Intermediate Biomedical Engineering interview questions

150 questions with answers (BME-151 to BME-300). Try each one aloud before reading the answer.

Anatomy & Physiology for Engineers

BME-151 · Calculate the K⁺ equilibrium potential if [K⁺]o = 5 mM and [K⁺]i = 140 mM at 37 °C.

Using the Nernst equation, E = (RT/zF) ln(Co/Ci) ≈ 61.5 log₁₀(5/140) mV = 61.5 × (−1.447) ≈ −89 mV.

BME-152 · Why is the resting membrane potential close to E_K rather than E_Na?

At rest the membrane is far more permeable to K⁺ than to Na⁺, because of leak channels. The Goldman–Hodgkin–Katz equation weights each ion's equilibrium potential by its permeability, so the result lies near E_K (about −90 mV) rather than E_Na (about +60 mV).

BME-153 · What is the Goldman–Hodgkin–Katz equation used for?

It calculates the membrane potential when several ions are permeant: V = 61.5 log₁₀[(P_K[K]o + P_Na[Na]o + P_Cl[Cl]i)/(P_K[K]i + P_Na[Na]i + P_Cl[Cl]o)] mV at 37 °C. Chloride terms are inverted because Cl⁻ is an anion.

BME-154 · Why does the AV node delay conduction?

Its slow conduction, about 0.1 s, lets the atria finish contracting and fill the ventricles before ventricular systole begins. It also limits how fast the ventricles respond to atrial tachyarrhythmias.

BME-155 · How is mean arterial pressure estimated from systolic and diastolic values?

MAP ≈ DBP + (SBP − DBP)/3, because diastole lasts longer than systole. For 120/80 mmHg, MAP ≈ 80 + 40/3 ≈ 93 mmHg.

BME-156 · What is the Frank–Starling mechanism?

Within physiological limits, greater end-diastolic volume (preload) stretches the myocardial fibres and increases stroke volume. This automatically matches ventricular output to venous return.

BME-157 · How does myelination increase nerve conduction velocity?

Myelin raises membrane resistance and lowers capacitance, so the action potential jumps between nodes of Ranvier (saltatory conduction). Large myelinated fibres conduct at up to about 120 m/s, compared with about 0.5–2 m/s for unmyelinated fibres.

BME-158 · Why is the long refractory period of cardiac muscle important?

The plateau phase, from Ca²⁺ influx, makes the absolute refractory period last almost as long as contraction. This prevents tetanic contraction and allows the ventricles to relax and refill between beats.

BME-159 · How do you calculate arterial oxygen content?

CaO₂ = 1.34 × Hb × SaO₂ + 0.003 × PaO₂ (mL O₂/dL). With Hb 15 g/dL, SaO₂ 0.98 and PaO₂ 100 mmHg: 19.7 + 0.3 ≈ 20 mL O₂/dL. Nearly all oxygen is carried bound to haemoglobin.

BME-160 · What shifts the oxygen–haemoglobin dissociation curve to the right?

Increased CO₂, increased H⁺ (lower pH, the Bohr effect), higher temperature and increased 2,3-BPG. A right shift lowers Hb's affinity for O₂ and helps unload oxygen in active tissues.

BME-161 · What is GFR and how is it measured?

Glomerular filtration rate is the plasma volume filtered per minute, about 125 mL/min in adults. It is measured from the clearance of a freely filtered, non-reabsorbed marker such as inulin, or estimated clinically from serum creatinine.

BME-162 · How does the baroreceptor reflex regulate blood pressure?

Stretch receptors in the carotid sinus and aortic arch sense pressure and signal the medulla. A rise in BP increases vagal output and reduces sympathetic output, lowering heart rate, contractility and vascular resistance. This is a fast negative-feedback loop.

BME-163 · How are compliance and resistance defined for vessels and lungs?

Compliance C = ΔV/ΔP shows how easily a structure distends. Resistance R = ΔP/Q shows opposition to flow. Lung compliance is about 100–200 mL/cmH₂O in healthy adults and falls with fibrosis or ARDS.

Biomedical Instrumentation

BME-164 · Why do biopotential amplifiers need very high input impedance?

Electrode–skin impedance can be tens of kΩ to MΩ. A low input impedance forms a voltage divider that attenuates the signal, and impedance imbalance converts common-mode voltage into a differential error. Input impedances above 10–100 MΩ are typical.

BME-165 · Calculate the gain of a three-op-amp instrumentation amplifier with R = 25 kΩ, R_G = 1 kΩ and a unity-gain difference stage.

G = (1 + 2R/R_G) × 1 = 1 + 50/1 = 51.

BME-166 · What does a CMRR of 100 dB mean in practice?

Ad/Ac = 10^(100/20) = 10⁵. With a differential gain of 1000, the common-mode gain is 0.01, so 1 V of common-mode interference appears as only 10 mV at the output, or 10 µV referred to the input.

BME-167 · What is a driven right-leg circuit?

A feedback circuit that senses the common-mode voltage on the measurement electrodes, inverts and amplifies it, and drives it back into the body through the right-leg electrode. This reduces common-mode interference and limits current for safety.

BME-168 · What is the difference between polarizable and non-polarizable electrodes?

