20-Bio-A1 Biomaterials and Biocompatibility · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2014 — 04-Bio-A1 Biomaterials and Biocompatibility. Three hours, open book, any non-communicating calculator. Seven questions of equal value (20 marks each, 100 marks total); five constitute a complete paper and only the first five appearing in the answer book are marked. All seven are solved here, because this set is a study resource rather than an examination script. Every question is qualitative/descriptive (materials selection, surface science, host response) rather than numerical, except Question 7, which asks for an engineering interpretation of a small stress–strain data set — that question therefore quotes and reasons from the given numbers while still answering in flowing prose, as the question itself calls for discussion rather than a computed final answer.
Reference texts (the books an open-book candidate should have on the desk for this subject):
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
A matrix intended to keep genetically engineered skin cells alive, functional, and protein-secreting while covering a burn wound must satisfy several biological requirements simultaneously. It must be cytocompatible (no cytotoxic monomer, catalyst, or degradation product, per ISO 10993-5) and provide a surface or bulk chemistry the cells can actually attach to — either through inherent cell-adhesive ligands (a natural-derived scaffold such as collagen or a decellularized dermal matrix) or through added adhesion peptides (e.g. RGD motifs grafted onto a synthetic hydrogel). It needs appropriate porosity and pore interconnectivity so that nutrients, oxygen, and metabolic waste (and, importantly here, the secreted therapeutic protein) can diffuse freely between the cells and the wound bed, since the graft has no vasculature of its own until host capillaries ingrow. Its degradation rate must be tuned to the timescale of wound healing and host tissue ingrowth — degrading too fast strands the cells before they establish a functional layer, too slow and the scaffold blocks remodelling and vascularization. It must maintain a moist wound-healing environment (a controlled moisture-vapour transmission rate) without desiccating the cells or macerating the wound, and it must be sterilizable by a method (e.g. gamma, e-beam, or ethylene oxide at a validated dose) that does not damage the still-living cell-bearing construct or denature the pre-seeded therapeutic capability. Finally, it should be minimally immunogenic so that the host does not mount a rejection response against the graft before the therapeutic benefit is delivered.
Before the polymer choice can be finalized, the cell biologists should be asked: (1) the cell type and its normal in-vivo niche (keratinocyte vs. fibroblast vs. a co-culture), since different skin cell types are adapted to different natural ECM ligands (laminin/collagen IV for keratinocytes at a basement membrane, collagen I for dermal fibroblasts); (2) the cells' preferred attachment ligand and required substrate stiffness, because many anchorage-dependent cells are mechanosensitive and will not proliferate or differentiate correctly outside a narrow stiffness window; (3) culture conditions (temperature, pH, osmolarity, serum dependence, doubling time) needed to keep the matrix's processing (e.g. cross-linking chemistry, solvent residues) compatible with cell survival; (4) the kinetics and mechanism of protein secretion (constitutive vs. inducible, how much protein per cell per day, and whether the protein needs to diffuse out immediately or can be retained/concentrated locally) so the matrix's diffusive properties can be matched to the therapeutic dosing need; (5) the immunogenicity of the genetically modified cells themselves (are they autologous, allogeneic, or xenogeneic), which determines whether the matrix must also provide any immunoisolation function; and (6) the required in-vivo residence time of functional cells, which sets the target degradation window for the scaffold.
Once the biological requirements above are known, the physical/mechanical selection criteria are: tensile strength and elasticity approximating native skin (skin is a compliant, non-linearly elastic, anisotropic tissue), so the graft can be handled surgically and can conform to a mobile wound bed (e.g. over a joint) without tearing or restricting movement; suture or staple retention strength, since the graft must be mechanically fixed to the wound margin; flexibility/conformability to irregular burn topography; swelling behaviour and dimensional stability on hydration, so the graft does not shrink or buckle once it takes up wound exudate; porosity and pore-size distribution (a mechanical/structural property as much as a biological one, controlling both cell infiltration and mechanical anisotropy); degradation-driven loss of mechanical integrity over time, which must not outpace the host's own new collagen deposition or the wound will dehisce; and moisture vapour transmission rate, a physical property that must sit in the range of healthy skin (too high desiccates, too low macerates). Handling properties relevant to the operating room — ease of cutting, non-adherence to gloves, and shelf stability of the pre-seeded product under required storage/transport conditions — complete the practical selection criteria.
A gelatin–chitosan or collagen–glycosaminoglycan porous sponge, cross-linked to a controlled degradation half-life of a few weeks and seeded with the engineered keratinocytes, illustrates the combination in practice: natural-ECM-derived chemistry for cell attachment, tuned porosity for nutrient/protein diffusion, and a degradation rate matched to expected re-epithelialization time.
