives of Internal Medicine. 1989;149:1514-1520

[2] Higgins M, Kannel W, Garrison R, Pinsky J, Stokes J III. Hazards of obesity—The Framingham experience. Acta Medica Scandinavica. Supplementum. 1988;723:23-36 [3] Rosen ED, Spiegelman BM. What we talk about when we talk about fat. Cell. 2014;

[4] Avogaro A, De Kreutzenberg SV. Mechanisms of endothelial dysfunction in obesity.

• 1 green or yellow pepper baked in the oven and peeled

Snack—160 kcal

Dinner—290 kcal

• Pepper

Sandwich with ham

Author details

References

Katarzyna Korybalska

156:20-44

• 1 big tomato (200 g)


**1. Introduction**

Who in biology has not heard of *Escherichia coli*? Known to many as the fundamental model microbe and perhaps model organism, *E. coli* is the cornerstone of many important findings in molecular biology and other areas of cell physiology. Perhaps even the first chemo-organoheterotroph had a similar mass composition as *E. coli*, providing the hits necessary to understand the evolution of modern bacteria [1]. Also called the "workhorse" of molecular biology for its fast growing rate in chemically defined media and extensive molecular tools available for different purposes, *E. coli* is considered the most important model organism of them all. Important findings and Nobel Prizes in biology have been developed in *E. coli*. For instance,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

cracking the genetic code [2], unveiling the nature of DNA replication [3], the groundbreaking advances on gene organization and regulation or as we love to call 'the operon' [4, 5], important evidence for the basis of mutations and ultimately to the evolution of organisms [6, 7], and finally, the achievement of a genetically modified organism [8] that skyrocketed several applications of the enormous capacity for manipulating this organism, rendering *E*. *coli* as a key player in biotechnology (for an excellent review, see Ref. [9]).

*E. coli* is part of the normal microbiota of mammals, rendering the predominant facultative microbe of the gastrointestinal tract and is currently a hot debate on the impact on normal

*Escherichia coli* as a Model Organism and Its Application in Biotechnology

http://dx.doi.org/10.5772/67306

255

This organism lacks many interesting features for biotechnology, such as growing at extreme temperatures or pH, the capacity to degrade toxic compounds, pollutants, or difficult to degrade polymers [23]. But as we will see later, this bacterium is capable of doing amazing

*E. coli* harbors a genome with particular features such as a strikingly organized structure, remnents of many phages, and insertion sequences (IS) and a high transport capacity toward the cytoplasm. In 1997, the complete genome sequence of the K-12 strain was obtained, and a myriad of research was catapulted from that moment on [24]. The complete genome contains a single circular duplex molecule composed of 4,639,221 bp. Regarding its structure, protein-coding regions correspond to 87.8% of the genome, while 0.8% encodes for stable RNAs, and 0.7% consists of noncoding repeats. The remaining 11% encodes for regulatory and other functions. Nevertheless, nearly 34% (1431) proteins are considered orphan or without defined molecular function but in a recent study, it was demonstrated that by homology with distant phylogenetical relationships, they may play a role in defined molecular pathways or processes [25]. From the orphan set in *E. coli*, at least 446 contain some molecular signature that can assess their molecular role. The fact that such a vast portion of the genome remains uncharacterized for defined molecular pathways or roles paves the way for future basic research that may have a strong impact on applied research and development. For biotechnological applications, growth optimization is also a required feature of genetically modified organisms. For *E. coli*, it has been recently shown that growth can be manipulated by removing nonessential genomic sequences that also can lead to the design of important biotechnological strains for industrial purposes [26]. The advancement of genomic, transcriptomic, and proteomic technologies has led to the development of several online resources for the analysis of molecular and physiological

microflora establishment and their role in disease [22].

aspects of *E. coli*. Some examples are given in **Table 1**.

**Name URL Description\***

PortEco http://www.porteco.org A resource for knowledge and data

EcoliWiki http://ecoliwiki.net/ Community-based pages about

EcoGen 3.0 http://ecogene.org/ A database dedicated to analyzing

https://ecocyc.org A comprehensive database joining

and phages

together genomic information with biochemical features of *E. coli*

of the biology of *E. coli* including plasmids, mobile genetic elements,

everything related to the biology of

the nonpathogenic *E. coli*

and comparing genomic and transcriptomic data

*Escherichia coli* **databases**

and Metabolic Pathways

EcoCyc: Encyclopedia of *E. coli* Genes

things and its only limit is our imagination.

The main reasons why *E. coli* is the organism of choice extends and is not limited to its fast growth in chemically defined media; relative cheap culture media; does not form aggregates; industrial scalability; several molecular tools for manipulation; extensive knowledge of its genetics and genomics; extensive knowledge on its transcriptome, proteome, and metabolome, and several strains are considered biosafety 1, which renders it ideal even for teaching and school demonstrations (for example, Bio-Rad transformation kits, URL: http://www.biorad.com/es-mx/category/pglo-plasmid-gfp-kits).

Typical *E. coli*, a Gram-negative bacillus bears a rod-shaped, measures only about 1 μm long by 0.35 μm wide, although this can vary considerably depending on the strain and its conditions, there are studies regarding different mutations that affect its size and length considerably. A recent paper showed that this bacterium can grow up to 750 μm in length. By random Tn10 insertion, they found this particular mutant has an insertion at genes *ybd*N and *ybd*M whose function remains uncertain and another mutation that remains to be identified [10]. Interestingly, this mutant strain gives rise to extremely long cells that are viable (capable of cell division) and retain metabolic activity that can be useful for studies involving intracellular localization or optimize cell-surface contact.

In terms of ecology, *E. coli* is a facultative aerobe (either respiration takes place in the presence of oxygen or fermentation in its absence), which bears a sensor for oxygen presence (redox state in the quinone pool) and can activate or repress the required metabolic enzymes, depending on oxygen levels [11–13]. Also, *E. coli* and other Enterobacteriaceae can be the first organisms to colonize human infant intestine due to this capacity to metabolize oxygen or ferment because this facultative bacteria will consume the remaining oxygen in this environment so that other strict anaerobic bacteria are capable of colonizing the intestine rendering the normal microbiota found in humans and also thrive as part of the normal microbiota [14].

Phylogenetically, *E. coli* is a member of the Enterobacteriaceae and is closely related to pathogens such as *Salmonella*, *Klebsiella*, *Serratia*, and the infamous *Yersinia pestis*, which causes plague. Although *E. coli* is mostly harmless, pathogenicity islands have been identified and associated with pathogenesis in *E. coli* resulting in strains that colonize different tissues [15].

The building blocks of *E. coli* consists of about 55% protein, 25% nucleic acids, 9% lipids, 6% cell wall, 2.5% glycogen, and 3% other metabolites [16–18], which for biotechnological applications is important, since carbon flux is often a problematic issue to address in order to generate a novel metabolic pathway or to enhance a current functioning pathway. Also, carbon flux is tightly regulated by sophisticated regulatory networks that require modeling and a basic understanding of the regulatory mechanisms in order to manipulate them and achieve desired goals [19–21].

*E. coli* is part of the normal microbiota of mammals, rendering the predominant facultative microbe of the gastrointestinal tract and is currently a hot debate on the impact on normal microflora establishment and their role in disease [22].

cracking the genetic code [2], unveiling the nature of DNA replication [3], the groundbreaking advances on gene organization and regulation or as we love to call 'the operon' [4, 5], important evidence for the basis of mutations and ultimately to the evolution of organisms [6, 7], and finally, the achievement of a genetically modified organism [8] that skyrocketed several applications of the enormous capacity for manipulating this organism, rendering *E*. *coli* as

The main reasons why *E. coli* is the organism of choice extends and is not limited to its fast growth in chemically defined media; relative cheap culture media; does not form aggregates; industrial scalability; several molecular tools for manipulation; extensive knowledge of its genetics and genomics; extensive knowledge on its transcriptome, proteome, and metabolome, and several strains are considered biosafety 1, which renders it ideal even for teaching and school demonstrations (for example, Bio-Rad transformation kits, URL: http://www.bio-

Typical *E. coli*, a Gram-negative bacillus bears a rod-shaped, measures only about 1 μm long by 0.35 μm wide, although this can vary considerably depending on the strain and its conditions, there are studies regarding different mutations that affect its size and length considerably. A recent paper showed that this bacterium can grow up to 750 μm in length. By random Tn10 insertion, they found this particular mutant has an insertion at genes *ybd*N and *ybd*M whose function remains uncertain and another mutation that remains to be identified [10]. Interestingly, this mutant strain gives rise to extremely long cells that are viable (capable of cell division) and retain metabolic activity that can be useful for studies involving intracel-

In terms of ecology, *E. coli* is a facultative aerobe (either respiration takes place in the presence of oxygen or fermentation in its absence), which bears a sensor for oxygen presence (redox state in the quinone pool) and can activate or repress the required metabolic enzymes, depending on oxygen levels [11–13]. Also, *E. coli* and other Enterobacteriaceae can be the first organisms to colonize human infant intestine due to this capacity to metabolize oxygen or ferment because this facultative bacteria will consume the remaining oxygen in this environment so that other strict anaerobic bacteria are capable of colonizing the intestine rendering the normal microbiota found in humans and also thrive as part of the normal microbiota [14]. Phylogenetically, *E. coli* is a member of the Enterobacteriaceae and is closely related to pathogens such as *Salmonella*, *Klebsiella*, *Serratia*, and the infamous *Yersinia pestis*, which causes plague. Although *E. coli* is mostly harmless, pathogenicity islands have been identified and associated with pathogenesis in *E. coli* resulting in strains that colonize different tissues [15]. The building blocks of *E. coli* consists of about 55% protein, 25% nucleic acids, 9% lipids, 6% cell wall, 2.5% glycogen, and 3% other metabolites [16–18], which for biotechnological applications is important, since carbon flux is often a problematic issue to address in order to generate a novel metabolic pathway or to enhance a current functioning pathway. Also, carbon flux is tightly regulated by sophisticated regulatory networks that require modeling and a basic understanding of the regulatory mechanisms in order to manipulate them and

a key player in biotechnology (for an excellent review, see Ref. [9]).