A perfectly polarizable electrode, such as platinum, passes current only by displacement (capacitive), with no charge crossing the interface. A non-polarizable electrode, such as Ag/AgCl, lets charge cross freely. Non-polarizable electrodes have lower motion artifact and more stable potentials.

BME-169 · What is half-cell potential and why does it matter?

The potential that develops at an electrode–electrolyte interface, about +0.22 V for Ag/AgCl relative to the standard hydrogen electrode. Mismatched electrodes create DC offsets of up to hundreds of mV that the amplifier must tolerate without saturating.

BME-170 · How does a pulse oximeter work?

It shines red (about 660 nm) and infrared (about 940 nm) light through tissue. Oxyhaemoglobin and deoxyhaemoglobin absorb these wavelengths differently, so the ratio R = (AC/DC)red / (AC/DC)IR of the pulsatile signals is mapped to SpO₂ with an empirical calibration curve.

BME-171 · How does an oscillometric blood pressure monitor work?

As the cuff deflates, arterial pulsations create pressure oscillations in it. The cuff pressure at maximum oscillation amplitude approximates MAP. Systolic and diastolic pressures are estimated with algorithm-specific amplitude ratios.

BME-172 · Why is a Wheatstone bridge used with strain gauges?

Strain gauges produce very small resistance changes (ΔR/R ≈ GF × ε). The bridge converts these into a differential voltage near zero at balance. Using four active arms (full bridge) maximizes sensitivity and compensates for temperature.

BME-173 · How is cardiac output measured by thermodilution?

A cold bolus is injected into the right atrium through a pulmonary artery catheter and a thermistor records the temperature-time curve downstream. CO is inversely proportional to the area under the curve (Stewart–Hamilton equation).

BME-174 · What is the principle of an electromagnetic blood flowmeter?

Faraday's law: blood moving through a magnetic field induces a voltage e = B·L·v across the vessel, where L is the vessel diameter. Electrodes measure this voltage, which is proportional to mean velocity and therefore to flow.

BME-175 · Why is galvanic isolation used in patient-connected amplifiers?

It breaks any conductive path between the patient and mains-referenced circuitry, limiting leakage and fault currents to the patient. It is implemented with transformer, optical or capacitive isolation barriers.

BME-176 · What is impedance pneumography?

Measuring respiration by injecting a small high-frequency current, typically tens of kHz, through thoracic electrodes, often the ECG electrodes. Chest impedance changes with lung air volume, giving respiratory rate.

Biosignal Processing (ECG, EEG, EMG)

BME-177 · Why is a 50/60 Hz notch filter used in ECG systems?

To remove powerline interference coupled capacitively and magnetically from mains wiring. Mains interference sits inside the ECG bandwidth and often exceeds the cardiac signal, so a low-pass filter cannot remove it.

BME-178 · What are the main noise sources in an ECG?

Powerline interference, baseline wander (respiration, below about 0.5 Hz), muscle (EMG) noise, motion artifacts, electrode contact noise and electrosurgical interference.

BME-179 · Why do diagnostic and monitoring ECG bandwidths differ?

Diagnostic ECG (about 0.05–150 Hz) preserves ST-segment and QRS detail. Monitoring mode (about 0.5–40 Hz) suppresses baseline wander and muscle noise for stable rhythm display, but can distort the ST segment.

BME-180 · How does the Pan–Tompkins algorithm detect QRS complexes?

It band-pass filters the signal to about 5–15 Hz, differentiates it, squares it, and applies a moving-window integrator about 150 ms wide. Adaptive thresholds with search-back for missed beats then locate the R peaks.

BME-181 · How do you calculate heart rate from standard ECG paper?

At 25 mm/s, a small box is 0.04 s and a large box is 0.2 s. HR = 300 / (number of large boxes between R waves) = 1500 / (number of small boxes), or HR = 60/RR in seconds.

BME-182 · Calculate the QTc using Bazett's formula for QT = 400 ms at HR = 75 bpm.

RR = 60/75 = 0.8 s. QTc = QT/√RR = 0.400/√0.8 = 0.400/0.894 ≈ 0.447 s, or 447 ms.

BME-183 · What is aliasing and how is it prevented?

Frequency components above fs/2 fold back into lower frequencies as false signals. An analog anti-aliasing low-pass filter before the ADC, with sufficient sampling rate margin, prevents it.

BME-184 · How do FIR and IIR filters compare for biosignals?

FIR filters can have exact linear phase, which preserves waveform morphology, and are always stable, but need many taps. IIR filters are efficient but have non-linear phase. Offline, zero-phase forward–backward filtering removes the phase distortion.

BME-185 · How does ensemble averaging improve SNR?

Averaging N stimulus-locked trials adds the coherent signal and averages out uncorrelated noise. Amplitude SNR improves by √N, so 100 sweeps give a 10× (20 dB) improvement. This is standard for evoked potentials.

BME-186 · How is surface EMG processed to estimate muscle activation?

Band-pass filter (about 20–450 Hz), full-wave rectify, then low-pass filter or compute a moving RMS to get the linear envelope. Normalize to maximum voluntary contraction for comparison across subjects.

BME-187 · Why is the Fourier transform limited for biosignals, and what are the alternatives?

It assumes stationarity and gives no timing information. Biosignals such as EEG and HRV are non-stationary. The short-time Fourier transform and wavelet transform give time–frequency representations.

