Bioactive potential of natural biomaterials: identification, retention and assessment of biological properties

Abstract

Biomaterials have had an increasingly important role in recent decades, in biomedical device design and the development of tissue engineering solutions for cell delivery, drug delivery, device integration, tissue replacement, and more. There is an increasing trend in tissue engineering to use natural substrates, such as macromolecules native to plants and animals to improve the biocompatibility and biodegradability of delivered materials. At the same time, these materials have favourable mechanical properties and often considered to be biologically inert. More importantly, these macromolecules possess innate functions and properties due to their unique chemical composition and structure, which increase their bioactivity and therapeutic potential in a wide range of applications. While much focus has been on integrating these materials into these devices via a spectrum of cross-linking mechanisms, little attention is drawn to residual bioactivity that is often hampered during isolation, purification, and production processes. Herein, we discuss methods of initial material characterisation to determine innate bioactivity, means of material processing including cross-linking, decellularisation, and purification techniques and finally, a biological assessment of retained bioactivity of a final product. This review aims to address considerations for biomaterials design from natural polymers, through the optimisation and preservation of bioactive components that maximise the inherent bioactive potency of the substrate to promote tissue regeneration.

Fig. 1

Examination of functional sites on macromolecules from sequentially based cell recognition to tertiary, 3-dimensional architectures that influence cell signalling. Cell surface receptors recognise material motifs and activate specific signalling cascades in response. CD44 cluster of differentiation 44, CS chondroitin sulphate, DDR1/2 discoidin domain receptors 1/2, DS dermatan sulfate, GPCR G-protein coupled receptor, HA hyaluronic acid, hOSCAR human osteoclast associated receptor, HS heparan sulphate, ITAM immunoreceptor tyrosine based activation motif, Jak/STAT Janus kinase/signal transducer and activator of transcription, MAPK mitogen-activated protein kinase, MyD88 myeloid differentiation primary response 88, NFAT nuclear factor of activated T-cells, NF-κB nuclear factor kappa-light-chain-enhancer of activated B cells, PI3K/Akt phosphatidylinositol 3-kinase/protein kinase B, PG proteoglycan, Rho/ROCK Rho/Rho-associated coiled-coil kinase, TGF-β transforming growth factor Beta, TLR toll-like receptor

Fig. 2

Summary of material design and processing methods that retain bioactivity. The first panel describes initial material characterisation to reveal crucial motifs present within the native tissue or starting material. Middle panel: further physical and chemical cross-linking must aim to conserve natural properties, avoiding modification of active motifs and sequences. The secondary structural organisation must be considered when modifying a starting material, and final products should mimic the mature network of the target tissue. Isolation and processing may aim to retain functional motifs associated with natural PTMs, while tissue processing and decellularisation techniques conserve many biological functions. PTMs post-translational modifications. Third panel: final evaluation is performed using functional assays and cell–matrix interaction arrays to confirm retained biological activity. ITAM immunoreceptor tyrosine-based activation motif, Jak/STAT- Janus kinase/signal transducer and activator of transcription, MAPK mitogen-activated protein kinase, NFAT nuclear factor of activated T-cells, NF-κB nuclear factor kappa-light-chain-enhancer of activated B cells, PTMs post-translational modifications, RHOA-ROCK Rho/Rho-associated coiled-coil containing protein kinase, CD3ζ cluster of differentiation 3ζ, TGF-β transforming growth factor Beta

Fig. 3

A fully high-throughput (HT) microfluidic platform for the generation of novel multi-material, multi cross-linking 3D cell-laden gradients as screening libraries (a). This high-throughput can be used to fabricate gradient libraries of polymer concentration to investigate the variable resultant cell phenotype to optimise material properties (b), or to optimise the cross-linking of materials and method of cross-linking to investigate cell–matrix interactions (c). Such a gradient platform allows for rapid screening of adhesion-related responses to determine material formulations for optimal function. In this study, cell-interactions were promoted with increasing UV cross-linking. Reproduced by permission of The Royal Society of Chemistry- Mater. Horizons (2020)

Fig. 4

Fluorescence microscopy images of bone marrow-derived MSCs positive for calcein (a) alkaline phosphatase (ALP) (b) (in black) on biomaterial formulations rendering highest ALP signal ratios and control (protein-free) conditions, after 1 day of cell culture. c Heat map representation of ALP present in all scaffolds cultured under static and dynamic conditions, for 1 day of cell culture. d Dentograms with hierarchical clustering of the average cell viability values for each formulation and combination of materials. e Calculated effects of a 3-factor model for each protein and protein combinations in ALP production and detected calcein under static and dynamic culture conditions. This figure was reproduced with permission from Elsevier (2020)