*Enhanced Liposuction - New Perspectives and Techniques*

tissue up to the depth of 5 cm. The Lipo-coagulation, results in liquefaction of the adipose tissue, hemostasis and stimulated contraction of adjacent vertical, oblique and horizontal fibers of the FSN, that connects the overlying soft tissue to the underlying muscle.

**Figure 20** shows thermal profile created by RFAL cannula inside porcine tissue.

The temperature around the internal electrode is 70 °C. The volume exposed to high temperature around the cannula. The tissue between internal and external electrode is exposed to directional RF flowing between the electrodes.

Computer simulation shows similar thermal profile (**Figure 21**) to the measured thermography.

One of the advantages of RF energy is that it is can be delivered into the body though the very tiny sub-millimeter cannula. That allows to minimize incision and mechanical trauma at treatment of such delicate zones as face and neck [33]. Large size cannula results in higher non-uniformity and especially for subcutaneous fat.

**Figure 20.** *Thermal profile in the tissue created by RFAL device.*

**Figure 21.**

*Computer simulation of temperature field created by RFAL device.*


**3. Bioinks**

Bioink is a term given to the precursor material used during printing. The main component of the bioink usually consists of a hydrogel. Hydrogels are used as they are highly hydrated, enable cell encapsulation and provide a cushioning effect during fabrication. Furthermore, hydrogels have adaptable rheological properties making them suitable for processing and printing [15]. Hydrogels used in tissue engineering applications are either naturally derived or synthetically derived polymers.

#### **3.1 Naturally derived hydrogels**

Collagen, gelatin (denatured collagen), silk, alginate and chitosan are the most commonly used naturally derived polymers in bioprinting with collagen being used in 26% of the bioprinting-relevant literature [26]. Natural polymers provide great bioactivity and high adhesiveness, resulting in high cell viability and proliferation. However, a lack of mechanical competence and reproducibility limit their use in a non-composite form.

#### **3.2 Synthetically derived hydrogels**

Aliphatic polyesters and poly(ethylene glycol) are the most commonly used synthetic polymers in bioprinting [27]. Synthetic polymers have a high printing fidelity and controllable degradation kinetics and mechanical properties. However, their inert properties result in low cell viability and proliferation rate [28]. Aliphatic polyesters used in tissue engineering are usually poly(lactic acid), poly(glycolic acid) and poly(caprolactone). Refer to **Table 1** for a comparison of their different properties.

#### **3.3 Characteristics**

When processing bioinks, three main parameters must be considered: viscosity of the precursor hydrogel, gelation and network stiffness post-processing [30].

#### **Table 1.**

*Correlation between water affinity and degradation rate for aliphatic polyesters.*

The latter two points can affect cell viability, proliferation, migration and even differentiation. The three parameters are mainly affected by the type of polymer, its concentration and molecular weight. Other factors that can have an adverse effect on cells are the shear stress induced during printing and the time period during which the cells are exposed to non-physiological conditions (fabrication time).

Rheology of the bioink, particularly viscosity, is an important consideration for printing. Viscosity affects printing fidelity and also influences the shear stresses induced during printing. High shear stresses during printing can damage cells in nozzle-based techniques leading to higher cell apoptotic activity. Each printing technique requires a different viscosity range; inkjet-based printers require low viscosity solutions [31], extrusion-based printers require a high viscosity [9], while laser-assisted bioprinting requires a medium viscosity [5]. However, stereolithographic techniques are not as limited by hydrogel viscosity ranges. Recent research has shown that the sheer stresses induced by the applied pressure in extrusion printing can be alleviated if the bioinks are prepared as isotonic solutions [32].

The objective to have a 3D structure laden with cells that are viable, proliferative and differentiative, along with a structure that exhibits mechanical competence, stability and biologically relevant complexity is dependent on the bioink and the printing process. Ultimately, the type of polymer, its chemical properties, viscosity, gelation, stiffness and fabrication time will affect the cells' status and the print fidelity.


**4. Irradiation pretreatment and its mechanisms**

Among various physical methods, irradiation is considered an attractive method for biomass and biofiber pretreatment. In biomass irradiation process, biomass and biofiber is exposed to high- energy radiations such as ultrasonic waves, microwaves, γ-rays, and electron beam. The irradiation effect on the biomass and process mechanisms varies according to the method applied. Generally, radiation processing technology is defined as: a radiolysis reaction, which uses γ-rays from radioisotopes such as cobalt-60 or cesium-137, or an electron beam produced by an electron accelerator to induce degradation of cellulose. In this process, the high energy radiation generated could change the characteristic of cellulosic biomass including: enhance specific surface area, reduce the degree of polymerization and crystallinity of cellulose, hydrolysis of hemicellulose, and partial depolymerization of lignin [26–29]. Typically, the irradiation pretreatment mechanism mode significantly depends on the technology applied during the pretreatment process. The effect of irradiation pretreatment is assessed base on the reducing sugar production during enzymatic saccharification and the solid residues left after pretreatment. The effectiveness of the treatment depends on several factors such as frequency of radiations, time of exposure, composition of the biomass, and resistance to the radiations by medium between radiations and biomass [30, 31]. Besides, the pretreatment combination used of the irradiation pretreatment and chemical treatment also gives a significant effect on the reducing sugar production during the enzymatic saccharification process [32, 33]. The detailed explanation on the effect of irradiation pretreatment on the biomass and biofiber structure and functional group is described in the next section.

#### **4.1. Type of irradiation pretreatments**

There are four different irradiation pretreatment methods that is commonly being used to pretreat biomass prior to enzymatic saccharification process. The irradiation methods are gamma-ray irradiation, electron-beam irradiation, microwave, and ultrasonication. Afore‐ mentioned in the previous section, the pretreatment mechanism of each process is different according to the method applied.