BME-188 · What are common time- and frequency-domain HRV measures?

Time domain: SDNN (overall variability) and RMSSD (short-term, vagal). Frequency domain: LF power (0.04–0.15 Hz), HF power (0.15–0.4 Hz, respiratory and vagal) and the LF/HF ratio.

BME-189 · What is the input-referred resolution of a 16-bit ADC spanning ±2.5 V after a gain of 500?

The input range is ±2.5 V/500 = ±5 mV, or 10 mV in total. LSB = 10 mV/2¹⁶ ≈ 0.15 µV, which is ample for ECG.

Medical Imaging

BME-190 · How do MRI and CT compare?

CT uses ionizing X-rays, is fast and widely available, and is excellent for bone, lungs, acute bleeding and trauma. MRI uses magnetic fields and RF with no ionizing radiation and gives superior soft-tissue contrast (brain, spine, joints), but it is slower and costlier.

BME-191 · What is the principle of ultrasound imaging?

A transducer sends short acoustic pulses into tissue. Echoes reflect from interfaces with different acoustic impedance (Z = ρc). Depth comes from time of flight, d = c·t/2 with c ≈ 1540 m/s, and echo amplitude sets pixel brightness.

BME-192 · An echo returns 65 µs after the pulse is sent. How deep is the reflector?

d = c·t/2 = 1540 × 65×10⁻⁶ / 2 ≈ 0.050 m, or 5.0 cm.

BME-193 · What is the Larmor frequency of protons at 3 T?

f = (γ/2π) × B₀ = 42.58 MHz/T × 3 T ≈ 127.7 MHz. At 1.5 T it is about 63.9 MHz.

BME-194 · What are T1 and T2 relaxation, and how do you weight images toward each?

T1 (spin–lattice) is the recovery of longitudinal magnetization. T2 (spin–spin) is the decay of transverse magnetization. T1-weighted images use short TR and short TE, so fat is bright and fluid dark. T2-weighted images use long TR and long TE, so fluid is bright.

BME-195 · How does CT reconstruct an image from projections?

Each projection is a line integral of attenuation (the Radon transform). Filtered back-projection applies a ramp filter and smears the projections back across the image. Modern scanners also use iterative reconstruction to reduce noise at lower dose.

BME-196 · Why is coupling gel needed in ultrasound?

Air's acoustic impedance (about 0.0004 MRayl) is far below that of tissue (about 1.6 MRayl). With R = ((Z₂ − Z₁)/(Z₂ + Z₁))², nearly 99.9% of the energy would reflect at an air gap. Gel removes the air layer.

BME-197 · Calculate the Doppler shift for f₀ = 5 MHz, blood velocity 0.5 m/s and insonation angle 60°.

f_D = 2f₀v cosθ / c = 2 × 5×10⁶ × 0.5 × 0.5 / 1540 ≈ 1.62 kHz, which falls in the audible range.

BME-198 · What is the trade-off between ultrasound frequency and penetration?

Higher frequency shortens the wavelength and improves axial and lateral resolution. However, attenuation rises roughly linearly with frequency (about 0.5 dB/cm/MHz in soft tissue), which limits imaging depth.

BME-199 · How do photoelectric absorption and Compton scattering differ in diagnostic X-ray?

Photoelectric absorption scales roughly as Z³/E³ and provides most of the contrast between bone, iodine and soft tissue. Compton scattering dominates in soft tissue at typical energies; it adds scatter that reduces contrast and is reduced with anti-scatter grids.

BME-200 · What do kVp and mAs control in X-ray imaging?

kVp sets photon energy, which controls penetration and subject contrast (higher kVp gives lower contrast). mAs sets the number of photons, which controls image noise and patient dose linearly, without changing beam quality.

BME-201 · If μ = 0.2 cm⁻¹, what fraction of a beam passes through 10 cm, and what is the half-value layer?

I/I₀ = e^(−μx) = e^(−2) ≈ 0.135, or 13.5%. HVL = 0.693/μ ≈ 3.47 cm.

BME-202 · How does SPECT differ from PET?

SPECT detects single gamma photons, for example 140 keV from ⁹⁹ᵐTc, through physical collimators, so it is less sensitive and has lower resolution. PET detects coincident 511 keV annihilation photon pairs with electronic collimation, which gives higher sensitivity and allows quantification.

Biomaterials

BME-203 · What is stress shielding?

When a stiff implant carries most of the load (Ti about 110 GPa or CoCr about 210 GPa, versus cortical bone at about 20 GPa), the surrounding bone is under-loaded. By Wolff's law it then resorbs, which can lead to loosening or periprosthetic fracture.

BME-204 · Why is 316L stainless steel low-carbon?

"L" means carbon at or below about 0.03%. Low carbon limits chromium carbide precipitation at grain boundaries (sensitization), which would otherwise deplete chromium locally and cause intergranular corrosion.

BME-205 · What is the Vroman effect?

Competitive, sequential protein adsorption on a biomaterial surface. Abundant, fast-diffusing proteins such as albumin adsorb first and are later displaced by proteins with higher affinity, such as fibrinogen and HMWK. This shapes the subsequent cell and coagulation response.

BME-206 · How does bioactive glass bond to bone?