254 *Escherichia coli* Escherichia coli - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications

rad.com/es-mx/category/pglo-plasmid-gfp-kits).

lular localization or optimize cell-surface contact.

achieve desired goals [19–21].

This organism lacks many interesting features for biotechnology, such as growing at extreme temperatures or pH, the capacity to degrade toxic compounds, pollutants, or difficult to degrade polymers [23]. But as we will see later, this bacterium is capable of doing amazing things and its only limit is our imagination.

*E. coli* harbors a genome with particular features such as a strikingly organized structure, remnents of many phages, and insertion sequences (IS) and a high transport capacity toward the cytoplasm. In 1997, the complete genome sequence of the K-12 strain was obtained, and a myriad of research was catapulted from that moment on [24]. The complete genome contains a single circular duplex molecule composed of 4,639,221 bp. Regarding its structure, protein-coding regions correspond to 87.8% of the genome, while 0.8% encodes for stable RNAs, and 0.7% consists of noncoding repeats. The remaining 11% encodes for regulatory and other functions. Nevertheless, nearly 34% (1431) proteins are considered orphan or without defined molecular function but in a recent study, it was demonstrated that by homology with distant phylogenetical relationships, they may play a role in defined molecular pathways or processes [25]. From the orphan set in *E. coli*, at least 446 contain some molecular signature that can assess their molecular role. The fact that such a vast portion of the genome remains uncharacterized for defined molecular pathways or roles paves the way for future basic research that may have a strong impact on applied research and development. For biotechnological applications, growth optimization is also a required feature of genetically modified organisms. For *E. coli*, it has been recently shown that growth can be manipulated by removing nonessential genomic sequences that also can lead to the design of important biotechnological strains for industrial purposes [26].

The advancement of genomic, transcriptomic, and proteomic technologies has led to the development of several online resources for the analysis of molecular and physiological aspects of *E. coli*. Some examples are given in **Table 1**.

Why plasmids are the basis for genetic engineering? By surveying the literature and commercial sources catalogs, there is a myriad of applications for plasmids: cloning, mutagenesis, protein fusion and overexpression, shuttle vectors from bacteria to a diverse range of hosts, among others. Plasmids must be first presented and then we provide some features that are relevant regarding the importance of plasmids in molecular biology and biotechnology.

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Plasmids are extrachromosomal molecules that are self-replicative and sometimes provide interesting features to its host. The term was first coined by Joshua Lederberg in 1952 referring to genetic elements in bacteria that remained as an independent molecule from the chromosome at any stage of their replication cycle [28]. The definition was further refined to all the autonomously replicating DNA molecules to avoid including viruses. These molecules are present not only in eubacteria but also are found in Archea and some lower eukaryotic organisms [29]. In nature, many bacteria contain self-replicating DNA molecules that can be harnessed for molecular biology applications. *E. coli* plasmids were the first ones to be exten-

In the 1970s, the first generation of cloning plasmids was created, and from that moment on, research in the biological area was enriched with a powerful tool. Plasmids must contain several important features to be used in research: proper size for ease to transform or transfect, selection markers, a replication origin, regulatory elements to control expression, and transcription termination. All features are important when designing a plasmid vector for the desired application, the reader can imagine the goal, and there will always be a way to create the molecular tool for achieving such a goal, and that is possible due to the basic structure of most plasmids used in molecular biology and their modularity [31]. In **Table 2**, we summarize some of the most important features (modules) that plasmids must have in order to serve for different applications. We point out that sequence composition and structure, copy number, selection marker, and special features such as reporter proteins or regulatory elements are the most important features in a plasmid and can influence the outcome of the desired application.

**Name Type of element Characteristics**

ColE1 Replication origin Generates 15–20 copies of each

p15A Replication origin Low copy number replication origin,

plasmid molecule. Colicin production. Related to plasmids that confer immunity to phage infections [32]. Found in low copy plasmids such as pBR322 [33]. There are mutations in this replication origin that leads to high copy number plasmids, such as pUC series that can render up to 700 copies per cell

estimated in 18–22 copies per cell [36]. This type of replication origin is often found in pACYC and its

[34, 35]

derivative vectors

sively modified for such purposes [30].

*Escherichia coli* **plasmids**

**Table 1.** Relevant *E. coli* resources and databases.

Recently, using the Genome Conformation Capture technique, it was revealed that the linear organization of the genome is also true for the 3D structure, rendering neighboring genes to form small factories that are coregulated or coexpressed and showed a higher probability of forming protein-protein interactions. This organization represents two important aspects of bacterial genomes, first, the compactness in the 3D space, containing pathways in a nonrandom distribution, and second, genes that are closer to each other tend to be coexpressed and form protein-protein interactions favoring the concept of transcription factor even in microbial cells [27].

In the following sections, we will review what we consider the modern tools for genetic engineering *E. coli*, and the future for this microbe that can be considered the toolbox for molecular biology and may be the answer to many problems that humanity may face in the future.


**5.1 Drug-delivery systems**

Next, some most recent examples are gathered where polymeric and lipid nanocapsules are used to carry a great variety of drugs. In **Table 2** some recent relevant studies are compiled.


**4. Conclusions**

The body of evidence that intends to measure the economic and financial repercussions of the Efficient Market Hypothesis (EMH) on cryptocurrencies has accumulated in an increasing pace. There is already a significant bulk of academic work that provides evidence in favor of ine fficiency in digital currency markets and primarily in the Bitcoin market.

Weak-form e fficiency that shows whether prices reflect the information contained in the past series of prices has been tested in the great majority of the thirty-eight studies under scrutiny. The largest number of academic papers examined advocate that the EMH does not hold. Long memory in cryptocurrency time series is detected as dependence from past returns is revealed. A battery of tests

from a wide spectrum are employed such as the Hurst exponent, the Bartels test, the variance ratio test and its specifications among others. Thereby, investors can use past information to predict future returns. This enables speculators to exercise profitable trading strategies suffering only very low risk.

It should be noted that long-range dependence that leads to inefficiency is found to fade out as time passes in the Bitcoin markets as well as in the cryptocurrencies market in general. This provides useful feedback and generates an even more vivid debate about the future of digital currencies. These coins constitute extremely sophisticated investment assets that have attracted an overwhelmingly upcoming number of investors and are expected to become a cornerstone in finance.

The present study fills a gap in relevant literature by providing an overall perspective of the efficiency characteristics and profit opportunities in digital currency markets. Therefore, this systematic survey enlightens policymakers, academics, investors and the economic press about the profitability dynamics inhibited in the markets of these innovative liquidity forms. Avenues for future research include the mapping of the efficiency-measuring dynamics of every methodology adopted in the relevant literature. Moreover, the connection of investor sentiment with the speed of adjustment of cryptocurrency markets towards validity of the Efficient Market Hypothesis should pave the way for new empirical examinations and systematic surveys.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.


**5.2.3 Loads**

158 Recent Advances in Arthroplasty

Fig. 1. Fatigue testing. The 3D finite element model developed in the analysis

Fig. 2. Implant attached to bone. Finite element model developed in the analysis

Abaqus 6.4.5.

All the models were created, analyzed and afterwards the results were processed using

In all cases for fatigue testing, the applied load was the load specified in the standard, of 2300 N. The load was applied on the upper surface of the piece that applys the load. For implant attached to bone the applied load was an arbitrary load of 1000 N in the

acetabular component of the prosthesis. Besides the pre-tensioning bolt load was simulated.

### **5.2.4 Results**

### **a. Fatigue testing:**

Due to the assembly load between the stem components, high levels of compressive contact stress were obtained. Therefore, and to wean the study of the influence of these stresses, we considered in the analysis the stress components S33, which would be responsible for a possible material fatigue. The simulation results obtained for the three models using different materials are given in Table 7.

Figure 3 shows the S33 stress distribution in the implant made of beta titanium alloy.

(\*) The indicated compression component was mainly due to the contact stresses.

Table 7. Simulation results for the three models

Fig. 3. S33 stress distribution (MPa)

Whereas the applied load varies between zero and 2300 N, the minimum stresses are related to those produced by the preload of the bolt connecting both parts of the implant, and the maximum stresses are due to the load of 2300 N.

