By Jessica Lu
Tissue engineering is a field that aims to regenerate tissues that have been damaged through disease, injury or trauma. This is done by taking cells from the body and seeding them into an appropriate scaffold, which serves as a template to grow new tissue. Then the cell-seeded scaffolds are either implanted directly to the injured site, or first cultured to grow tissues in vitro before implantation. The step of choosing a suitable scaffold is highly important for tissue engineering. The scaffold needs to mimic the ECM (extracellular matrix), the structure in which cells naturally reside (O’Brien, 2011). Although decellularized ECMs (extracellular matrices) have long been used as scaffolds in tissue engineering, they have a number of drawbacks. Hence, there is increasing desire for synthetic scaffolds to be developed (Kyburz & Anseth, 2015). This is challenging because of the complex structure and function of the ECM.
The drawbacks of using decellularized ECMs as scaffolds in tissue engineering are numerous. They can have significant variation between batches, are inherently bioactive and may elicit unwanted immune responses if implanted in vivo (Unal & West, 2020). Furthermore, the process of decellularization also depletes decellularized tissues from the proteoglycans (PGs) and glycosaminoglycans (GAGs). PGs and GAGs connect cells to collagen bundles or elastic fibres and are responsible for a range of cellular responses. GAGs also bind and store large amounts of water which allows matrices to withstand high compressive forces. As a result, the loss of PGs and GAGs causes a significant detriment to ECM structure. (Hinderer, Layland & Schenke-Layland, 2016). These drawbacks have led to great interest in engineering synthetic mimics of the ECM.
A synthetic scaffold used in tissue engineering needs to fulfil several requirements. A successful scaffold needs to be biocompatible, i.e. cells must adhere, function normally and begin to proliferate, as well as avoid a dramatic immune response (O’Brien, 2011). The ECM regulates such important cell processes through bidirectional signalling with cells using protein receptors and growth factors (Papavasiliou, 2012). Secondly, the scaffold should biodegrade, as the goal of tissue engineering is to eventually let the body’s own cells to replace the scaffold. The by-products of degradation must be non-toxic. Furthermore, a scaffold would ideally have mechanical properties that resemble where they are implanted. For example, tissue engineering scaffolds for bone should be strong and weight-bearing. Finally, it is critically important that scaffolds allow cells to penetrate and allow waste products to diffuse out (O’Brien, 2011).
One promising synthetic ECM material is hydrogel. Hydrogels are crosslinked polymers which absorb large quantities of water without dissolving. Poly(ethylene glycol) (PEG), poly(vinyl alcohol) (PVA), and poly(2-hydroxyethyl methacrylate) (PHEMA) are the most common polymers used in 3D tissue engineering (Unal & West, 2020). Their ability to absorb water has two benefits. Firstly, it gives the structure the mechanics of soft tissue. Secondly, this allows easy diffusion of molecules secreted by cells (Kyburz & Anseth, 2015). Hydrogels are inherently biologically inactive, making them a good option for implantable grafts, patches and organs. Furthermore, cells can be encapsulated in 3D hydrogels, mimicking the in vivo microenvironment better than in 2D. There are also strategies to allow hydrogels to biodegrade, either hydrolytically or in response to cell-secreted enzymes (Unal & West, 2020).
As most synthetic hydrogels are bioinert, bioactive molecules usually need to be provided for cell-scaffold interaction. Proteins and peptides may be immobilised in the scaffold. The proteins chosen are engineered to mimic those in the ECM and greatly affect cell behaviour. A huge consideration is cell adhesion, as this is usually essential for cell survival as well as cell migration, proliferation and differentiation. Shorter, cell-adhesive peptides are derived from native ECM proteins such as fibronectin, vitronectin and laminin and introduced to the scaffold. Shortened peptides are used instead of the full protein to prevent an immune response. Growth factors and other bioactive molecules should also be provided (sometimes immobilised to the scaffold), as they regulate important cellular behaviours (Unal & West, 2020). For example, vascular endothelial growth factor (VEGF) strongly induces angiogenesis. VEGF has been immobilised into PEG hydrogels and found to promote tubulogenesis, the first step to developing microvasculature (Leslie-Barbick, Moon & West, 2009).
