liver support system liver support system a liver support system or diachysis is a type of therapeutic device to assist in performing the functions of the liver. such systems focus either on removing the accumulating toxins (liver dialysis), or providing additional replacement of the metabolic functions of the liver through the inclusion of hepatocytes to the device (bioartificial liver device). this system is in trial to help people with acute liver failure (alf) or acute-on-chronic liver failure. liver support systemspecialtyhepatology the primary functions of the liver include removing toxic substances from the blood, manufacturing blood proteins, storing energy in the form of glycogen, and secreting bile. the hepatocytes that perform these tasks can be killed or impaired by disease, resulting in hepatic insufficiency. a sudden onset of life-threatening hepatic insufficiency is known as acute liver failure (alf) which can be seen in person with previously diseased liver or a healthy one. etymology the word diachysis derives from the greek word, διάχυσησ, which means "diffusion" the word dialysis derives from the greek word, διάλυσις, which means "dissolution" liver failure classification for hepatic insufficiency in hyperacute and acute liver failure, the clinical picture develops rapidly with progressive encephalopathy and multiorgan dysfunction such as hyperdynamic circulation, coagulopathy, acute kidney injury and respiratory insufficiency, severe metabolic alterations, and cerebral edema that can lead to brain death. in these cases the mortality without liver transplantation (ltx) ranges between 40-80%. ltx is the only effective treatment for these patients although it requires a precise indication and timing to achieve good results. nevertheless, due to the scarcity of organs to carry out liver transplantations, it is estimated that one third of patients with alf die while waiting to be transplanted. on the other hand, a patient with a chronic hepatic disease can suffer an acute decompensation of liver function following a precipitating event such as variceal bleeding, sepsis and excessive alcohol intake among others that can lead to a condition referred to as acute-on-chronic liver failure (aclf). both types of hepatic insufficiency, alf and aclf, can potentially be reversible and liver functionality can return to a level similar to that prior to the insult or precipitating event. ltx has shown an improvement in the prognosis and survival with severe cases of alf. nevertheless, cost and donor scarcity have prompted researchers to look for new supportive treatments that can act as “bridge” to the transplant procedure. by stabilizing the patient's clinical state, or by creating the right conditions that could allow the recovery of native liver functions, both detoxification and synthesis can improve, after an episode of alf or aclf. three different types of supportive therapies have been developed: bio-artificial, artificial and hybrid liver support systems (table 2). table 2: liver support systems bio-artificialartificialhybrids elad extracorporeal liver assist device mars molecular adsorbent recirculating system hepat-assist blss bioartificial liver support system prometheus fpsa fractionated plasma separation and adsorption system tecla-halss teca-hybrid artificial liver support system rfb radial flow bioreactor spad single-pass albumin dialysis mels modular extracorporeal liver support amc-bal bioartificial liver sepet selective plasma filtration therapy - bioartificial liver devices liver support systemspecialtyinternal medicine bioartificial liver devices are experimental extracorporeal devices that use living cell lines to provide detoxification and synthesis support to the failing liver. bio-artificial liver (bal) hepatassist 2000 uses porcine hepatocytes whereas elad system employs hepatocytes derived from human hepatoblastoma c3a cell lines. both techniques can produce, in fulminant hepatic failure (fhf), an improvement of hepatic encephalopathy grade and biochemical parameters. potential side effects that have been documented include immunological issues (porcine endogenous retrovirus transmission), infectious complications, and tumor transmigration. other biological hepatic systems are bioartificial liver support (blss) and radial flow bioreactor (rfb). detoxification capacity of these systems is poor and therefore they must be used combined with other systems to mitigate this deficiency. today, its use is limited to centers with high experience in their application. a bioartificial liver device (bal) is an artificial extracorporeal liver support (els) system for an individual who is suffering from acute liver failure (alf) or acute-on-chronic liver failure (aclf). the fundamental difference between artificial and bal systems lies in the inclusion of hepatocytes into the reactor, often operating alongside the purification circuits used in artificial els systems. the overall design varies between different bal systems, but they largely follow the same basic structure, with patient blood or plasma flow through an artificial matrix housing hepatocytes. plasma is often separated from the patient’s blood to improve efficiency of the system, and the device can be connected to artificial liver dialysis devices in order to further increase the effectiveness of the device in filtration of toxins. the inclusion of functioning hepatocytes in the reactor allows the restoration of some of the synthetic functions that the patient’s liver is lacking. early history the first bioartificial liver device was developed in 1993 by dr. achilles a. demetriou at cedars-sinai medical center. the bioartificial liver helped an 18-year-old southern california woman survive without her own liver for 14 hours until she received a human liver using a 20-inch-long, 4-inch-wide plastic cylinder filled with cellulose fibers and pig liver cells. blood was routed outside the patient's body and through the artificial liver before being returned to the body. dr. kenneth matsumara's work on the bal led it to be named an invention of the year by time magazine in 2001. liver cells obtained from an animal were used instead of developing a piece of equipment for each function of the liver. the structure and function of the first device also resembles that of today's bals. animal liver cells are suspended in a solution and a patient's blood is processed by a semipermeable membrane that allow toxins and blood proteins to pass but restricts an immunological response. development advancements in bioengineering techniques in the decade after matsumara's work have led to improved membranes and hepatocyte attachment systems. cell sources now include primary porcine hepatocytes, primary human hepatocytes, human hepatoblastoma (c3a), immortalized human cell lines and stem cells. use the purpose of bal-type devices is not to permanently