Titanium as a Biomaterial for Implants 161

the loads. Comparing the stemmed models, from the point of view of stress and displacements that were produced in the bone, with the titanium Ti35Nb7Zr5Ta implant, the behaviour of the whole bone-implant is nearer to the natural bone, and the effect of

Fig. 6. S33 Stress in the bone, stemmed Ti35Nb7Zr5Ta implant model

It is well known that porosity decreases the Young's modulus of a material, thus it could be thought as a means to reduce stress shielding. It is difficult to get the properties for FEA analysis but it could be used some approach to represent the Young's modulus of a material with a given fraction of porosity. However, the effect the porosity on fatigue strength discourage using fully porous material in joint arthroplasty implants because porous metal alone does not provide sufficient mechanical strength to sustain the physiological loads

Acero, J.; Calderón, J.; Salmeron, J.; Verdaguer, J. & Consejo, C. (1999). The behaviour of

Cameron, H.; Pilliar, R. & Macnab, I. (1976). The rate of bone ingrowth into porous metal.

Currey, J. (1998). Mechanical properties of vertebrate hard tissues. *Proceedings of the*

titanium as a biomaterial: microscopy study of plates and surrounding tissues in facial osteosynthesis. *Journal of Cranio-maxillofacial Surgery,* Vol.27, No.2, (April

*Journal of Biomedical Materials Research,* Vol.10, No.2, (March 1976), pp. 295-302,

*Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine,* Vol.212,

**5.3 Fully porous material in hip implants**

1999), pp. 117-123

ISSN 00219304

No.6, (1998), pp. 399-411, ISSN 09544119

(Ryan et al., 2006).

**6. References**

stress shielding was the smaller.

Using the mechanical properties of the materials, were drawn Goodman diagrams and plotted the points corresponding to the applied loading (Figures 4 and 5). From them, it could be seen that the implants have unlimited duration of life with the geometry, loads and properties considered. However, it can be seen in the diagram for the stainless steel implant that the point representing working conditions is located near the boundary of the safe zone.

### Fig. 4. Goodman's diagram. Ti6Al4V

It should be noted that the environment in which the implant should work would influence the final fatigue behaviour.

#### **b. Implant fixed to bone:**

In Figure 6 the S33 stress is shown for the bone, from the analysis of the stemmed Ti35Nb7Zr5Ta implant. Comparing whit the model without the implant, in the stemmed ones it was observed a change in the stress pattern, because the stem shielded the bone from

Fig. 5. Goodman's diagram. 316L and Ti35Nb7Zr5Ta

Using the mechanical properties of the materials, were drawn Goodman diagrams and plotted the points corresponding to the applied loading (Figures 4 and 5). From them, it could be seen that the implants have unlimited duration of life with the geometry, loads and properties considered. However, it can be seen in the diagram for the stainless steel implant that the point representing working conditions is located near the boundary of the safe zone.

It should be noted that the environment in which the implant should work would influence

In Figure 6 the S33 stress is shown for the bone, from the analysis of the stemmed Ti35Nb7Zr5Ta implant. Comparing whit the model without the implant, in the stemmed ones it was observed a change in the stress pattern, because the stem shielded the bone from

316 L

Fig. 4. Goodman's diagram. Ti6Al4V

Fig. 5. Goodman's diagram. 316L and Ti35Nb7Zr5Ta

Ti35Nb7Zr5

the final fatigue behaviour. **b. Implant fixed to bone:**  the loads. Comparing the stemmed models, from the point of view of stress and displacements that were produced in the bone, with the titanium Ti35Nb7Zr5Ta implant, the behaviour of the whole bone-implant is nearer to the natural bone, and the effect of stress shielding was the smaller.

Fig. 6. S33 Stress in the bone, stemmed Ti35Nb7Zr5Ta implant model


**1.1 Coastal applications for UAVs**

The affordability and capability of UAV surveys to rapidly produce spatially extensive and high-resolution terrain models has been a boon to geoscientific research over the past decade (e.g., [33]). The majority of which has used Structure from Motion (SfM) photogrammetry. This technique generates automatic tie points (ATPs) between overlapping images and triangulates their position to produce a 3-D point cloud [34, 35]. The utility of UAVs and SfM have been well documented; however, difficulties remain in accurately capturing areas of topographic complexity, water boundaries, and vegetation [33]. The latter two can be particularly problematic in coastal environments due to variability in tidal ranges and/or wave run up on the seaward boundary [36, 37] and in the density of seasonally intermittent or established vegetation across the beach and foredune [38–41]. Other common problems that arise in coastal environments include wet or low texture surfaces that limit image recognition [41], maximum operational wind conditions of � < 25 km/h [37, 42], and regulations that restrict UAV flights in the vicinity of pedestrians [37].

Due to these difficulties, the breadth of UAV research monitoring sandy coastlines has been relatively limited. Research testing the quality of SfM derived point clouds to those produced by more traditional methods such as terrestrial laser scanning (TLS) has found good agreement in areas with limited vegetation height or density [39, 41]. Furthermore, the geo-referencing of UAV point clouds using survey ground control points (GCPs) have been able to obtain centimeter scale error over non-vegetated surfaces [36, 38–40, 43]. However, the elevational error over vegetated surfaces can be an order of magnitude or higher, ranging from the 10–100-cm scale and is largely controlled by vegetation density and canopy height [41]. To date, only a small number of UAV coastal monitoring studies have quantified topographic or volumetric changes of beach-dune morphology in response to storm events [37], storm seasons and human management [38, 44], or annual cycles [40, 45].

55 km/h and gusts of up to 93 km/h were recorded at the Stanhope meteorological station (SMS), located 9 km east of BB. It must be noted that SMS records wind conditions at 3 m above the surface, or 7 m below standard meteorological recordings, and likely underrepresent the peak wind speeds associated with SD. Addition-

*Brackley Beach (A), Prince Edward Island National Park, extends 6 km in an east to west orientation and is backed by highly continuous foredune. Following the storm Dorian, a flattened beach profile (B), significant*

An integrated aerial LiDAR topographic and bathymetric (topobathy) survey was conducted at Prince Edward Island National Park between the 4th and 7th of July 2019. The LiDAR data was obtained through CBCL Limited with permissions from Parks Canada, as part of the Federal Transportation Risk Assessment Initiative. A Leica Chiroptera II dual sensor Topobathy LiDAR, utilizing an infrared (240 kHz) and green laser (35 kHz), seamlessly captured 291 m swaths of both the

ally, there are no offshore buoys on the north coast of PEI to record marine conditions associated with SD; however, significant wave heights of 7–8 m and a storm surge of 1.2 m was forecasted by the Dalcoast-HFX model [48]. Peak storm surge levels, coincident with a 0.8 m high tide recorded on the north coast of PEI at 2 a.m. on September 8th, are estimated to be 2 m above mean sea level (MSL). An initial site assessment of SD's impact on BB observed modification of the beachdune morphometry including a flattened beach profile (**Figure 3B**), scarping of the entire frontal slope (i.e., from the dune toe to dune crest) of the foredune at the eastern section of BB (**Figure 3C**), and a continuous 1–2 m scarp across the majority

of the mid and western sections of BB (**Figure 3D** and **E**).

*foredune erosion (C), a continuous scarp developed alongshore (D and E).*

*Monitoring Storm Impacts on Sandy Coastlines with UAVs*

*DOI: http://dx.doi.org/10.5772/intechopen.91459*

**2.1 Pre-SD baseline survey**

**71**

**Figure 3.**

UAV and SfM applications have displayed the potential to increase the efficiency and rapidity of coastal monitoring. In order for this potential to be fully realized, difficulties arising during data collection, processing, and analysis must be addressed when developing systematic and repeatable survey strategies. Recently, UAV coastal research have also used multi-spectral (e.g., [46]) and LiDAR sensors (e.g., [47]); however, this chapter will focus specifically on UAV and SfM systems. The remainder of this chapter will be an introduction to basic quality control measures, handling of survey and environmental uncertainties, and reporting of topographic and volumetric changes associated with a storm event. Furthermore, the persistent challenges of coastal monitoring with UAVs and prospective solutions will be discussed. Ultimately, the aim of this chapter is to present a repeatable methodology to confidently report topographic change and highlight future methodological advances that are still needed to improve upon current coastal monitoring strategies using UAVs.

### **2. Study site**

Brackley Beach (BB; 46°250 50″ N 63°110 50″ W) is a part of the Prince Edward Island National Park, located on the north shore of Prince Edward Island (PEI), Canada (**Figure 3A**). BB extends �6 km alongshore in an east to west orientation and is backed by a highly continuous foredune. On September 7th and 8th BB was impacted by the post-tropical storm Dorian (SD). Sustained N–NNE winds of up to *Monitoring Storm Impacts on Sandy Coastlines with UAVs DOI: http://dx.doi.org/10.5772/intechopen.91459*

#### **Figure 3.**

**1.1 Coastal applications for UAVs**

pedestrians [37].

[40, 45].

ing strategies using UAVs.

