20-Bio-A1 Biomaterials and Biocompatibility · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, May 2018 — 04-Bio-A1 Biomaterials and Biocompatibility. Three hours, open book, any non-communicating calculator. Six questions of equal value (20 marks each, 100 marks total for a complete paper); five constitute a complete exam paper and only the first five appearing in the answer book are marked. All six are solved here, because this set is a study resource rather than an examination script. Most questions require an essay-format answer (materials selection, host response, surface/mechanical characterization); Question 6 additionally asks for an engineering interpretation of a small stress–strain data set, so it quotes and reasons from descriptive statistics computed from the given numbers while still answering in the flowing prose the question calls for.
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.
This question includes a full stress-strain data table. Both curves are reproduced below from the printed paper (Figures 6.1 and 6.2) and are read directly from them.
Three replicate samples per material give the stress and percent strain at maximum load; the accompanying stress-strain curves show the shape of each material's full loading path to failure.
| Material | Stress at max load (MPa) | % strain at max load |
|---|---|---|
| Material 1 | 1.464 / 1.684 / 1.671 | 61.44 / 57.5 / 54.0 |
| Material 2 | 14.418 / 13.482 / 13.5 | 506.3 / 668.8 / 588.6 |
[Figure not reproduced: Material 1 stress-strain curve as printed on the May 2018 examination paper. See the official exam paper or the cited reference text.]
[Figure not reproduced: Material 2 stress-strain curve as printed on the May 2018 examination paper. See the official exam paper or the cited reference text.]
Note on the data (assumption stated). Each plotted curve is evidently one representative specimen, not the mean of the three. The Material 2 curve (≈14.4 MPa at ≈505% strain) coincides with the first tabulated replicate (14.418 MPa, 506.3%). The Material 1 curve’s peak stress (≈1.45 MPa) matches its first replicate (1.464 MPa), but its peak strain (≈122%) is about twice the tabulated 54–61%; the paper does not explain the difference. The three-replicate table is therefore used for the quantitative comparison and the curves only for curve shape (yielding, stiffening, failure mode). No conclusion below depends on which Material 1 strain is used: Material 2 is about 4× (curve basis) to 10× (table basis) more extensible either way.
Averaging the three replicates, Material 1 fails at a mean stress of about 1.61 MPa and a mean strain of about 57.6%, whereas Material 2 fails at a mean stress of about 13.8 MPa and a mean strain of about 587.9% — roughly 8.6× the strength and 10.2× the extensibility of Material 1. Beyond the raw numbers, the curve shapes reveal a more fundamental mechanical difference. Material 1 shows a low-modulus rise (secant modulus of the order of 1 MPa over the first 20% strain) that is nearly linear to about 60% strain and then stiffens progressively (concave-up, J-type), typical of an elastomeric or collagenous soft-tissue-like material undergoing progressive fibre/chain straightening and stiffening with strain, followed by an abrupt, near-vertical drop to zero stress at failure — i.e. no yield point and no post-peak plastic region, a brittle-like failure mode despite the material's overall softness and extensibility. Material 2, by contrast, shows a very much stiffer initial region (secant modulus to the yield knee of the order of 50 MPa, roughly 50× that of Material 1) up to a clear yield knee at ≈10 MPa and only ≈25–35% strain, then a short plateau, followed by an extended strain-hardening region in which the material continues to carry increasing stress over a very large additional strain before finally fracturing — a ductile, energy-absorbing failure mode with a large post-yield safety margin (visible plastic deformation) before ultimate failure.
These differences are essential for material selection because they map directly onto different mechanical roles a soft-tissue biomaterial must play. A material's toughness (energy absorbed to failure, the area under its stress-strain curve) is far larger for Material 2, both because of its much higher stress and because of its enormous strain range, so Material 2 can absorb far more mechanical energy before failing. Material 1's abrupt, warning-free failure is a significant selection concern for any load-bearing application, since a clinician or the tissue itself gets no mechanical warning (no yielding) before catastrophic loss of integrity; Material 2's extended strain-hardening region, by contrast, gives a large window of visible deformation as a warning sign before fracture. Conversely, Material 1's much lower stiffness and strength may be exactly what is wanted where a very compliant, low-load-bearing material is required and where its lower strength is never approached in service.
Material 1’s low initial modulus and strain-stiffening J-type response are qualitatively the behaviour of native compliant soft tissues such as skin, blood-vessel wall and bladder, whose collagen fibres straighten and are progressively recruited to carry load. That makes it attractive where compliance matching matters more than strength — for example a compliant blood-contacting layer of a small-diameter vascular graft (the compliance-mismatch problem of Question 2), a soft-tissue filler or bulking agent, or a soft skin-substitute or cell-scaffold matrix — always provided the in-service stresses stay well below its ≈1.6 MPa failure stress, because it fails abruptly and without warning. Material 2, roughly 50× stiffer initially, 8.6× stronger and very extensible, with ductile strain hardening, suits load-bearing, high-toughness soft-tissue devices: tendon or ligament augmentation, hernia-repair mesh, sutures, or a reinforcing outer wrap on a graft — applications where strength, toughness and a gradual, visible failure matter more than compliance. Its stiffness would be a drawback wherever it must match a compliant host tissue (it would recreate exactly the stiff-graft compliance mismatch), and its post-yield deformation is permanent, so a design must keep cyclic service stresses well below the ≈10 MPa yield knee.
Before either material could actually be applied to living tissue, several properties beyond this single-pull-to-failure test would need to be established: fatigue/cyclic loading behaviour, since physiological tissues are loaded repeatedly rather than pulled once to failure, and a material's single-cycle strength says nothing about its cyclic fatigue life; viscoelastic behaviour (creep and stress relaxation), since soft tissues and their replacements are rarely loaded at a single fixed strain rate; anisotropy, since native soft tissues (and many candidate biomaterials, e.g. fibre-reinforced constructs) have direction-dependent properties that a single uniaxial curve does not capture; hydration dependence, since mechanical properties measured dry can differ substantially from the same material tested wet, at body temperature, in a physiological environment; and, of course, biocompatibility, degradation behaviour, and suture retention strength, none of which are addressed by a stress-strain curve alone but are equally necessary before either material could be used in a device.
A small-diameter vascular graft (Question 2) shows why neither material alone is the answer. A Material-1-like compliant, strain-stiffening wall would best match the native artery (addressing the anastomotic intimal hyperplasia discussed in Question 2(a)), but its low strength and abrupt failure would demand a large safety factor against burst under pulsatile pressure; a Material-2-like polymer supplies that strength and a gradual failure but is far too stiff to match the artery. Many composite graft designs therefore combine the two — a compliant inner layer inside a strong, extensible reinforcing outer layer — which is exactly the trade-off these two data sets expose.
