Biomedical Engineering Fundamentals: Biomechanics, Biomaterials and Physiology for Engineers
This domain checks whether you can turn mechanics, materials science and quantitative physiology into defensible device decisions. Questions are usually short calculations wrapped in a clinical scenario (implant, catheter, prosthetic) plus a judgement call on material selection.
Mechanics of tissue and implants
- Engineering stress σ = F/A and strain ε = ΔL/L0; Young's modulus E = σ/ε only in the linear elastic region.
- Typical values: cortical bone E ≈ 15–20 GPa with ultimate tensile strength ≈ 100–150 MPa (stronger in compression); cancellous bone 0.1–2 GPa. 316L stainless steel ≈ 193 GPa, Ti-6Al-4V ≈ 110 GPa, Co-Cr alloy ≈ 230 GPa.
- Modulus mismatch between implant and bone drives stress shielding and periprosthetic bone resorption.
- Soft tissues are viscoelastic: they creep, relax and show hysteresis, so rate-dependent behaviour means quasi-static data cannot justify an impact or high-rate design.
- Vessel wall tension follows the Laplace relation T = P·r for a thin cylinder, so doubling aneurysm diameter doubles wall tension at the same pressure.
Biomaterials selection
- Judge on three axes: mechanical match, corrosion or degradation behaviour, and biocompatibility evidence under ISO 10993.
- 316L is strong and cheap but vulnerable to pitting and crevice corrosion in chloride environments; titanium relies on a stable TiO2 passive layer; Co-Cr is chosen for wear couples; UHMWPE crosslinking improves wear resistance but reduces fatigue and fracture toughness.
- Hydrolytically degradable polyesters (PLA, PLGA) must have a degradation half-life matched to the healing timeline of the host tissue.
Quantitative physiology
- Cardiac output CO = HR × SV; mean arterial pressure ≈ DBP + (SBP − DBP)/3.
- Poiseuille flow Q ∝ r⁴, so a 20% reduction in lumen radius reduces flow to about 41% of baseline.
Worked example
A 12 mm diameter femoral stem carries 3 kN. Area = π(0.006)² = 1.131 × 10⁻⁴ m². Stress = 3000 / 1.131 × 10⁻⁴ ≈ 26.5 MPa. Against a Ti-6Al-4V yield near 850 MPa the static safety factor is about 32, which is irrelevant: the governing case is fatigue at 10⁷ cycles (endurance ≈ 500 MPa), and even that passes, so the real design problem is the 110 GPa versus ~17 GPa modulus gap producing stress shielding. Show that reasoning chain, not just the number.
Traps that cost marks
- Quoting a static safety factor when fatigue or wear governs.
- Comparing a ceramic's compressive strength with a tensile loading case (ceramics are weak in tension).
- Ignoring Poisson effects and interference fit when computing press-fit stresses.
- Treating 2D culture or bulk-material data as directly transferable to a loaded, hydrated implant.