Brackley Beach (BB; 46°250

**2. Study site**

**70**

The affordability and capability of UAV surveys to rapidly produce spatially extensive and high-resolution terrain models has been a boon to geoscientific research over the past decade (e.g., [33]). The majority of which has used Structure from Motion (SfM) photogrammetry. This technique generates automatic tie points (ATPs) between overlapping images and triangulates their position to produce a 3-D point cloud [34, 35]. The utility of UAVs and SfM have been well documented;

complexity, water boundaries, and vegetation [33]. The latter two can be particularly problematic in coastal environments due to variability in tidal ranges and/or wave run up on the seaward boundary [36, 37] and in the density of seasonally intermittent or established vegetation across the beach and foredune [38–41]. Other common problems that arise in coastal environments include wet or low texture surfaces that limit image recognition [41], maximum operational wind conditions of � < 25 km/h [37, 42], and regulations that restrict UAV flights in the vicinity of

Due to these difficulties, the breadth of UAV research monitoring sandy coastlines has been relatively limited. Research testing the quality of SfM derived point clouds to those produced by more traditional methods such as terrestrial laser scanning (TLS) has found good agreement in areas with limited vegetation height or density [39, 41]. Furthermore, the geo-referencing of UAV point clouds using survey ground control points (GCPs) have been able to obtain centimeter scale error over non-vegetated surfaces [36, 38–40, 43]. However, the elevational error over vegetated surfaces can be an order of magnitude or higher, ranging from the 10–100-cm scale and is largely controlled by vegetation density and canopy height [41]. To date, only a small number of UAV coastal monitoring studies have quantified topographic or volumetric changes of beach-dune morphology in response to storm events [37], storm seasons and human management [38, 44], or annual cycles

UAV and SfM applications have displayed the potential to increase the efficiency and rapidity of coastal monitoring. In order for this potential to be fully realized, difficulties arising during data collection, processing, and analysis must be

addressed when developing systematic and repeatable survey strategies. Recently, UAV coastal research have also used multi-spectral (e.g., [46]) and LiDAR sensors (e.g., [47]); however, this chapter will focus specifically on UAV and SfM systems. The remainder of this chapter will be an introduction to basic quality control measures, handling of survey and environmental uncertainties, and reporting of topographic and volumetric changes associated with a storm event. Furthermore, the persistent challenges of coastal monitoring with UAVs and prospective solutions will be discussed. Ultimately, the aim of this chapter is to present a repeatable methodology to confidently report topographic change and highlight future methodological advances that are still needed to improve upon current coastal monitor-

50″ N 63°110

Island National Park, located on the north shore of Prince Edward Island (PEI), Canada (**Figure 3A**). BB extends �6 km alongshore in an east to west orientation and is backed by a highly continuous foredune. On September 7th and 8th BB was impacted by the post-tropical storm Dorian (SD). Sustained N–NNE winds of up to

50″ W) is a part of the Prince Edward

however, difficulties remain in accurately capturing areas of topographic

*Spatial Variability in Environmental Science - Patterns, Processes, and Analyses*

*Brackley Beach (A), Prince Edward Island National Park, extends 6 km in an east to west orientation and is backed by highly continuous foredune. Following the storm Dorian, a flattened beach profile (B), significant foredune erosion (C), a continuous scarp developed alongshore (D and E).*

55 km/h and gusts of up to 93 km/h were recorded at the Stanhope meteorological station (SMS), located 9 km east of BB. It must be noted that SMS records wind conditions at 3 m above the surface, or 7 m below standard meteorological recordings, and likely underrepresent the peak wind speeds associated with SD. Additionally, there are no offshore buoys on the north coast of PEI to record marine conditions associated with SD; however, significant wave heights of 7–8 m and a storm surge of 1.2 m was forecasted by the Dalcoast-HFX model [48]. Peak storm surge levels, coincident with a 0.8 m high tide recorded on the north coast of PEI at 2 a.m. on September 8th, are estimated to be 2 m above mean sea level (MSL). An initial site assessment of SD's impact on BB observed modification of the beachdune morphometry including a flattened beach profile (**Figure 3B**), scarping of the entire frontal slope (i.e., from the dune toe to dune crest) of the foredune at the eastern section of BB (**Figure 3C**), and a continuous 1–2 m scarp across the majority of the mid and western sections of BB (**Figure 3D** and **E**).

#### **2.1 Pre-SD baseline survey**

An integrated aerial LiDAR topographic and bathymetric (topobathy) survey was conducted at Prince Edward Island National Park between the 4th and 7th of July 2019. The LiDAR data was obtained through CBCL Limited with permissions from Parks Canada, as part of the Federal Transportation Risk Assessment Initiative. A Leica Chiroptera II dual sensor Topobathy LiDAR, utilizing an infrared (240 kHz) and green laser (35 kHz), seamlessly captured 291 m swaths of both the topographic surface and near shore bathymetry, up to 5 m water depth. The survey was flown at an altitude of 400 m above the ground surface and maintained a sampling density of 2.72 points per meter (p/m) for the bathymetric and 18.6 p/m for the topographic surfaces. Point clouds were then geo-referenced using a total of 141 ground control points with a root mean square error (RMSE) of 0.05 m for the vertical transformation. The point cloud data was then classified for surficial and supra-surficial elements (e.g., vegetation, water surface, etc.). Following classification of the point cloud, only bathymetric and topographic surface classes were maintained to produce a 1 m 1 m 'bare earth' or digital terrain model (DTM) that will serve as a pre-SD baseline for comparisons to post-SD UAV surveys.

removal if elevated error values are recorded. Common causes of error can include GPS survey (e.g., limited number of satellites or line of sight), pedestrian interference during UAV flights, or user identification error during post processing.

BB East) using the commercial SfM software Pix4D. Alternative SFM software including Agisoft Metashape [36, 39, 40, 42–45] and Fledermaus v-7 [38] have been previously used and described. The remainder of this section will focus on the general Pix4D workflow used in this study. Initially, a target number of 10,000 ATPs were generated from each overlapping image. Next, Automatic Aerial Triangulation (AAT), Bundle Block Adjustment (BBA), and camera calibration were optimized for all images. Uncalibrated cameras, a result of errors in the internal (e.g. vibrations) or external (e.g., position and orientation) camera parameters, were deactivated to remove potential topographic deformation during point cloud generation.

*Monitoring Storm Impacts on Sandy Coastlines with UAVs*

*DOI: http://dx.doi.org/10.5772/intechopen.91459*

Furthermore, flight lines on the periphery of the survey grid had difficulty finding a sufficient number of ATPs due to areas of homogenous vegetation (**Figure 4B**), selecting prominent features such as treetops from multiple angles (**Figure 4B**), or breaking waves in the nearshore (**Figure 4C**). As a result, severe over- or under-estimation of the surface can occur by misclassifying reference elevation values. In an attempt to reduce survey error, images were clipped to the primary area of interest (i.e., the beach-dune zone). This ensured an optimal number of ATPs present in each image and removed potential edge or 'bowl' effects that can distort the point cloud in areas away from the GCP locations [40]. Next, GCPs that were clearly visible in the flight images were zoomed into and marked at their centroid (**Figure 4D**) to ensure proper pixel selection. GCPs that were blurry (**Figure 4E**), warped, displayed limited contrast, or not entirely visible were not selected because un-proper pixel selection can also introduce error during geographic transformation. Marked GCPs were then used for geographic conversion resulting in a vertical RMSE of 0.028 and 0.021 for BB West and East, respectively.

**3.2 DSM and DTM generation and environmental uncertainty**

2.65 108 points with a density of 2000 points per m2

resolution for direct comparisons to the pre-SD LiDAR DTM.

**73**

Following initial processing, a point cloud was generated and consisted of

classified using Pix4D's predefined class groups including ground, road surface, high vegetation, building, and human made objects, in order to improve DTM filtering. Point interpolation was completed using an inverse distance weighting (IDW) approach to generate a universal DSM (i.e., retaining all elevational classes) and filtered DTM (i.e., retaining only the ground elevation class) at a 1 m 1 m

Prior to measuring topographic changes between surfaces, an initial assessment of the quality of the LiDAR DTM and UAV DSM and DTM displayed significant irregularities (**Figure 5A**). For instance, the LiDAR DTM was generated using green laser that can penetrate up to 5 m depth and full wave form infrared laser that can penetrate the vegetation canopy. This provides a fully integrated bathymetric and topographic surface transitioning from the nearshore—to backshore zones (**Figure 5A**). SfM is not able to penetrate the water column and has difficulty measuring ground points in vegetated areas. As a result, the SfM DSM captures noise above the surface that is associated with wave breaking and run up in the nearshore and foreshore zones (**Figure 5A** and **B**). In the backshore, the vegetated crest and lee slope is overestimated on average by 0.5–1m(**Figure 5A**) and represents variability in vegetation density and canopy height (**Figure 5B**). Alternatively, the standard Pix4D DTM filtering method almost completely removed the foredune in areas that recorded significant scarping. **Figure 5A** shows

. The point cloud was

Images of all connected flight grids were collectively processed (i.e., BB West and


**8. Conclusions**

Titanium dioxide (TiO2) is one among the wide band gap metal oxide semiconducting materials used in thin films that have received much attraction due to its sensing properties, dielectric properties, antireflective coatings, good physical and chemical stability, high refractive index, low absorption, low cost, non-toxicity, high electron mobility, longer electron life time, low recombination losses and ease in preparation.

Titanium dioxide (TiO2), also known as titanium (IV) oxide or titania, is the naturally occurring oxide of titanium. Nanoscaled titanium dioxide (TiO2) in thin layer or nanopowder forms appertains to the most extensively studied semiconductors. This metal oxide is a promising semiconductor frequently used due to its nontoxicity, chemical stability, photocatalytic activity and low cost [37, 38]. Especially, thin films as the nanostructured electrode materials have become very important in the fields of photovoltaics, energy storage, sensing, photo-electro-catalysis, etc.

