Biomedical Engineering

110 flashcards by MentisLibrary

Hemodynamics, biomechanics, biomaterials, bioinstrumentation, medical imaging, biotransport, cell electrophysiology, tissue engineering and device regulation, for undergraduates and engineers moving into medical devices.

What is the Hagen-Poiseuille law, and why does vessel radius dominate it?
Steady laminar flow through a rigid tube is proportional to the pressure drop and to the fourth power of the radius. A small change in radius swamps every other variable.
How is vascular resistance defined, and what is the systemic value?
As the pressure drop across a bed divided by the flow through it. Systemic vascular resistance is computed from mean arterial pressure, right atrial pressure and cardiac output.
How do you calculate mean arterial pressure from a cuff reading?
Diastolic pressure plus one third of the pulse pressure, because the heart spends roughly two thirds of the cycle in diastole at rest.
How is cardiac output computed, and what is a normal value?
Heart rate multiplied by stroke volume. Indexing it to body surface area gives cardiac index, which allows comparison between patients of different size.
Why is blood a non-Newtonian fluid?
Red cells aggregate into rouleaux at low shear and deform and align at high shear, so apparent viscosity falls as shear rate rises. Plasma alone is Newtonian.
What is the Fahraeus-Lindqvist effect?
Apparent blood viscosity falls as tube diameter decreases below about 300 micrometres, because red cells migrate to the axis and leave a low viscosity plasma layer at the wall.
State the law of Laplace for a cylindrical vessel and say what it explains.
Wall tension is the product of transmural pressure and radius, so wall stress also depends on thickness. A dilated vessel carries far more wall stress at the same pressure.
What is wall shear stress, and why do endothelial cells care about it?
The tangential drag the flowing blood exerts on the endothelium. Endothelial cells sense it and respond by producing nitric oxide, aligning with flow and staying quiescent.
What does the Womersley number describe?
The ratio of unsteady inertial forces to viscous forces in pulsatile flow. A high value means the velocity profile is blunt and out of phase with the pressure gradient rather than parabolic.
Is flow in the aorta laminar or turbulent?
Mostly laminar, but the peak systolic Reynolds number exceeds the usual transition value, so brief turbulence occurs at peak ejection and settles again in diastole.
What does the Windkessel model represent?
The elastic aorta as a compliance in parallel with a peripheral resistance. It stores blood in systole and discharges it in diastole, converting pulsatile ejection into near steady capillary flow.
How is pulse wave velocity related to arterial stiffness?
By the Moens-Korteweg relation: velocity rises with the square root of elastic modulus and wall thickness and falls with radius and density. It is a direct clinical measure of stiffness.
What is hematocrit, and how does it affect flow?
The volume fraction of red cells in blood. Raising it increases oxygen carrying capacity but also viscosity, so there is an optimum rather than a maximum.
Why is a bioprosthetic heart valve chosen over a mechanical one, or the reverse?
Mechanical valves last decades but require lifelong anticoagulation because of thrombogenic surfaces and shear. Bioprosthetic valves avoid anticoagulation but calcify and degrade, typically in 10 to 20 years.
How are stress and strain defined for biological tissue?
Stress is force per unit area and strain is fractional deformation. Their ratio in the linear region is the elastic modulus, though most tissues are only approximately linear.
How stiff and strong is cortical bone?
Its longitudinal elastic modulus is about 17 to 20 GPa, with ultimate compressive strength near 170 MPa and tensile strength near 130 MPa. It is stronger in compression than in tension.
What is stress shielding, and which implant property drives it?
Bone unloaded by a much stiffer implant resorbs according to Wolff's law. The driver is the mismatch in elastic modulus between the implant and the bone it bridges.
State Wolff's law in engineering terms.
Bone remodels its density and trabecular orientation in response to the mechanical loads it habitually carries. Load it more and it densifies; unload it and it resorbs.
What does viscoelasticity mean for a tissue, and what three behaviours reveal it?
The material response depends on loading rate as well as magnitude. Creep under constant load, stress relaxation under constant deformation, and hysteresis on cyclic loading all follow.
How do the Maxwell and Kelvin-Voigt models differ?
Maxwell puts a spring and dashpot in series, so it relaxes stress but creeps without limit. Kelvin-Voigt puts them in parallel, so it creeps to a limit but cannot relax stress instantaneously.
What is the aggregate modulus of articular cartilage, and why is it so low?
About 0.5 to 0.9 MPa. Cartilage carries load mainly through pressurised interstitial fluid, so its solid matrix stiffness is low while its instantaneous load capacity is high.
How large is the ground reaction force in walking and running?
About 1.2 times body weight at the peaks of walking and 2 to 3 times body weight in running. It is measured directly with a force plate.
Why is the hip joint reaction force several times body weight in single leg stance?
The abductor muscles act on a short moment arm compared with the long arm of body weight about the hip centre, so they must pull hard and their force adds to the joint load.
What are the phases of the gait cycle and their proportions?
Stance occupies about 60 percent of the cycle and swing about 40 percent, with two double support periods that vanish as walking becomes running.
What is inverse dynamics in gait analysis?
Working from measured motion and ground reaction forces back to the net joint moments and forces, segment by segment, using Newton and Euler equations.
Describe the Hill muscle model.
A contractile element with force-length and force-velocity properties, in series with an elastic element and in parallel with a passive elastic element. It is the workhorse of musculoskeletal simulation.
Why does eccentric contraction produce more force than concentric?
The force-velocity relationship is asymmetric: lengthening a contracting muscle recruits passive and cross bridge resistance, giving forces up to 1.5 to 1.8 times the isometric maximum.
How is fatigue life of an orthopaedic implant estimated?
From the stress-life curve of the alloy combined with the expected cycle count. A hip stem sees roughly one million gait cycles per year, so a 20 year design life demands a high cycle fatigue analysis.
What does biocompatibility actually mean, and what standard governs testing?
The ability of a material to perform with an appropriate host response in a specific application. It is not a property of the material alone but of the material, the site and the duration.
Why is Ti-6Al-4V so widely used in implants?
It combines a modulus closer to bone than steel or cobalt chrome, excellent corrosion resistance from a self repairing titanium dioxide passive layer, and good strength to weight.