In body fluid, 45S5 Bioglass exchanges Na⁺ and Ca²⁺ for H⁺. A silica-rich gel layer forms, and calcium phosphate precipitates on it and crystallizes into hydroxycarbonate apatite. Collagen and cells bond to this apatite layer.

BME-207 · How does PLGA degrade, and how can the degradation rate be tuned?

Its ester bonds hydrolyse into lactic and glycolic acid, which are metabolized. The rate depends on the LA:GA ratio (50:50 is fastest), molecular weight, crystallinity and device size. Acidic products can autocatalyse bulk degradation.

BME-208 · What is the difference between bulk and surface erosion?

In bulk erosion (PLA, PLGA), water penetrates faster than the bonds hydrolyse, so the whole device degrades and loses strength throughout. In surface erosion (polyanhydrides), degradation is confined to the surface, which gives near zero-order drug release.

BME-209 · What are superelasticity and shape memory in nitinol?

Superelasticity: above the austenite finish temperature (Af), stress induces martensite and up to about 8% strain recovers on unloading. Shape memory: a sample deformed in the martensitic state recovers its shape when heated above Af.

BME-210 · How are ISO 10993 tests selected?

They are chosen through a risk-based biological evaluation according to the nature of body contact (surface, externally communicating or implant) and its duration (limited ≤24 h, prolonged >24 h to 30 days, long-term >30 days). Endpoints include cytotoxicity, sensitization, irritation, systemic toxicity, genotoxicity, implantation and haemocompatibility.

BME-211 · Why is UHMWPE wear debris a concern?

Submicron to micron-sized wear particles activate macrophages, which release cytokines that drive osteoclastic bone resorption (osteolysis) and aseptic loosening. Highly cross-linked and vitamin E–stabilized UHMWPE reduces wear.

BME-212 · How do sterilization methods affect polymers?

Gamma and e-beam radiation can cause chain scission, cross-linking and oxidation, for example embrittling UHMWPE. Ethylene oxide is gentle but leaves residues that need aeration. Steam autoclaving (about 121–134 °C) damages many thermoplastics and hydrolysable polymers.

BME-213 · Which corrosion mechanisms matter for implants?

Pitting, crevice, galvanic (dissimilar metals), fretting (micromotion at modular junctions such as head–neck tapers), intergranular, stress-corrosion cracking and corrosion fatigue.

BME-214 · Why are hydrogels attractive for soft tissue and drug delivery?

They have high water content, tissue-like low modulus, tunable mesh size and good permeability to nutrients. Their cross-link density controls swelling, stiffness and the diffusion-controlled release of encapsulated drugs or cells.

BME-215 · How do drug-eluting stents reduce restenosis?

A polymer or polymer-free coating releases an antiproliferative drug (sirolimus, everolimus or paclitaxel) over weeks to months, inhibiting smooth-muscle proliferation and neointimal hyperplasia. Delayed endothelialization requires dual antiplatelet therapy.

Biomechanics

BME-216 · A 700 N axial load acts on a bone cross-section of 5 cm². What is the stress?

σ = F/A = 700 / (5×10⁻⁴ m²) = 1.4×10⁶ Pa, or 1.4 MPa.

BME-217 · A 100 mm tendon stretches by 3 mm. With E = 1.2 GPa, what are the strain and stress?

ε = 3/100 = 0.03 (3%). σ = Eε = 1.2×10⁹ × 0.03 = 36 MPa, assuming the linear region.

BME-218 · How do the Maxwell and Kelvin–Voigt viscoelastic models differ?

Maxwell (spring and dashpot in series) shows stress relaxation but creeps without limit. Kelvin–Voigt (in parallel) shows bounded creep but no stress relaxation. The standard linear solid combines both and better represents tissue.

BME-219 · By Poiseuille's law, how much does flow fall if a vessel's radius narrows by 20% at the same pressure drop?

Q = πr⁴ΔP/(8μL), so Q ∝ r⁴. The new flow is 0.8⁴ ≈ 0.41 of the original, a reduction of about 59%.

BME-220 · What does the law of Laplace say about vessel walls?

For a thin-walled cylinder, wall tension T = P·r and hoop stress σ = P·r/t. A dilated aneurysm has a larger r and a thinner wall, so wall stress rises, which drives further dilation and rupture risk.

BME-221 · Why is blood non-Newtonian?

At low shear rates, red cells aggregate into rouleaux and raise viscosity. At high shear rates, they disaggregate and align, so blood is shear-thinning. In large arteries, a Newtonian viscosity of about 3.5 mPa·s is a reasonable approximation.

BME-222 · The forearm is horizontal and holds 50 N at 35 cm from the elbow; the biceps inserts at 4 cm. Find the biceps force and joint reaction, ignoring forearm weight.

Taking moments about the elbow: F_m × 4 = 50 × 35, so F_m ≈ 438 N upward. The joint reaction is 438 − 50 ≈ 388 N downward on the forearm. The muscle force is about 9× the load.

BME-223 · Why is cortical bone strongest in compression?

Its mineral (hydroxyapatite) resists compression well, while collagen provides tensile strength and toughness. Typical ultimate strengths are about 190 MPa in compression, 130 MPa in tension and 70 MPa in shear.

BME-224 · What is the toe region of a tendon stress–strain curve?