TiO2 in crystallographic form of anatase has become an interesting candidate as an n-type photoanode due to its band gap (Eg = 3.2 eV), which is higher than that for the rutile phase (Eg = 3.0 eV) and has excellent efficiency to generate the electron-hole pairs [39, 40]. The preparation of the nanostructured electrode materials with highly uniform nanoparticles has been investigated by many groups [41]. The most commonly used method is the sol-gel technique utilizing the molecular templates. The main advantage of this purely chemical method lies in a possibility of layer preparation under laboratory conditions as well as the possibility to tailor TiO2 layer properties by varying preparation conditions. Nanoparticles with controlled chemical composition, size distribution, uniformity and dispersion can be readily synthesized using reverse micelles [42].

The TiO2 thin films have been introduced as electron transport layers (ETL) because of their large band gap (3.7 eV) and well-matched energy levels (valence band of ~ 8.1 eV and conduction band of ~ 4.4 eV). The requirements for ETL involve high electron mobility and transparency in the visible region to allow transmission of light into the active layer. These requirements limit the number of materials that have these characteristics, among which is the well-known and widely used titanium oxide [43].

Thus, in the present work, I have attempted to synthesis nanosized TiO2 particles with 7.7 nm, which is less than commercially available TiO2 powder (25 nm), and modified preparatory condition followed to prepare nanocrystalline TiO2 thin films with different molar concentrations. The results from the above study exhibits that the prepared nano TiO2 films with different molar concentrations are to enhance the optical, structural and electrical behavior of film which is suitable for photovoltaic applications.

**55**

**Author details**

Lourduraj Stephen

Tiruchirappalli, Tamil Nadu, India

provided the original work is properly cited.

Department of Physics, St. Joseph's College (Autonomous),

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: lourduraj82@gmail

*Titanium Dioxide Versatile Solid Crystalline: An Overview*

*DOI: http://dx.doi.org/10.5772/intechopen.92056*

#### **Conflict of interest**

The authors declare no conflict of interest.

*Titanium Dioxide Versatile Solid Crystalline: An Overview DOI: http://dx.doi.org/10.5772/intechopen.92056*

*Assorted Dimensional Reconfigurable Materials*

be readily synthesized using reverse micelles [42].

The authors declare no conflict of interest.

The TiO2 thin films have been introduced as electron transport layers (ETL) because of their large band gap (3.7 eV) and well-matched energy levels (valence band of ~ 8.1 eV and conduction band of ~ 4.4 eV). The requirements for ETL involve high electron mobility and transparency in the visible region to allow transmission of light into the active layer. These requirements limit the number of materials that have these characteristics, among which is the well-known and widely used titanium oxide [43]. Thus, in the present work, I have attempted to synthesis nanosized TiO2 particles with 7.7 nm, which is less than commercially available TiO2 powder (25 nm), and modified preparatory condition followed to prepare nanocrystalline TiO2 thin films with different molar concentrations. The results from the above study exhibits that the prepared nano TiO2 films with different molar concentrations are to enhance the optical, structural and electrical behavior of film which is suitable for photovoltaic applications.

Titanium dioxide (TiO2) is one among the wide band gap metal oxide semiconducting materials used in thin films that have received much attraction due to its sensing properties, dielectric properties, antireflective coatings, good physical and chemical stability, high refractive index, low absorption, low cost, non-toxicity, high electron mobility, longer electron life time, low recombination losses and ease in preparation. Titanium dioxide (TiO2), also known as titanium (IV) oxide or titania, is the naturally occurring oxide of titanium. Nanoscaled titanium dioxide (TiO2) in thin layer or nanopowder forms appertains to the most extensively studied semiconductors. This metal oxide is a promising semiconductor frequently used due to its nontoxicity, chemical stability, photocatalytic activity and low cost [37, 38]. Especially, thin films as the nanostructured electrode materials have become very important in the fields of photovoltaics, energy storage, sensing, photo-electro-catalysis, etc. TiO2 in crystallographic form of anatase has become an interesting candidate as an n-type photoanode due to its band gap (Eg = 3.2 eV), which is higher than that for the rutile phase (Eg = 3.0 eV) and has excellent efficiency to generate the electron-hole pairs [39, 40]. The preparation of the nanostructured electrode materials with highly uniform nanoparticles has been investigated by many groups [41]. The most commonly used method is the sol-gel technique utilizing the molecular templates. The main advantage of this purely chemical method lies in a possibility of layer preparation under laboratory conditions as well as the possibility to tailor TiO2 layer properties by varying preparation conditions. Nanoparticles with controlled chemical composition, size distribution, uniformity and dispersion can

**8. Conclusions**

**54**

**Conflict of interest**


*Article* **Towards More Sustainable Materials for Geo-Environmental Engineering: The Case of Geogrids**

**Alessio Cislaghi 1,2,\*, Paolo Sala 1, Gigliola Borgonovo 3, Claudio Gandolfi 1 and Gian Battista Bischetti 1,2**

**Abstract:** Plastic materials are widely used in geotechnical engineering, especially as geosynthetics. The use of plastic-based products involves serious environmental risks caused by their degradation. Innovative research has been focusing on biodegradable polymers of natural origin, especially on poly(lactic acid) (PLA), to reduce the use of plastics. This study aims to explore the potentiality of biopolymers for the production of geogrids, measuring the chemical and mechanical characteristics of raw materials and of prototype samples, similar to those available on the market. First, chemical composition and optical purity were determined by hydrogen nuclear magnetic resonance (1H-NMR) and polarimetry. Furthermore, samples of uniaxial and biaxial geogrids were custom-molded using a professional 3D printer. Mechanical properties were measured both on the filament and on the prototype geogrids. The maximum tensile resistance was 6.76 kN/m for the neat-PLA filament and 10.14 kN/m for uniaxial prototype geogrids produced with PLA-based polymer mixed with titanium dioxide. PLA-based materials showed higher tensile properties than polypropylene (PP), the most common petroleum derivative. Conversely, such biomaterials seem to be more brittle and with scarce elongation rate respect PP. Nonetheless, these results are encouraging and can support the use of PLA-based materials for innovative biodegradable geosynthetics production, especially if used in combination with live plants.

**Keywords:** geosynthetics; poly(lactic acid); biopolymers; geogrids; tensile strength

#### **1. Introduction**

In recent years, the European Union (EU) has developed a new economic paradigm to foster the challenge of sustainability: a Circular Economy approach instead of the classical Linear Economy [1]. A specific focus is on the plastics that are identified as a priority and committed itself to "prepare a strategy addressing the challenges posed by plastics throughout the value chain and taking into account their entire life-cycle". In 2015, the EU plastic demand was 49 millions of tons, about 20%of which for the Construction and Demolition (CaD) sector, which is one of the most challenging fields for reaching the Circular Economy Strategy objectives [2–4]. The main result of this political effort is that many European countries have implemented a framework that leads to a recycling rate of up to 90% as early as the end of 2016. However, this will not be enough because the European Commission is working on a revision of the essential requirements for the use of plastics in CaD to ensure by 2030, all plastics will be reusable or easily recycled. Plastics in CaD are mainly used to produce geosynthetics, i.e., polymeric products used in contact with soil or rock and/or other geotechnical materials and mainly applied in civil and environmental engineering. This term covers a wide range of products such as geotextiles, geomembranes, geogrids, geonets, geocomposites, etc. [5].

**Citation:** Cislaghi, A.; Sala, P.; Borgonovo, G.; Gandolfi, C.; Bischetti, G.B. Towards More Sustainable Materials for Geo-Environmental Engineering: The Case of Geogrids. *Sustainability* **2021**, *13*, 2585. https://doi.org/10.3390/ su13052585

Academic Editor: Giuseppe Cardile

Received: 22 January 2021 Accepted: 24 February 2021 Published: 28 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Most geosynthetics are fabricated from petroleum-derived polymers such as the polyolefin and polyester family. Polypropylene (PP) covers almost the whole total production (approximately 90%), whereas polyethylene (PE) accounts for the remaining part [5,6]. In the last 7 decades, the scientific effort has been focusing on the mechanical properties to make these materials more resistant to degradation by environmental stressors (e.g., air, ultra-violet (UV) radiation, bacteria) developing mineral fiber reinforced composites and using additives to improve their stability. Thus, for many applications, biodegradability was an undesirable property, because the manufactured products must guarantee long lifespan, low cost, and mechanical properties that ensure great effectiveness. However, this purpose inevitably drove to a clash with the ecological safety. In fact, polymers are heavily subject to a wide range of degradation processes. Over time and under common environmental conditions, plastic composites may degrade into micro-plastics particles (below 5 mm), which persist into the soil and cause negative environmental effects [7]. Moreover, another threat for the environment is the leaching of additives from the composites. This process is particularly complex because many factors influence the loss of additives, such as UV radiation, humidity, temperature and product thickness [8].

Thus, since the last decade, there has promptly been an increasing interest for the natural and biodegradable materials as the biopolymers to replace the synthetic ones in the worldwide market. This considerable interest in biopolymers is associated with an increase of environmental awareness that has led to the scientific and industrial community to search for a good and cost-effective alternative to the conventional materials in several engineering applications [9].

