GATE Biomedical Engineering Biomaterials Syllabus

Distinguish a bioinert from a bioactive material.

Bioinert materials (e.g., alumina, Ti) provoke minimal reaction and become encased in fibrous tissue; bioactive materials (e.g., bioglass, hydroxyapatite) chemically bond directly to living tissue via surface reactions.

What is bioactivity in the context of biomaterials?

The ability of a material to elicit a specific biological response at its surface, forming a direct chemical bond with surrounding tissue — typically a carbonated hydroxyapatite (HCA) layer bonding to bone.

What surface layer forms on bioactive glass that allows bonding to bone?

A carbonated hydroxyapatite (HCA) layer, formed via ion exchange, silica gel formation, and precipitation of $\ce{Ca}$ and $\ce{PO4}$ from body fluid.

What is the Bioglass 45S5 composition?

$45\%\ \ce{SiO2}$, $24.5\%\ \ce{CaO}$, $24.5\%\ \ce{Na2O}$, and $6\%\ \ce{P2O5}$ (by weight).

What is biodegradability of a biomaterial?

The capacity of a material to be broken down in the biological environment (by hydrolysis, enzymes, or cells) into products that are absorbed, metabolized, or excreted by the body.

By what chemical mechanism do polyesters like PLA and PGA degrade in the body?

By hydrolysis of the ester backbone bonds, cleaving the chains into smaller oligomers and finally monomeric acids that are metabolized.

Why does PGA degrade faster than PLA?

PGA lacks the methyl side group of PLA, so it is more hydrophilic and less sterically hindered; water penetrates and hydrolyzes the ester bonds faster (PGA degrades in weeks vs. months/years for PLA).

Differentiate bulk erosion from surface erosion in degradable polymers.

In bulk erosion water penetrates faster than it degrades, so the whole matrix degrades throughout (e.g., PLGA). In surface erosion degradation outpaces water diffusion, so the device erodes layer-by-layer from outside (e.g., polyanhydrides), keeping a constant geometry.

What is rheology?

The study of the deformation and flow of matter, especially the relationship between applied stress and resulting strain or strain rate in fluids and soft solids.

Define a Newtonian fluid and give its constitutive equation.

A fluid whose viscosity is independent of shear rate. Shear stress $\tau = \mu \dot{\gamma}$, where $\mu$ is the dynamic viscosity and $\dot{\gamma}$ the shear rate (e.g., water).

What is shear-thinning (pseudoplastic) behavior? Give a biological example.

Viscosity decreases with increasing shear rate. Blood and synovial fluid are shear-thinning, allowing easier flow at high shear.

State the power-law (Ostwald–de Waele) model for non-Newtonian fluids.

$\tau = K \dot{\gamma}^{\,n}$, where $K$ is the consistency index and $n$ the flow behavior index: $n<1$ shear-thinning, $n=1$ Newtonian, $n>1$ shear-thickening.

What does the Bingham plastic model describe and what is its equation?

A material that behaves rigidly until a yield stress is exceeded, then flows: $\tau = \tau_0 + \mu_p \dot{\gamma}$ for $\tau > \tau_0$, where $\tau_0$ is the yield stress and $\mu_p$ the plastic viscosity.

Define viscoelasticity and name the two ideal-element models.

Viscoelastic materials exhibit both viscous (dashpot) and elastic (spring) responses. The Maxwell model (spring + dashpot in series) and the Kelvin–Voigt model (spring + dashpot in parallel).

In dynamic mechanical analysis, what do the storage modulus $G'$ and loss modulus $G''$ represent?

$G'$ is the elastic (stored, recoverable) energy component; $G''$ is the viscous (dissipated) energy component. Their ratio gives $\tan\delta = G''/G'$, the damping/loss factor.

What is the working principle of Atomic Force Microscopy (AFM)?

A sharp tip on a flexible cantilever scans a surface; interatomic forces deflect the cantilever, which is measured by a laser reflected onto a photodiode, building a 3-D topographic map at near-atomic resolution.