The initial non-linear region, up to about 2% strain, where crimped collagen fibres straighten and the tendon stiffens progressively. The linear region follows, and failure occurs at roughly 8–10% strain.

BME-225 · What is inverse dynamics in gait analysis?

Calculating net joint moments and powers from measured kinematics (motion capture), external forces (force plates) and segment inertial properties, by solving the Newton–Euler equations segment by segment from the foot upward.

BME-226 · Estimate the Reynolds number in the aorta for ρ = 1060 kg/m³, mean v = 0.3 m/s, d = 25 mm and μ = 3.5 mPa·s.

Re = ρvd/μ = 1060 × 0.3 × 0.025 / 0.0035 ≈ 2270. This is near the transitional range, and peak systolic velocity can briefly push flow toward disturbed or turbulent conditions.

BME-227 · What is wall shear stress and why does it matter?

The tangential frictional stress of blood on the endothelium. For Poiseuille flow, τ = 4μQ/(πr³). Endothelial cells sense it, and regions of low or oscillatory shear, such as bifurcations, are prone to atherosclerosis.

BME-228 · How does pulse wave velocity relate to arterial stiffness?

The Moens–Korteweg equation gives c = √(Eh/(2ρr)), so stiffer, thicker-walled or narrower arteries carry pressure waves faster. Carotid–femoral PWV is a clinical marker of arterial stiffening with age and hypertension.

Therapeutic & Life-Support Devices

BME-229 · How does a pacemaker work?

Leads sense the intrinsic cardiac activity. If no beat occurs within the programmed interval, or the rhythm is too slow or AV conduction is blocked, the pulse generator delivers a brief, low-energy pulse (a few volts, about 0.4–1 ms) to depolarize the myocardium.

BME-230 · What do pacemaker codes such as VVI and DDD mean?

In the NBG code, position 1 is the chamber paced, position 2 the chamber sensed, position 3 the response to sensing (I = inhibit, T = trigger, D = both) and position 4 rate modulation (R). VVI paces and senses the ventricle and is inhibited by sensed beats. DDD is dual-chamber.

BME-231 · Why are biphasic defibrillation waveforms preferred?

Reversing current polarity partway through the shock achieves equal or higher first-shock success at lower energy (typically 120–200 J versus 360 J for monophasic). This reduces myocardial injury and allows smaller devices.

BME-232 · How much energy is stored in a 32 µF defibrillator capacitor charged to 5 kV?

E = ½CV² = 0.5 × 32×10⁻⁶ × (5000)² = 400 J. The energy delivered is lower because of internal resistance and truncation.

BME-233 · How does synchronized cardioversion differ from defibrillation?

Cardioversion times the shock to the R wave, avoiding the vulnerable period on the T wave that could induce VF. It is used for organized rhythms with a pulse (AF, flutter, stable VT). Defibrillation is unsynchronized and used for VF and pulseless VT.

BME-234 · What is the difference between volume-control and pressure-control ventilation?

Volume control delivers a set tidal volume, and airway pressure varies with compliance and resistance. Pressure control applies a set inspiratory pressure, and tidal volume varies. Pressure control limits barotrauma, but volume must be monitored.

BME-235 · What is lung-protective ventilation?

Using a low tidal volume (about 6 mL/kg predicted body weight), limiting plateau pressure (below about 30 cmH₂O) and applying adequate PEEP. This minimizes volutrauma, barotrauma and atelectrauma, especially in ARDS.

BME-236 · How do infusion pumps prevent free flow and detect occlusion?

Anti-free-flow mechanisms clamp the set whenever it is removed from the pump. Upstream and downstream pressure sensors detect occlusions, and ultrasonic air-in-line detectors stop infusion if bubbles appear.

BME-237 · Why does electrosurgery use high frequency, and how do monopolar and bipolar modes differ?

Above about 100 kHz, current does not stimulate nerves or muscle (no faradic effect) and only heats tissue. Monopolar mode passes current from a small active tip through the body to a large return pad. Bipolar mode confines current between the two jaws of the forceps.

BME-238 · What is the difference between veno-venous and veno-arterial ECMO?

VV ECMO drains and returns blood on the venous side and supports gas exchange only, for respiratory failure. VA ECMO returns blood into an artery and supports both circulation and gas exchange, for cardiac or cardiopulmonary failure.

BME-239 · Why does a hemodialyzer use countercurrent flow?

Blood and dialysate flow in opposite directions, so a concentration gradient is maintained along the whole fibre length. This maximizes diffusive clearance of urea, creatinine and potassium compared with co-current flow.

BME-240 · How does laser wavelength determine the tissue effect?

The CO₂ laser (10.6 µm) is strongly absorbed by water, so it gives shallow, precise cutting and ablation. Nd:YAG (1064 nm) penetrates deeper and is used for coagulation. The ArF excimer laser (193 nm) photoablates the cornea in LASIK.

Tissue Engineering

BME-241 · Why are scaffold porosity and pore size important?

High (often above 90%), interconnected porosity allows cell migration, nutrient diffusion, vascularization and waste removal. Optimal pore size depends on tissue; for bone ingrowth roughly 100–400 µm is commonly cited.

BME-242 · What limits the thickness of engineered tissue without vasculature?

Oxygen diffusion. Cells more than about 100–200 µm from a capillary or nutrient source become hypoxic. Thick constructs develop necrotic cores unless they are perfused or vascularized.

