20-Bio-B5 Rehabilitation Engineering · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, May 2013 — 04-Bio-B5 Cell and Tissue Engineering (the exam's own header). Three hours, open book, any Casio/Sharp non-communicating calculator. Six questions of equal value (20 marks each, 120 marks printed); five constitute a complete paper and only the first five appearing in the answer book are marked (100 marks total). All six are solved here, because this set is a study resource rather than an examination script. Every question is essay/descriptive (design, regulatory, and mechanistic reasoning in tissue engineering) with no numerical data to compute — the marking-scheme arithmetic is the only concrete number in the paper.
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.
i) Fibronectin. Fibronectin is a large adhesion glycoprotein bearing the RGD (Arg-Gly-Asp) integrin-binding motif; incorporating it into the collagen scaffold provides an additional, well-characterized cell-attachment ligand alongside collagen's own (weaker, integrin-α1β1/α2β1-mediated) binding sites. The expected effect is improved cell adhesion, spreading, and migration into and through the scaffold — fibronectin is also chemotactic for fibroblasts, so it would be expected to accelerate cell infiltration and, downstream, the rate at which seeded cells begin depositing their own ECM. Because fibronectin is itself an early provisional-matrix protein in natural wound healing, its addition also nudges the construct's biology toward a wound-healing-like remodelling programme.
ii) Hyaluronic acid (HA). HA is a large, highly hydrophilic glycosaminoglycan with no fixed cell-binding sequence of its own (its main cellular effects are via CD44 and RHAMM receptor signalling rather than integrin binding). Adding HA to the collagen scaffold increases water retention and swelling (raising the construct's hydration and osmotic pressure, which can loosen scaffold pore structure and reduce mechanical stiffness), and promotes cell migration and proliferation via CD44-mediated signalling, particularly favouring a more motile, less differentiated (early wound-healing-like) fibroblast phenotype. HA is also anti-adhesive to some degree at high molecular weight (it creates a hydrated, low-friction pericellular space), so at high concentration it can paradoxically reduce firm cell–matrix attachment even as it promotes migration — the net effect on construct performance depends on HA molecular weight and concentration.
iii) Elastin. Elastin (or its soluble precursor tropoelastin, typically incorporated as cross-linked elastin fragments or co-electrospun with collagen) provides elastic recoil and mechanical compliance under cyclic loading that pure collagen, being a comparatively stiff, low-extensibility protein, does not supply on its own. This is expected to make the construct better able to withstand repeated stretch-relaxation cycles without permanent deformation — important for any tissue that experiences cyclic mechanical loading (vascular wall, skin, lung). Elastin also signals through the elastin-binding protein / GXXPG-motif pathway, which can modulate fibroblast phenotype and matrix-protein synthesis, generally favouring a more quiescent, organized matrix-deposition programme rather than the highly proliferative, disorganized deposition associated with fibrotic scarring.
Beyond individual matrix proteins, whole auxiliary systems can be added. The most specific and broadly useful addition is a controlled growth-factor delivery system: for example, vascular endothelial growth factor (VEGF) encapsulated in biodegradable poly(lactic-co-glycolic acid) (PLGA) microspheres embedded within the collagen scaffold, engineered (via polymer molecular weight and lactide:glycolide ratio) to release VEGF over a sustained period of one to several weeks as the microspheres degrade. The expected effect is accelerated, sustained pro-angiogenic signalling that drives host neovascularization into the construct on a timescale matched to cell/tissue ingrowth, directly addressing the mass-transfer/vascularization limitation discussed in Questions 1–3 — a construct beyond a few hundred microns thick will otherwise develop a hypoxic, non-viable core before the host's own angiogenic response can catch up. A second, complementary system would be gene delivery to the seeded cells (e.g., a plasmid or lentiviral vector encoding a matrix protein or growth factor, delivered ex vivo before seeding or embedded in the scaffold for in-situ transduction, as elaborated in Question 6(b)) so the cells themselves become a local, sustained production source rather than relying solely on an exogenously loaded, finite depot. Where mechanical performance is the priority instead, a controlled cross-linking system (e.g., a titratable glutaraldehyde, EDC/NHS, or genipin cross-linking step) tunes the scaffold's degradation rate and stiffness independently of its composition, allowing the engineer to match scaffold persistence to the expected rate of host ECM replacement (the same matching principle raised for the foreign body response in Question 3).
A VEGF-microsphere-loaded, fibronectin-functionalized collagen scaffold, with cross-linking tuned so the scaffold's mechanical integrity persists only slightly longer than the expected vascular ingrowth and fibroblast-remodelling timeline, illustrates how the additive (a) and systems (b) answers combine into a single coherent construct design rather than being independent add-ons.