Name the three main imaging modes of AFM.

Contact mode (tip drags in continuous contact), non-contact mode (tip oscillates just above the surface), and tapping/intermittent-contact mode (tip oscillates and lightly touches each cycle).

What is a key advantage of AFM over electron microscopy for biological samples?

AFM works in air or liquid (physiological conditions) without vacuum, coating, or staining, giving true 3-D nanoscale topography and the ability to image live, hydrated samples and measure forces.

Which force law governs AFM tip–sample interaction, and what regions does it have?

The Lennard-Jones potential, $U(r) = 4\varepsilon\left[\left(\dfrac{\sigma}{r}\right)^{12} - \left(\dfrac{\sigma}{r}\right)^{6}\right]$, with long-range attraction (van der Waals) and short-range repulsion.

What is the fundamental difference between electron microscopy and light microscopy?

Electron microscopy uses a beam of electrons (much shorter wavelength) instead of light, achieving far higher resolution; the de Broglie wavelength $\lambda = \dfrac{h}{p} = \dfrac{h}{\sqrt{2 m e V}}$ shrinks with accelerating voltage.

How does the de Broglie relation set the resolution limit of electron microscopes?

Resolution scales with wavelength $\lambda = h/p$. Accelerating electrons to high voltage gives picometer-scale $\lambda$, far below visible light ($\sim 500\ \text{nm}$), enabling atomic resolution.

What does SEM (Scanning Electron Microscopy) measure and what kind of image does it give?

SEM scans a focused electron beam over a sample surface and detects secondary and backscattered electrons, producing a high-depth-of-field, 3-D-like image of surface topography and morphology.

Why must most samples be coated (e.g., with gold) before SEM imaging?

Non-conductive samples accumulate charge under the electron beam, distorting the image; a thin conductive coating (Au, Pt, C) dissipates charge and enhances secondary-electron emission.

What is the working principle of Transmission Electron Microscopy (TEM)?

A high-energy electron beam is transmitted through an ultra-thin specimen ($<100\ \text{nm}$); transmitted/diffracted electrons form a magnified projection image revealing internal structure at atomic resolution.

State two key differences between SEM and TEM.

SEM detects reflected/secondary electrons from the surface for 3-D topography at lower resolution; TEM transmits electrons through a thin section giving 2-D internal structure at much higher (atomic) resolution and higher accelerating voltage.

Why does TEM require ultra-thin specimens?

Electrons must pass through the sample; if it is too thick they are absorbed or scattered multiply, blurring the image. Sections are typically $<100\ \text{nm}$ thick.

What signals besides the image can an electron microscope generate for elemental analysis?

Characteristic X-rays detected by EDS/EDX (energy-dispersive X-ray spectroscopy) for elemental composition, plus backscattered electrons giving atomic-number (Z) contrast.

What does Fourier Transform Infrared (FTIR) spectroscopy measure?

It measures absorption of infrared light by molecular bonds at specific vibrational frequencies, identifying functional groups and chemical bonds from the resulting spectrum.

What molecular condition must be met for a vibration to be IR active?

The vibration must cause a net change in the molecular dipole moment, $\dfrac{\partial \mu}{\partial Q} \neq 0$. Symmetric vibrations with no dipole change are IR inactive.

Why is FTIR called 'Fourier transform' spectroscopy?

It uses a Michelson interferometer to collect all IR frequencies simultaneously as an interferogram; a mathematical Fourier transform converts that time/path-difference signal into the frequency-domain (wavenumber) spectrum, giving the Fellgett (multiplex) and Jacquinot advantages.

In what units is the FTIR x-axis given, and how does it relate to wavelength?

In wavenumber $\bar{\nu}$ ($\text{cm}^{-1}$), where $\bar{\nu} = \dfrac{1}{\lambda} = \dfrac{\nu}{c}$. Wavenumber is directly proportional to vibrational energy.