BME-243 · How should scaffold degradation be matched to tissue regeneration?

The degradation rate should match the rate of new matrix formation. Degrading too fast causes loss of mechanical support. Degrading too slowly impedes remodelling and can cause chronic inflammation or stress shielding.

BME-244 · How do perfusion bioreactors improve tissue growth?

They force medium through the pores of the scaffold, improving oxygen and nutrient delivery to the core. They also apply fluid shear, which can stimulate matrix production, for example by osteoblasts. This gives more uniform cell distribution than static culture.

BME-245 · Which growth factors are commonly used in tissue engineering?

BMP-2 and BMP-7 for bone, VEGF for angiogenesis, TGF-β (especially TGF-β3) for chondrogenesis, FGF-2 for proliferation, and PDGF for wound healing and recruitment of smooth-muscle cells.

BME-246 · What are the minimal criteria that define mesenchymal stem cells?

Plastic adherence in culture. Expression of CD73, CD90 and CD105, and lack of CD45, CD34, CD14 or CD11b, CD79α or CD19, and HLA-DR. Ability to differentiate into osteoblasts, chondrocytes and adipocytes in vitro.

BME-247 · How does substrate stiffness influence stem cell fate?

Cells sense matrix elasticity through focal adhesions and cytoskeletal tension. On soft matrices (around 1 kPa), MSCs tend toward neurogenic lineages. On intermediate matrices (around 10 kPa), they tend toward myogenic lineages. On stiff matrices (around 30 kPa or more), they tend toward osteogenic lineages.

BME-248 · What are common scaffold fabrication techniques?

Solvent casting with particulate leaching, freeze-drying, gas foaming, thermally induced phase separation, electrospinning and additive manufacturing (FDM, stereolithography, selective laser sintering, bioprinting).

BME-249 · What determines bioink printability?

Shear-thinning viscosity for extrusion, rapid gelation or cross-linking to hold shape, and mechanical integrity, balanced against cell viability under nozzle shear and biocompatibility. Stiffer inks print better but often reduce cell function.

BME-250 · What is mechanotransduction and how is it exploited in tissue engineering?

The conversion of mechanical stimuli (strain, shear, pressure) into biochemical signals through integrins, ion channels and the cytoskeleton. Bioreactors apply cyclic strain to vascular or tendon constructs and compression to cartilage to improve matrix organization.

BME-251 · How can engineered tissues be vascularized?

By delivering angiogenic factors (VEGF), co-culturing with endothelial cells to form pre-vascular networks, printing sacrificial channels, using decellularized vascular trees, or implanting the construct first at a well-vascularized site (in vivo prevascularization).

BME-252 · What are the trade-offs between autologous and allogeneic cell sources?

Autologous cells avoid immune rejection, but they are limited in number, need a harvest procedure with donor-site morbidity, and delay treatment. Allogeneic cells can be made in advance in large batches as off-the-shelf products, but they risk immune responses and need donor screening.

Biosensors

BME-253 · What reaction underlies a first-generation glucose biosensor?

Glucose + O₂ → (GOx) → gluconolactone + H₂O₂. The H₂O₂ is oxidized at a Pt electrode at about +0.6 V relative to Ag/AgCl, giving current proportional to glucose. Drawbacks are oxygen dependence and interference at the high potential.

BME-254 · How do mediators improve second-generation glucose sensors?

Artificial redox mediators such as ferrocene derivatives or ferricyanide shuttle electrons from the enzyme's FAD centre to the electrode, replacing O₂. This lowers the operating potential, which reduces interference and removes oxygen dependence.

BME-255 · How do Michaelis–Menten kinetics limit a biosensor's linear range?

v = Vmax[S]/(Km + [S]), which is linear only when [S] ≪ Km. Adding a diffusion-limiting outer membrane lowers the local substrate concentration and extends the linear range to clinically high glucose levels.

BME-256 · What is the Nernstian slope of a pH electrode?

2.303RT/F per pH unit: about 59.16 mV/pH at 25 °C and about 61.5 mV/pH at 37 °C. The slope is temperature dependent, so temperature compensation is required.

BME-257 · How does a surface plasmon resonance biosensor work?

Polarized light excites surface plasmons in a thin gold film at a specific resonance angle. Binding of molecules at the surface changes the local refractive index and shifts this angle. This allows label-free, real-time measurement of binding kinetics.

BME-258 · What does the Sauerbrey equation describe?

For a quartz crystal microbalance, Δf = −2f₀²Δm / (A√(ρ_q μ_q)): the resonant frequency decreases linearly with added rigid mass per unit area. It does not hold for thick, viscoelastic films.

BME-259 · A blank has σ = 0.02 µA and the calibration slope is 0.5 µA/mM. What are the LOD and LOQ?

LOD = 3σ/slope = 0.06/0.5 = 0.12 mM. LOQ = 10σ/slope = 0.2/0.5 = 0.4 mM.

BME-260 · Why do CGM readings lag behind blood glucose?

CGMs measure interstitial fluid glucose. Glucose must diffuse from capillaries into interstitial fluid, and sensor processing adds delay, so readings typically lag blood by about 5–15 minutes. The lag is most noticeable when glucose is changing rapidly.

BME-261 · What is the ISO 15197 accuracy criterion for glucose meters?