replace liver functions, but to serve as a supportive device, either allowing the liver to regenerate properly upon acute liver failure, or to bridge the individual's liver functions until a transplant is possible. function bals are essentially bioreactors, with embedded hepatocytes (liver cells) that perform the functions of a normal liver. they process oxygenated blood plasma, which is separated from the other blood constituents. several types of bals are being developed, including hollow fiber systems and flat membrane sheet systems. various types of hepatocytes are used in these devices. porcine hepatocytes are often used due to ease of acquisition and cost; however, they are relatively unstable and carry the risk of cross-species disease transmission. primary human hepatocytes sourced from donor organs present several problems in their cost and difficulty to obtain, especially with the current lack in transplantable tissue. in addition, questions have been raised about tissue collected from patients transmitting malignancy or infection via the bal device. several lines of human hepatocytes are also used in bal devices, including c3a and hepg2 tumour cell lines, but due to their origin from hepatomas, they possess the potential to pass on malignancy to the patient. there is ongoing research into the cultivation of new types of human hepatocytes capable of improved longevity and efficacy in a bioreactor over currently used cell types, that do not pose the risk of transfer of malignancy or infection, such as the hepz cell line created by werner et al.. hollow fibre systems similar to kidney dialysis, hollow fiber systems employ a hollow fiber cartridge. hepatocytes are suspended in a gel solution such as collagen, which is injected into a series of hollow fibers. in the case of collagen, the suspension is then gelled within the fibers, usually by a temperature change. the hepatocytes then contract the gel by their attachment to the collagen matrix, reducing the volume of the suspension and creating a flow space within the fibers. nutrient media is circulated through the fibers to sustain the cells. during use, plasma is removed from the patients blood. the patient's plasma is fed into the space surrounding the fibers. the fibers, which are composed of a semi-permeable membrane, facilitate transfer of toxins, nutrients and other chemicals between the blood and the suspended cells. the membrane also keeps immune bodies, such as immunoglobulins, from passing to the cells to prevent an immune system rejection. cryogel-based systems currently, hollow-fibre bioreactors are the most commonly accepted design for clinical use due to their capillary-network allowing for easy perfusion of plasma across cell populations. however, these structures have their limitations, with convectional transport issues, nutritional gradients, non-uniform seeding, inefficient immobilisation of cells, and reduced hepatocyte growth restricting their effectiveness in bal designs. researchers are now investigating the use of cryogels to replace hollow-fibres as the cell carrier components in bal systems. cryogels are super-macroporous three-dimensional polymers prepared at sub-zero temperatures, by the freezing of a solution of cryogel precursors and solvent. the pores develop during this freezing process – as the cryogel solution cools, the solvent begins to form crystals. this causes the concentration of the cryogel precursors in the solution to increase, initiating the cryogelation process and forming the polymer walls. as the cryogel warms, the solvent crystals thaw, leaving cavities that form the pores. cryogel pores range in size from 10-100 µm in size, forming an interconnected network that mimics a capillary system with a very large surface area to volume ratio, supporting large numbers of immobilised cells. convection mediated transport is also supported by cryogels, enabling even distribution of nutrients and metabolite elimination, overcoming some of the shortcomings of hollow-fibre systems. cryogel scaffolds demonstrate good mechanical strength and biocompatibility without triggering an immune response, improving their potential for long-term inclusion in bal devices or in-vitro use. another advantage of cryogels is their flexibility for use in a variety of tasks, including separation and purification of substances, along with acting as extracellular matrix for cell growth and proliferation. immobilisation of specific ligands onto cryogels enables adsorption of specific substances, supporting their use as treatment options for toxins, for separation of haemoglobin from blood, and as a localised and sustained method for drug delivery. jain et al. investigated poly(an-co-nvp) and poly(nipaam)-chitosan cryogels as hepatocyte carriers in a bal system. they found that cell-seeded bioreactors had reduced the levels of bilirubin and ammonia in the plasma (by 58.9 ± 6.7% and 61.2 ± 7%, respectively) and increased the level of albumin by 31.1 ± 28% compared to the control group. this demonstrated that the hepg2 cells in the bioreactor were able to carry out detoxifying and synthetic functions of normal liver cells. however, after 3 hours of plasma circulation, hepatocyte function deteriorated rapidly. this was likely due to the toxic effect of aclf plasma on hepatocytes, or nutrient and oxygen depletion. damania et al. took this system a step further, utilising hepg2 seeded poly(nipaam)-chitosan cryogels in a bioreactor, along with an activated charcoal cloth used as a filter. as seen with the artificial bal devices, activated charcoal is useful in the filtration of liver toxins, including ammonia and bilirubin. the researchers then incorporated the bioreactor into a rat model, connecting rats with induced liver failure to the system and passing their blood across a plasma-separation membrane, with the plasma running through the seeded cryogel and charcoal filter. they compared results against rats with induced liver failure who were not connected to a bioreactor. measuring levels of ammonia, urea, bilirubin, albumin, and ast over a 3-hour time period, they found a reduction in bilirubin, ast, and urea levels and an increase in albumin levels, suggesting the bioreactor is functioning in its removal of toxins and synthesis of new proteins. in addition, they ran the same experiment using an acellular bioreactor, and found that bilirubin and ast decreased over time, but to a lesser degree than the seeded bioreactor, showing that the detoxification results of the seeded reactor were due to a combination of both the hepg2 cells, and the activated charcoal cloth. however, in the cell-seeded reactor there was an increase in the levels of ammonia, which the researchers stated could be attributed to these toxins having a detrimental effect on the cells over time, reducing their function. clinical studies there have been numerous clinical studies