1. Bioengineered Scaffolds
Liver tissue engineering (LTE) holds the potential of restoring in part or total functions of the liver, with the aim of treating acute or chronic liver diseases. The ultimate goal of LTE is to reproduce a completely functional liver to be transplanted into affected patients or be used as an extracorporeal device. Tissue engineering techniques include cell seeding or implanting cells into scaffolds with biodegradable properties and structures capable of maintaining three-dimensional (3D) cell growth. Typically, these scaffolds comprise an artificial matrix or ECM, along with a selection of polymers that offer mechanical support for 3D cell proliferation. LTE technical approaches are dependent on the use of adult hepatocytes since these cells are anchorage-dependent cells. Hepatocytes are also regarded as being very sensitive to the ECM environment for maintenance of their differentiated functions and viability. Therefore, in order to produce an effective hepatocyte cell culture, LTE requires an appropriate ECM environment
[1][2].
Scaffold material selection will have a vital effect on the success of the liver tissue engineering technical approach. The scaffold should be capable of providing sufficient support for growing tissue and surface topography for successful cell attachment. Scaffolds should also be designed to provide channels for cell migration. Animal-extracted ECM scaffolds have the advantage of supplying binding sites to enable integrin-mediated cell adhesion. However, Hammond et al., 2006 discussed several problems associated to this particular type of scaffold, which include: low mechanical strength, not being immediately scaleable and experiencing interbatch variability
[3]. Thus, biodegradable polymers are becoming increasingly popular within LTE. These polymers have been identified to behave more predictably in vitro, which is why they are more susceptible to being modified to improve cell-surface attachment. Additionally, they have the capability to be constructed into complex micro-scaffolds and degrade to form natural metabolites.
Biodegradable polymers, which include polylactic lactic acid (PLA), poly (L-lactic) acid (PLLA) polyglycolic acid (PGA) and PDMS are frequently used to produce a scaffold (
Figure1) that provides a microenvironment comparable to the in vivo environment
[4][5]. Li et al., 2013 have reported that this microenvironment includes a high supply of oxygen and nutrients, a 3D ECM, as well as high cell density. These biomaterials have been exhibited to support viable hepatocyte populations, which demonstrates their excellent biocompatibility
[6]. Hammond et al., 2006, stated how various surface-modification procedures are available to increase the cell-surface adhesion rate of these biomaterials, whilst not causing any adverse effects to their bulk properties
[3].
Figure 1. Scanning electron microscopy (SEM) micrograph (2000×) of poly (L-lactic) acid (PLLA) fibres produced from electrospinning
[5].
Jain et al., 2013 have assessed different hydrophilic polymers which have been used in the production of bioengineered scaffolds for liver reconstruction
[7]. Chitosan has been identified to have a likeness to glycosaminoglycan, thereby causing it to be a popular matrix for hepatocyte culture. Chitosan scaffolds manufactured as composites, nanofibers, and hydrogels are commonly used for the maintenance of hepatocytes in vitro. Being a hydrophilic polymer, chitosan has the ability to promote spheroid formation within hepatocytes. Hybrid scaffolds comprised of chitosan with collagen have been used effectively for hepatocyte differentiation and spheroid formation. Scaffolds constructed from alginate, as seen in
Figure 2, have been utilised to microencapsulate or cultivate hepatocytes to develop implantable constructs
[5][8]. Additionally, alginate is also a hydrophilic polymer, and as a result it stimulates spheroid formation. This consequentially increases cellular interactions, along with hepatocyte function. In order to create favourable growth conditions for hepatocyte culture, porous alginate scaffolds are produced to contain approximately 90% porosity and a pore size of 100 µm. They are capable of encouraging spheroid formation due to low cell adherence to the substrate.
Figure 2. SEM of the cross–section of alginate scaffold
[8].
Recent studies have demonstrated structure optimisation of porous and mesh scaffolds manufactured from biomaterials that retain low cell-adhesion strength. Altering the surface features of the selected biomaterial enables the possibility to further improve function of the scaffold. Hepatocytes exhibit different behaviour on a monolayer of a polymer, in contrast to a porous or mesh configuration on the same polymer. Research conducted by Edgar et al., 2016 demonstrated that porous scaffolds comprise interconnected micropores with hydrophilic characteristics and exceptional fluid absorption, resulting from their large surface area to volume ratio
[9]. Moreover, their mechanically poor architecture, flexibility, and degradability, allow porous scaffolds to be a useful application for wound repair. Mesh scaffolds have become relatively popular within the TE community because of their ability to show a structural architecture similar to that of natural soft tissue. Mesh scaffolds are produced via electrospinning nanofibers that are composed of the desired biomaterial. These nanofibers are woven to form a 3D structure capable of supporting an environment for cellular adhesion and proliferation
[3].
Furthermore, one of the most significant challenges to LTE is to manufacture a porous scaffold with high mechanical strength, along with having the ability to maintain vascularisation. Numerous studies emphasise that the function of the scaffold is determined by the following main factors: pore numbers, pore size, and pore connectivity. Size of pores could have a significant influence on cell migration, as extremely large pores may diminish vascularisation. In comparison, pores that are smaller than 100 nm could affect diffusion of nutrients and waste. Inefficient diffusion of nutrients could lead to decreased viability of implanted cells and overall failure of the implanted device. Therefore, the porosity must be suitably balanced with the chosen biomaterial, in addition to their mechanical features and cellular influence
[10][11]. The following table (
Table 1) summarizes biomaterials utilized for liver tissue engineering scaffolds.
Table 1. Summary of biomaterials used for scaffolds in liver tissue engineering.