At least 95% of results must fall within ±15 mg/dL of the reference for concentrations below 100 mg/dL, and within ±15% at or above 100 mg/dL. Separately, 99% must fall within zones A and B of the consensus error grid.

BME-262 · How does an ISFET work?

An ion-sensitive field-effect transistor replaces the metal gate with an ion-sensitive insulator in contact with the solution and a reference electrode. Surface charge from H⁺ or other ions shifts the threshold voltage, so the drain current varies with pH.

BME-263 · How do aptamers compare with antibodies as bioreceptors?

Aptamers are nucleic-acid strands selected by SELEX. They are chemically synthesized, have little batch variation, are thermally stable, can be regenerated and are easily modified. Antibodies often have higher affinity and broader validation, but are costlier and less stable.

BME-264 · How is electrochemical impedance spectroscopy used in biosensors?

A small AC signal is applied over a frequency range with a redox probe such as ferro/ferricyanide. Target binding hinders electron transfer and increases the charge-transfer resistance (Rct) in the Randles equivalent circuit. This enables label-free detection.

Regulatory Standards & Quality

BME-265 · How is device classification determined under the EU MDR?

Annex VIII applies 22 rules based on invasiveness, contact duration (transient below 60 min, short-term up to 30 days, long-term above 30 days), whether the device is active, the body site, and special cases such as software and nanomaterials. If several rules apply, the highest class applies.

BME-266 · How do 510(k), De Novo and PMA differ?

A 510(k) shows substantial equivalence to a predicate and leads to clearance. De Novo classifies novel low-to-moderate-risk devices that have no predicate. PMA requires valid scientific evidence, usually clinical, of safety and effectiveness for Class III devices and leads to approval.

BME-267 · What are the main steps of the ISO 14971 risk management process?

Risk analysis (intended use, hazards, hazardous situations, risk estimation), risk evaluation against acceptability criteria, risk control, evaluation of overall residual risk with benefit–risk analysis, a risk management review, and production and post-production monitoring.

BME-268 · What is the priority order for risk control options?

First, inherently safe design and manufacture. Second, protective measures in the device or manufacturing process, such as alarms and interlocks. Third, information for safety, such as warnings and training. Lower-priority measures should not replace feasible higher-priority ones.

BME-269 · What are the Design History File, Device Master Record and Device History Record?

The DHF documents the design and development history. The DMR, called the medical device file in ISO 13485, contains specifications and procedures for making the device. The DHR records how each lot or unit was actually produced.

BME-270 · What is CAPA?

Corrective and preventive action: a systematic process that collects quality data (complaints, nonconformances, audits), investigates root causes, implements actions to eliminate them or prevent occurrence, and verifies that the actions are effective.

BME-271 · What are the software safety classes in IEC 62304?

Class A: no injury or damage to health is possible. Class B: non-serious injury is possible. Class C: death or serious injury is possible. Higher classes require more rigorous documentation, architecture, unit verification and testing.

BME-272 · What does IEC 62366-1 require?

A usability engineering process: a use specification, user interface characteristics related to safety, identification of use-related hazards, formative evaluations during design, and summative validation with representative users performing critical tasks.

BME-273 · What does post-market surveillance involve under the EU MDR?

A proactive PMS system and plan, a PMS report (Class I) or Periodic Safety Update Report (Class IIa and above), post-market clinical follow-up, trend reporting, and vigilance reporting of serious incidents and field safety corrective actions.

BME-274 · What is a UDI?

A Unique Device Identifier. It contains a device identifier (UDI-DI, the model) and a production identifier (UDI-PI, such as lot, serial number and expiry date), in human- and machine-readable form. It is registered in GUDID in the US and EUDAMED in the EU.

BME-275 · How does FMEA relate to ISO 14971?

FMEA is a bottom-up analysis of component or process failure modes and their effects. It is a useful input to risk analysis but not sufficient alone, because it misses hazards that arise without failure (normal use, use errors). It is complemented by top-down fault tree analysis.

BME-276 · What is IEC 60601-1-2?

The collateral standard for electromagnetic compatibility of medical electrical equipment. It sets emission limits and immunity test levels (ESD, radiated RF, fast transients, surges, magnetic fields) based on the intended use environment, such as a hospital or home.

Clinical Engineering & Electrical Safety

BME-277 · Why is the microshock threshold so much lower than the macroshock threshold?

When current enters the heart directly through a catheter or pacing lead, the current density at the myocardium is very high. About 100 µA can induce VF, compared with about 100 mA through intact skin. Design limits for cardiac applied parts are therefore around 10 µA.

BME-278 · What are the IEC 60601-1 patient leakage current limits for type BF and CF applied parts?

Type BF: 100 µA in normal condition and 500 µA in single-fault condition. Type CF: 10 µA in normal condition and 50 µA in single-fault condition, for DC or AC patient leakage.

BME-279 · What is a single fault condition?

A condition in which one means of protection against a hazard is defective, such as an open protective earth or one supply conductor interrupted. Equipment must remain safe in normal condition and under any single fault.

BME-280 · How does a line isolation monitor work?

It continuously measures the impedance from each line of an isolated power system to ground and displays the total hazard current, meaning the current that would flow if a ground fault occurred. It alarms when this exceeds a set threshold, without interrupting power.