Among the fully biodegradable polymers, the poly(lactic acid) or polylactide (PLA) is certainty the most promising [10]. PLA is a thermoplastic, high-strength and high-modulus polymer [11,12] that has already used on large-scale production and commercialized for a wide range of fields such as food technology, medical engineering, pharmaceutical, packaging and agriculture thanks to its huge versatility [13]. Such biopolymer ensures several advantages in terms of sustainability and eco-compatibility: (i) PLA derives from renewable agriculture-based resources [14]; (ii) PLA decomposes into non-toxic substances, as water, carbon dioxide and humus [9,15] (iii) PLA degrades quite slowly [16]; and (iv) PLA is recyclable and compostable [17]. In addition, neat-PLA has been reinforced with a wide variety of natural fibers such as bamboo, banana, coir, cotton, flax, hemp, jute, kenaf, ramie, sisal, etc. [18,19], and is actually used for numerous applications in automotive, aerospace, construction, civil and the sports and leisure sectors [20–22]. Nevertheless, these biopolymers are not yet adopted in geotechnical and geo-environmental engineering, even if few studies focused on geocomposites products [23,24].

In this context, the present study aims to explore the potentiality and the possibility to use biodegradable polymers, even enhanced with natural fibers, to replace the common petroleum-derivatives polymers in the production of geosynthetics for geo-environmental and geotechnical applications. For pursuing the main objective, this research joins a detailed literature review with specific laboratory measurements to provide a comprehensive and integrated framework for assessing the usability in function of the geo-environmental application. Specific purposes are:

#### **2. Materials and Methods**

#### *2.1. State-of-the-Art: Neat-PLA and PLA-Based Materials*

PLA is a linear aliphatic thermoplastic polyester derived from lactic acid, one of the simplest chiral molecules, obtained by the fermentation of renewable and biodegradable plant species, such as corn or rice starch and sugar feedstock [25]. Lactic acid exists in form of two different stereo isomers: L- and D-lactic acid; however, when lactic acid is produced by fermentation, it is largely composed by L-isomer (PLLA) for 99.5%, whereas by D-isomer for 0.5% [18]. From lactic acid, PLA can be produced through different processes: direct condensation, azeotropic dehydrative condensation or ring-opening polymerization [26].

Physical and mechanical properties of neat-PLA are related to its composition (i.e., isomers), processing temperature, annealing, time and molecular weight [27]. Hardness, stiffness, crease, and melting points are linked to the crystallization behavior, which in turn depends on the stereochemistry and thermal history. Among all factors influencing the mechanical properties of neat-PLA, the polymerization process does not apparently cause significant variations [11]. Such mechanical properties have been largely investigated since the late 1990s, thus large amount of data is available from scientific literature, and producer's technical sheets (Table 1). On average, neat-PLA has good mechanical properties: a tensile strength (TS) from 23 to 70 MPa, a tensile modulus or Young's modulus (YM) from 2.3 to 3.8 GPa, and an elongation at break (EL) about 4%. Such variability mainly depends on the polymer characteristics, such as molecular weight and specific gravity. Since the neat-PLA cannot always meet all the requirements for different fields, a common solution consists of incorporating natural fibers for improving specific properties [28–33]. However, a significant gap appears when comparing the measurements of mechanical properties on the neat-PLA with PLA blended with natural fibers, especially in terms of data availability. Most of studies have been recently carried out since a decade ago and have been summarized in Table 2.

The results of these studies underlined how the mechanical properties of the biocomposites can be significantly altered by constituent structures, contents, and production process. In several cases, the addition of natural fibers to neat-PLA improved the tensile resistance of the polymer up to 80–100% (Figure 1). Plackett et al. (2003) [34] and Ben and Kihara (2007) [35] observed an increase of TS from 55 up to more than 100 MPa reinforcing neat-PLA with jute fibers and kenaf fibers, respectively. Excellent improvement of mechanical resistance from 7 to 17 MPa occurred for a neat-PLA blended with corn husk and silane [36]. In addition, satisfactory results, in terms of TS, were obtained in several studies using fibers derived by herbaceous and woody plants. Molding neat-PLA and other natural fibers, Graupner et al. (2009) [37] observed an increase of TS between 37% and 100% using cotton and hemp fibers, according to the fiber content. Similar results were found using the ramie and flax fibers: TS increases of +36% and of +23%, respectively [38,39]. Interesting results were obtained using also fibers derived from synthetic cellulose as rayon. Despite such encouraging results, in other cases, conversely, fiber addition to neat-PLA led to a decrease of tensile strength if compared with pure polymer, for example wood-flour addition decreased PLA TS from 67 MPa to 40–57 MPa [40]. Also blends with milkweed, lesquerella, or cuphea were not successful in enhancing mechanical properties of PLA.

**Figure 1.** Change in percent in terms of Tensile Strength and Elastic Modulus measured between neat-PLA and PLA-based materials.

**Table 1.** Physical and mechanical characteristics of neat-PLA [41]. SG is the specific gravity, EL is the elongation at break, TS is the tensile strength and YM is the Young's modulus.

**Table 2.** Main characteristics of PLA-based composites collected from the literature. C is the fiber content, PT is the processing temperature, TS is the tensile strength, YM is the Young's modulus, and EL is the elongation at break. In bold font, the measurements related to neat-PLA. The values in bold refer to neat-PLA.

**Table 2.** *Cont.*

**Table 2.** *Cont.*

**Table 2.** *Cont.*

\*\* indicates that the biomaterials were alkali-treated.

#### *2.2. Tested Biomaterials*

This study conducted a series of analysis on 6 different biomaterials and 2 petroleumderivates. The selection was carried out considering the availability on the market, adaptability for the 3D printing process (i.e., consumables for 3D printers), eco-compatibility, and slow degradation rate. This choice allowed to produce uniaxial and biaxial prototype geogrids using a 3D printing processes. Based on the requirements, a total of 6 materials in form of 1.75 mm diameter filament were selected:

Moreover, PP (Renkforce©, Conrad Electronic, Hirschau, Germany) and polyethylene terephthalate glycol PETG (XYZprinting©, Taipei, Taiwan) filaments were tested and used for producing prototype geogrids for a comparison with biopolymer-based filaments and prototype samples.

#### *2.3. Chemical and Mechanical Characterization of Biomaterials*

A detailed characterization was carried out on the selected biomaterials. First, the chemical characterization of PLA-based samples was obtained through optical purity analysis and 1H-NMR spectroscopy to determinate enantiomeric ratio of the two lactic acid stereoisomers composing the PLA-based samples, and their structure. Specific optical rotation ([α]D) was registered with a JASCO P-2000 polarimeter, at 25 ◦C with a wavelength of 589 nm. PLA-based samples were solubilized in chloroform (CHCl3) at a concentration of 1.00 g/dL. If necessary, some solutions were filtered with polytetrafluoroethylene syringe filters to avoid the insoluble particulate to interfere with the polarimetric analysis. Optical purity was calculated referring to the [α]D value of pure poly (L-lactic acid). Equation (1) defines the percentage of optical purity (OP):

$$\text{OP}(\%) = \frac{[\alpha]\_{589}^{25}}{-156} \cdot 100 \tag{1}$$

where [α] is the specific rotation observed and −156 is the specific rotation for L-lactic acid enantiopure PLA, at a concentration of 1 g/dL at 25 ◦C [65]. NMR spectra were recorded at 25 ◦C with a Bruker AV 600 spectrometer at 600 MHz. Samples were prepared solubilizing PLA in CDCl3 in a range of concentration of 3–6 mg/mL.

Mechanical tests were performed using a Universal Testing Machine (MTS Criterion Model 44, MTS Systems Corporation, Eden Prairie, MN, USA) equipped with universal clamps (with a distance between the jaws of 10 cm) that avoided sample damage at the clamping points according to ASTM D6637 standard. Tensile force (in N) was exerted by a system of gears at a rate of 10 mm/min and it was measured as a function of the strain by a load cell (F.S. = 5000 N). The tensile force at the point of rupture was taken as the peak load, and TS (MPa) was calculated by dividing the breaking force by the cross-sectional area of the filament (mm2). For model geogrids, TS (kN/m) was calculated by dividing the breaking force by the sample width.

#### *2.4. Prototypes Building and Testing*

In geotechnical engineering research, the geosynthetics and in particular geogrids have been tested into physical model experiments: from the small-scale and large-scale laboratory tests to full-scale field tests. Certainly, full-scale model tests are more representative of field conditions, but they are very expensive and inevitably present difficulties associated with the model preparation [66,67]. In the literature, small-scale test results are compared to those obtained by full-scale ones [68–70]. Moreover, 3D printing has become a revolutionary technology and has proven to be an important tool for many fields of engineering applications. Thanks to the flexibility of this technology, 3D printers quickly transform theoretical design concepts into prototypes, on which it is possible to verify their feasibility. The pioneering studies that underlined how 3D printing can support geotechnical engineering are recent [71–74]. This technology allows better understanding of the mechanical behavior of innovative elements of geotechnical structures testing materials, geometries, and positions. Recently, Stathas et al. (2017) [74] using this technology, successfully fabricated and tested standard prototypes of model geogrids according to the

technical specifications of the geotechnical literature [70,75]. The TS of model geogrids used in 1:N-reinforced soil models must be 1/N2 of that of full-scale geogrids when the test is carried out under 1-g condition, or n/N2 of that of prototype (small-scale geogrids) under n-g condition in centrifuge tests. Please note that N represents the scale of a reduced model and n is the level of gravity that the model is subjected to; usually, n is set equal to N [76].