For bone regeneration, ceramic scaffolds such as hydroxyapatite (HA) and tri-calcium phosphate (TCP) are often used. Their structural similarity to bone gives them excellent biocompatibility. In fact, HA is a primary constituent of bone. Despite this, their clinical applications are limited. Ceramics are brittle and difficult to shape for implantation. In addition, after implantation, the new bone formed in porous HA cannot sustain the mechanical loads during remodelling. Finally, the degradation rate of HA after implantation is difficult to control (O’Brien, 2011).
Composite scaffolds have also been investigated. There have been attempts to incorporate ceramics into polymer-based scaffolds (O’Brien, 2011). For example, poly-L-lactic acid and poly-L-lactic-co-glycolic acid have been mixed with the ceramics hydroxyapatite and β-tricalciumphosphate, in an attempt to improve the mechanical strength of polymer scaffolds for biodegradable bone pins (Damadzadeh et al., 2010). Other composite scaffolds are comprised of synthetic polymers combined with natural polymers. Although they have some potential, composite scaffolds still have problems with biocompatibility or biodegradability (O’Brien, 2011).
The increased control with synthetic ECMs makes them desirable over naturally occurring materials. But their use comes with the need to better understand and mimic what happens in nature. For the future, more research needs to be done on how cells interact with the ECM to create better mimics. Hydrogels in particular are promising, but work needs to be done to make them more biocompatible if they are to be used clinically in tissue engineering. Also, whilst hydrogels mimic the mechanics of soft tissue well, finding a suitable synthetic scaffold with the mechanical strength for bone or cartilage remains challenging.
Damadzadeh, B., Jabari, H., Skrifvars, M., Airola, K., Moritz, N. & Vallittu, P. K. (2010) Effect of ceramic filler content on the mechanical and thermal behaviour of poly-l-lactic acid and poly-l-lactic-co-glycolic acid composites for medical applications. Journal of Materials Science: Materials in Medicine. 21 (9), 2523-2531. Available from: doi: 10.1007/s10856-010-4110-9.
Hinderer, S., Layland, S. L. & Schenke-Layland, K. (2016) ECM and ECM-like materials — Biomaterials for applications in regenerative medicine and cancer therapy. Advanced Drug Delivery Reviews. 97 260-269. Available from: doi: 10.1016/j.addr.2015.11.019.
Kyburz, K. A. & Anseth, K. S. (2015) Synthetic mimics of the extracellular matrix: how simple is complex enough? Annals of Biomedical Engineering. 43 (3), 489-500. Available from: doi: 10.1007/s10439-015-1297-4.
Leslie-Barbick, J. E., Moon, J. J. & West, J. L. (2009) Covalently-immobilized vascular endothelial growth factor promotes endothelial cell tubulogenesis in poly(ethylene glycol) diacrylate hydrogels. Journal of Biomaterials Science.Polymer Edition. 20 (12), 1763-1779. Available from: doi: 10.1163/156856208X386381.
O’Brien, F. J. (2011) Biomaterials & scaffolds for tissue engineering. Materials Today. 14 (3), 88-95. Available from: doi: 10.1016/S1369-7021(11)70058-X.
Papavasiliou, G. (2012) Synthetic PEG Hydrogels as Extracellular Matrix Mimics for Tissue Engineering Applications. In: Sonja Sokic (ed.).Biotechnology. [e-book] Rijeka, IntechOpen. pp. Ch. 8. Available from: doi: 10.5772/31695.
Unal, A. Z. & West, J. L. (2020) Synthetic ECM: Bioactive Synthetic Hydrogels for 3D Tissue Engineering. Bioconjugate Chemistry. Available from: doi: 10.1021/acs.bioconjchem.0c00270.