BME-281 · What is the difference between MOOP and MOPP?

Means of operator protection and means of patient protection are insulation, creepage, clearance and protective earth requirements in IEC 60601-1. MOPP is stricter. For example, at mains voltage, 2 MOPP require a 4000 Vac dielectric test and 8 mm creepage, compared with 3000 Vac for 2 MOOP.

BME-282 · What is return electrode monitoring in electrosurgery?

A split return pad lets the generator continuously measure contact impedance between the two halves. If the pad partially peels and impedance rises, the generator alarms and stops output, preventing burns at the return site.

BME-283 · How is risk-based maintenance scheduling decided?

Each device is scored on factors such as clinical function, physical risk to patients if it fails, maintenance requirements and incident history (for example with the Fennigkoh–Smith method). The score decides whether to include it in planned maintenance and how often.

BME-284 · What are the key steps in a medical device incident investigation?

Secure and quarantine the device and accessories unaltered. Preserve logs and settings. Interview staff. Reconstruct the event. Test with the manufacturer if needed. Identify root and contributing causes, including human factors. Report to the manufacturer or regulator as required. Implement and track corrective actions.

BME-285 · How does body resistance affect shock current?

Dry skin resistance can be about 100 kΩ, while wet or broken skin can drop to about 1 kΩ. Internal body resistance is about 500 Ω. At 120 V, a 1 kΩ path gives I = V/R = 120 mA, which is enough to cause VF.

BME-286 · What protective earth resistance does IEC 60601-1 allow?

0.1 Ω between the protective earth terminal or appliance inlet and accessible protected parts. 0.2 Ω when measured from the mains plug pin of a non-detachable power cord. This is tested with a high current to stress the connections.

BME-287 · Why does EMC matter for medical devices in hospitals?

Hospitals contain strong emitters (electrosurgery, MRI, radios, phones, wireless networks) alongside sensitive low-level measurements (EEG, ECG). Interference can corrupt readings, trigger false alarms or disrupt pumps and ventilators, so devices must meet emission and immunity requirements.

BME-288 · What metrics measure clinical engineering performance?

Equipment uptime, MTBF, MTTR, PM completion rate, repeat-repair rate, response time, use-error incidents and the cost-of-service ratio (service cost divided by acquisition cost).

Healthcare Informatics & AI

BME-289 · How do HL7 v2 and FHIR differ?

HL7 v2 uses pipe-delimited event messages (segments such as MSH and PID) over interfaces such as MLLP, with considerable site-specific variation. FHIR defines granular resources accessed through modern RESTful APIs with JSON or XML, making web and mobile integration easier.

BME-290 · What are the levels of interoperability?

Foundational (data can be transmitted), structural (common format and syntax), semantic (shared meaning through vocabularies such as SNOMED CT and LOINC) and organizational (governance, policy and workflow alignment).

BME-291 · How is DICOM metadata organized?

As data elements identified by (group, element) tags with a value representation. For example, (0010,0010) is Patient's Name and (0008,0060) is Modality. Images are organized by patient, study, series and instance UIDs.

BME-292 · What methods does HIPAA allow for de-identification?

Safe Harbor, which removes 18 specified identifiers such as names, small geographic units, most date elements and record numbers, with no actual knowledge of re-identification risk. Expert Determination, in which a qualified expert certifies that the re-identification risk is very small.

BME-293 · A test has 90% sensitivity and 90% specificity, and disease prevalence is 1%. What is the PPV?

Per 1000 people: TP = 9 and FP = 0.1 × 990 = 99. PPV = 9/(9 + 99) ≈ 8.3%. Low prevalence makes most positive results false positives.

BME-294 · What do the ROC curve and AUC represent?

The ROC curve plots sensitivity against 1 − specificity across all decision thresholds. AUC is the probability that a random positive case is ranked above a random negative case: 0.5 is chance and 1.0 is perfect discrimination.

BME-295 · Why must medical datasets be split at the patient level?

If images or records from the same patient appear in both training and test sets, the model can recognize patient-specific features. This leaks information and inflates performance. Splits should be by patient, and ideally validated externally by site or time.

BME-296 · How is class imbalance handled in medical ML?

Use stratified sampling, class-weighted or focal loss, oversampling (including SMOTE) or undersampling, and augmentation of the minority class. Report metrics that are robust to imbalance, such as sensitivity, PPV, F1 and PR-AUC, rather than accuracy alone.

BME-297 · What is transfer learning in medical imaging?

Starting from a network pretrained on a large dataset, such as natural images, and fine-tuning it on a smaller medical dataset. It speeds convergence and improves performance when labelled medical data are scarce.

BME-298 · What is federated learning?

Training a shared model across several institutions by exchanging model updates rather than raw patient data. Data stay on site, which aids privacy, but non-identical data distributions and update leakage remain challenges.

BME-299 · How does the IMDRF framework categorize SaMD risk?

It crosses the state of the healthcare situation (critical, serious or non-serious) with the significance of the information (treat or diagnose, drive clinical management, or inform clinical management) to give categories I to IV, with IV the highest impact.

BME-300 · What is LOINC used for?

Logical Observation Identifiers Names and Codes is a universal coding system for laboratory tests and clinical observations. It ensures that results such as "serum glucose" are identified consistently across systems.

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