20-Bio-A1 Biomaterials and Biocompatibility · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2016 — 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.
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 |
Averaging the three replicates, Material 1 fails at a mean stress of about 1.61 MPa and a mean strain of about 57.7%, 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 smooth, concave-up (J-shaped) rise 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 distinct, much stiffer initial near-linear region up to a clear yield point (≈10.5 MPa), 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 modulus, low strength, and brittle-like (non-yielding) failure make it best suited to low-load, non-structural soft-tissue applications — for example a soft, compliant filler, a low-load cell-scaffold matrix (Question 2), or a coating/lens-type material where the in-service stresses are always far below its failure stress and its brittleness is therefore never engaged. Material 2's high strength, very large extensibility, and ductile strain-hardening behaviour make it far better suited to load-bearing soft-tissue replacement applications that must withstand large, repeated deformation with a safety margin — for example a vascular graft wall (Question 3), a tendon/ligament augmentation device, a hernia-repair mesh, or a skin substitute (Question 2) subjected to stretching over a mobile wound bed.
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 tissue-engineered vascular graft wall (Question 3) would specifically need Material-2-like ductile, high-extensibility behaviour with a wide safety margin, since a graft that failed the way Material 1 does — abruptly, with no warning — under arterial pulsatile pressure would be catastrophic, reinforcing why the small-diameter graft problem discussed there is as much a mechanical-compliance problem as a haemocompatibility one.