For this study, two model geogrids were designed and tested on a scale of 1:5 so that could be used to investigate the behavior of geosynthetics-reinforced soil structures in 5× *g* centrifuge models. Uniaxial and biaxial geogrids were designed scaling to 1:5 the standard dimensions of full-scale geogrids available on the market. The prototype samples were produced by a Sharebot NG 2 printer using the manufacturing technology of the fused filament fabrication (a common 3D printing process for thermoplastic materials) and the printer feedstock is a continuous filament. The dimensions of the prototype samples were 50 mm width × 150 mm length, and 0.5 mm thick, with an accuracy of 0.1 mm. Mechanical tests were carried out on 1.75 mm diameter filaments and on 3D printed geogrids samples.

#### **3. Results**

#### *3.1. Chemical and Mechanical Characterization of Biomaterials*

The average [α]D values of samples containing PLA are showed in Table 3. Neat PLA showed a specific rotation value of −128.70 ± 1.40◦ that was the highest recorded followed by h-PLA and c-PLA with −110.05 ± 1.56◦ and −107.62 ± 0.16◦, respectively. In contrast, the lowest [α]D values were recorded for ht-PLA and ad-PLA, with −70.75 ± 1.01◦ and −35.50 ± 0.14◦ respectively. Optical purity values, for being calculated on the base of [α]D, presented the same trend: n-PLA showed the highest value of 81.96 ± 0.52% while ad-PLA the lowest, with 23.50 ± 0.05%.

**Table 3.** Results of chemical properties: specific rotation [α]D ( ◦) and optical purity OP (%).

The structure of n-PLA was confirmed by NMR analysis in all the samples. In the protonic spectrum a quartet and a doublet were observed at 5.15 and 1.57 ppm. Those signals are typical of the CH and CH3 groups of the polymeric chain. In the protonic spectrum, a weak quartet at 4.35 ppm was also visible and compatible with the terminal CH. The results obtained by h-PLA 1H-NMR spectrum showed other signals in the central region, between 3.3 and 4.5 ppm, and intense high-field peaks, whereas other weak high-field peaks that could refer to compound sugars, were evident at 0.86 ppm (triplet), at 1.23 ppm (singlet) and 1.52 ppm (singlet). Thanks to the filtration process, the protonic spectrum of sample ad-PLA showed higher resolution. Signals compatible with an organic compound part of the solution (about 10%) were detected. In particular, the double triplets resonating at 4.07 and 4.38 could be attributed to methylenic structures of a polyesteric chain (O-CH2-CH2-O-COO-) and the signal at 2.31 ppm could be ascribed to a methylene group adjacent to an esteric group (CH2-COO-). The ht-PLA spectra resulted comparable before and after filtration. The structure of PLA was confirmed but signals compatibles with lactide were detected at 1.7 and 5.0 ppm. No other organic compound was detected. The c-PLA and w-PLA samples showed similar protonic spectra overlapping the quartet signals that were detected at 5.17 and 5.24 ppm. Other signals between 1.3 and 2.7 ppm among with polyesteric chains signals (about 10%) were highlighted.

From the mechanical point of view, the measurements carried out with the tensile tests provided a preliminary characterization of these materials: the results are summarized in Table 4. The filaments of n-PLA showed the most tensile resistant than the others with a TS of 51.93 ± 2.28 MPa. Among the PLA-based polymers, the filaments of the additive-enriched PLA, i.e., ad-PLA and ht-PLA, provided a relevant tensile resistance ranging between 34 MPa to 49 MPa. Conversely, a gap was evident comparing n-PLA and the additive-enriched PLA with the natural fibers reinforced-PLA. In fact, h-PLA, c-PLA, and w-PLA showed an average of TS equal to 23 MPa, 28 MPa, and 29 MPa, respectively.

**Table 4.** Measurements of mechanical properties obtained by tensile tests: tensile strength (TS), Young modulus (YM) and elongation rate (EL).

#### *3.2. Prototypes Samples Building and Testing*

The prototypes were produced with a Sharebot NG2 3D printer with Fused Deposition Modeling technology overlapping two thin layers of fused material. The extrusion temperatures ranged between 190 and 220 ◦C although it depends on which biomaterial must be printed. All the prototype samples met the necessary demands of accuracy in dimension variability [74]. In particular, the error was ±0.06 mm along the planar directions, whereas was ±0.0025 mm in thickness. The excellent accuracy did not completely prevent the lack of defects, generally concentrated in the junctions. Figure 2 shows some examples of the prototype samples of geogrids produced using n-PLA and h-PLA.

**Figure 2.** Examples of (**a**) uniaxial and (**b**) biaxial prototype geogrids produced using n-PLA (black) and h-PLA (brown) prototype geogrids.

As expected, evident differences were observed conducting tests on uniaxial and biaxial prototype samples. For all materials, the uniaxial geogrids provide a more consistent tensile resistance than the biaxial ones. The increase of TS varies from 48% of the n-PLA to 215% in the case of ad-PLA. Conversely, such difference is attenuated in the case of PETG and PP samples. In terms of TS, good performance was provided by n-PLA and additivesenhanced PLA prototypes. In particular, n-PLA uniaxial geogrids were characterized by an average TS of 6.76 ± 0.19 kN/m and by a pronounced elastic deformation phase (Figure 3). Biaxial samples provided a lower value of TS (4.56 ± 0.75 kN/m) and showed a marked with a strain at break less than 2%.

**Figure 3.** Stress-strain curves of n-PLA prototype geogrids: the continuous line corresponds to the biaxial samples, whereas the dotted line to the uniaxial ones.

Considering all the biomaterials, ht-PLA is the more resistant in both uniaxial and biaxial cases with TS values of 10.14 ± 1.50 kN/m and 6.71 ± 0.15 kN/m respectively (Figure 4). However, it was meanwhile the stiffest one. Analyzing the stress-strain curves, the plastic deformation phase seems to be completely absent and the strain at break is slightly more than 2%. In the case of ad-PLA, uniaxial geogrids showed a good tensile resistance with 6.69 ± 1.90 kN/m with an elongation at break up to 8% (Figure 5). This condition represents the best performance in terms of the elongation rate than the other prototypes produced with other biomaterials. Biaxial samples provided a lower TS (2.12 ± 1.26 kN/m) and a significant lower strain at break. Samples of PLA-reinforced with natural fibers yielded lower values in terms of TS and the same strain rate at break, if compared to the neat biomaterial. Uniaxial prototypes made using h-PLA showed a TS of 4.44 ± 1.26 kN/m, while biaxial ones 2.73 ± 1.08 kN/m (Figure 6). The prototypes containing hemp fibers resulted about 35% less resistant and less elastic than the corresponding in neat-PLA. In addition, h-PLA was the worst resistant biomaterial, without showing a relevant improvement in terms of elasticity. Undoubtedly, a certain uncertainty was related to the difficulties for reproducing the geometry of the prototype samples during the 3D printing process. Such fact was probably caused by the extreme stiffness of this biomaterial. Weak resistance was shown by geogrid prototypes produced with w-PLA. They did not provide satisfactory results both in tensile resistance and elasticity, with a recorded TS of 3.5 ± 1.04 kN/m for uniaxial models and 2.84 ± 0.73 kN/m for biaxial ones (strain at break <2%) (Figure 7). Conversely, c-PLA uniaxial geogrids combine a moderate TS with an encouraging strain at break compared to the PLA samples blended with the other natural fibers (hemp and wood blends) (Figure 8). TS was 4.49 ± 1.22 kN/m, whereas the elongation at break was over 8%. Biaxial models produced variable stress-strain curves with lower resistant characteristics: TS was 1.93 ± 0.53 kN/m.

**Figure 4.** Stress-strain curves of ht-PLA prototype geogrids: the continuous line corresponds to the biaxial samples, whereas the dotted line to the uniaxial ones.

**Figure 5.** Stress-strain curves of ad-PLA prototype geogrids: the continuous line corresponds to the biaxial samples, whereas the dotted line to the uniaxial ones.

**Figure 6.** Stress-strain curves of h-PLA prototype geogrids: the continuous line corresponds to the biaxial samples, whereas the dotted line to the uniaxial ones.

**Figure 7.** Stress-strain curves of w-PLA prototype geogrids: the continuous line corresponds to the biaxial samples, whereas the dotted line to the uniaxial ones.

**Figure 8.** Stress-strain curves of c-PLA prototype geogrids: the continuous line corresponds to the biaxial samples, whereas the dotted line to the uniaxial ones.

For a comparison with the commonly used materials, additional mechanical tests were conducted on PETG and PP prototype geogrids. The results pointed out two opposite behaviors, both for uniaxial and biaxial samples. As shown in Figure 9, TS values were 5.03 ± 0.46 kN/m and 4.89 ± 0.46 kN/m respectively for uniaxial and biaxial geogrids, whereas the elongation at break never exceeded 3% in all cases. Such results are very similar in both the prototype configurations. On the other hand, PP geogrids were characterized by a lower TS at yield, 1.04 ± 0.08 kN/m, and by a long elastic deformation phase followed by a likewise plastic deformation phase (Figure 10) with an elongation at break of 15–20% for the uniaxial model and 30–40% for the biaxial.

Finally, the mechanical tests conducted on the plastic-based materials allowed to compare the results and to discuss the intrinsic properties of biomaterials. PLA-natural fibers models were, on average, 35% less resistant to tension when compared with n-PLA ones; so, apparently, the geogrids geometry reduced the gap among the TS values of the biomaterials measured on the filaments. The ht-PLA made geogrids were the most tensile resistant and the only ones that presented TS values greater than n-PLA. Although the encouraging observations, all the PLA-based geogrid prototypes are very stiff. In fact, despite PP prototypes showed very low resistance to tension stress, they ensure a significant elasticity that, in some cases, must be requested for several geoengineering applications.

**Figure 9.** Stress-strain curves of PETG prototype geogrids: the continuous line corresponds to the biaxial samples, whereas the dotted line to the uniaxial ones.

**Figure 10.** Stress-strain curves of PP prototype geogrids. In the inside panels, the stress-strain curves are zoomed to distinguish their characteristics. The continuous line corresponds to the biaxial samples, whereas the dotted line to the uniaxial ones.

#### **4. Discussion**

#### *4.1. An Overview of Biomaterials: From the Mechanical Properties to the Biodegradation*

Many authors have already demonstrated through observations and measurements how the PLA-based biopolymer provides better performance in terms of mechanical resistance than other plastic-derived materials as PP [25]. In the present study, the measurements confirmed that PLA-based materials, in the most of cases, are more resistant under tensile stress than PP and PETG. In addition, it is significant how PP showed the lowest TS. Conversely, as expected, PLA and its blends showed to be very brittle with a scarce elongation at break. All tested materials exhibited an average elongation rate at break approximately to 2%, 600 times smaller than PP. This evidence causes some restrictions in its application and especially in civil engineering where the materials have to guarantee plastic deformation under high stress [16].

Concerning the integration of natural fibers, a discrepancy is evident between additiveenriched PLA filaments (ad-PLA and ht-PLA) and PLA-natural fibers blends (h-PLA, c-PLA and w-PLA). The natural fibers that integrated the neat-PLA matrix showed a maximum TS (23–28 MPa) lower than the n-PLA, partially offset by an increase of elasticity (approximately 3 GPa of YM) [77]. Thus, the present study did not assure that integrating PLA matrix with natural fibers provides noticeable advantages in terms of physical and mechanical properties respect than the neat material.

This range of values agree with other experiences conducted on PLA-based materials (Table 2) and includes the effects due to several factors such as the technology of 3D printer, the production process, the constituent structure, and contents. In particular, the different fabrication processes (fused deposition modeling, powder bed fusion, inject printing, stereolithography, direct energy deposition, laminated object manufacturing) are of relevant importance [78–80]. Among this variety of methods, however, the fused filament fabrication, the most used in the maker community and adopted in the present study, offers many advantages among which a relatively low melting point and a low request of energy [81]. Nevertheless, 3D process can induce an excessive porosity and crystallization among the layers, affecting the quality of the final product. To reduce the defects and improve the mechanical properties, a heat treatment post-3D printing could be an effective solution [82].

Another element of discussion is the degradability of PLA-based materials. The degradation of PLA can occur through abiotic process by the hydrolysis and/or biotic process by bacterial, fungal, and enzymatic activities. PLA clearly shows its eco-compatible characteristics decomposing into water, carbon dioxide and humus [15,17,83]. Despite its eco-friendly nature, PLA is moderately resistant to degradation processes, although can be influenced by several factors, including PLA own properties, as molecular weight and crystallinity, and environmental factors, such as humidity, temperature, pH and UV light [26]. For civil engineering applications, it is important to verify the biodegradation into the soil. On this topic, several studies carrying out soil burial tests investigated the decomposition of PLA under different shapes and its composites [64,84]. Generally, 1-year burial leads to minimal degradation under natural conditions (25 ◦C) [85–87]. Generally, PLA degradation in soil is slower than in compost medium because the latter usually has a higher moisture content and temperature range, enhancing PLA hydrolysis and assimilation by thermophilic microorganisms [88]. In addition, Calmon et al. (1999) [89] monitored the biodegradability of PLA films over 20 months showing minimal degradation or finding clear signs only after 2 years.

#### *4.2. Applications in Geo-Environmental Engineering*

In the field of geo-environmental applications, the present study encourages the application of PLA-based materials to replace the most widely used petroleum-based materials as geosynthetics. They could be an excellent solution where it is required to perform a particular function for a limited time. A set of common applications can be: (i) *containment*, where it is important to limit the soil or sediment losses of a specific geometry; (ii) *protection layers*, where localized stresses cause damages to the soil surface; (iii) *soil reinforcement*, where the application consists of increasing the soil resistance to the sediment mobilization; (iv) *erosion control*, where surface erosion of soil particles is commonly due to surface water runoff and/or wind forces; and (v) *frictional interlayer*, where it is necessary to increase the friction at the interface between two different soil layers. Moreover, besides the optimum compatibility with the use in semi-natural and natural environment, PLA-based materials can promote the plant growth, especially the root system, through their degradation [90]. It is precisely such combination that could be the basis for designing a wide variety of soil-bioengineering measures. Among them, the geosynthetics reinforcing soil structures with vegetated face steep slopes remain the most common solution promoted by landscape engineers for the erosion control and the stabilization of the shallower soil layer [91,92]. To establish a permanent vegetation cover, seeding and live planting are the widely used method [93]. Seeding or hydro-seeding is traditionally recommended for a rapid slope-land revegetation [94], whereas the live planting that consists of installing unrooted cuttings vertically or perpendicularly to the slopes, is more expensive. Plant roots growing into the geogrids apertures can help to stabilize the soil shallower layer binding soil particles, increasing soil aggregation and anchoring the crossed soil mass to the fractures into the bedrock [93,95–99]. In addition, the vegetation cover minimizes the kinetic energy of raindrops, reduces runoff, and especially contrasts the soil erosion [100–104]. The vegetation contribution increases with increasing plant growth (and root system growth) and can compensate the degradation of the mechanical performance of PLA-based geogrids over the time. According to this proposal, adopting the PLA-based materials in the geoenvironmental engineering practice allows the pursuit of the geotechnical objective (i.e., soil stabilization, erosion control) and the ecological-environmental purposes.

#### **5. Conclusions**

In the present study, laboratory tests were conducted on some biodegradable biopolymer based on PLA to characterize their properties in view of their use for geosynthetics production, as an alternative to petroleum-based polymers. An investigation on the global market of the bioplastics revealed how the PLA-based materials are the most promising for a wide range of applications in different fields (e.g., packaging, automotive products, consumer handled items, medical devices, etc.). Among a wide variety of biodegradable materials available on the market, 6 different PLA-based materials and 2 petroleumderivatives were selected as the best candidate for the applications in geo-environmental engineering practice. Chemical and mechanical tests carried out on filaments (adapt for the 3D printing) showed a tensile resistance ranging from 23 to 52 MPa and an elastic modulus from 2.0 to 3.7 GPa with the presence of L-lactic acid that varies from 23.5 to 82.0%.

Furthermore, the production of uniaxial and biaxial small-scaled standard geogrids through a 3D printer allowed a robust comparison among them and among other materials as PETG and PP. As expected, the uniaxial geogrids were approximately 90% more resistant than the biaxial ones, considering all the tested cases. Among PLA-based, n-PLA and ht-PLA (i.e., with titanium dioxide) showed the best performance in terms of TS, 6.76 kN/m and 10.14 kN/m, respectively. Such values are approximately two times higher than the same prototype samples produced by PETG and are much higher than those guaranteed by PP geogrids. Despite the higher TS, PLA-based prototype samples were less elastic than those produced by PP that present an elongation at break up to 25%.

Finally, the biodegradable PLA or PLA-based polymers could be reliable and robust materials for the use in the field of geo-environmental (civil, geotechnical and soil-bio) engineering. Further investigations on their mechanical degradation under different field conditions (water, soil, rooted-soil, etc.) to verify their durability, will allow the establishment of whether these materials may actually represent an alternative to the petroleumbased products that cause serious impacts on the environment. Clearly, there is still a gap in terms of tensile strength between biopolymer-made and petroleum-based geosynthetics, but their use in combination with live plants, where the root system development initially supports and then gradually substitutes the biodegradable products in soil reinforcement, represents a promising research field.

**Author Contributions:** Conceptualization, A.C., P.S., C.G., and G.B.B.; methodology, A.C., P.S., and G.B.; software, A.C. and P.S.; validation, A.C., P.S. and G.B.; formal analysis, A.C., P.S. and G.B.; investigation, A.C., P.S. and G.B.B.; resources, C.G. and G.B.B.; data curation, A.C., P.S. and G.B.; writing—original draft preparation, A.C. and P.S.; writing—review and editing, A.C., P.S., G.B., C.G., and G.B.B; visualization, A.C. and P.S.; supervision, C.G. and G.B.B.; project administration, C.G. and G.B.B.; funding acquisition, C.G. and G.B.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was an integral part of the project TREE:HERO, acronym of "TREE distribution patterns: HillslopE failuRe prevention through forest management", entirely funded by Fondazione Cariplo (Italy; Ref. 2017–0714) in the framework of "Research dedicated to hydrogeological instability: a contribution to the prevision, prevention and risk mitigation".

**Acknowledgments:** The authors thank the Special Issue Editor Slobodan Mickovski and the two anonymous reviewers for their suggestions and comments.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**