20-Bio-B10 Biomechanical Device Design & Human Factors · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2015 — 04-Bio-B10 Analytical Biochemistry. Three hours, closed book, any non-communicating calculator. Six questions of equal value (20 marks each); five constitute a complete 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. Every question is essay/descriptive (technique principle, interpretation of an instrument trace or image), with no numerical calculation on this sitting.
Reference texts (the books a candidate should have reviewed 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.
The three most commonly used AFM operating modes are contact mode, non-contact mode, and tapping (intermittent-contact) mode.
In contact mode, the cantilever tip remains in continuous, gentle physical contact with the sample surface, operating in the repulsive part of the tip–sample force curve; a feedback loop holds either constant force (deflection) or constant height as the tip is raster-scanned. This gives high resolution and fast scanning, but the continuous lateral (shear/drag) contact force can damage soft samples (e.g., biological specimens) or drag/displace loosely bound features, and can wear the tip. In non-contact mode, the cantilever is oscillated at (or near) its resonant frequency a small distance above the surface, never touching it, and senses the weaker, longer-range attractive van der Waals forces through their effect on the oscillation's resonant frequency or amplitude; this avoids damaging the sample or tip but generally gives lower resolution and can be disrupted by a thin adsorbed contaminant/water layer that can trap the tip. Tapping mode is a middle ground: the cantilever oscillates at a larger amplitude near resonance and the tip briefly, intermittently touches the surface only at the bottom of each oscillation cycle; this greatly reduces the lateral shear forces of continuous contact mode (protecting soft samples) while still achieving resolution comparable to contact mode, which is why tapping mode is the most widely used mode for soft or biological samples.
AFM height (vertical, z-direction) resolution is exceptionally fine — typically sub-nanometre, down to the sub-Ångström to Ångström scale under good conditions — because the z-piezo and optical-lever detection system can resolve extremely small cantilever deflections, in principle sufficient to distinguish individual atomic step heights on a crystal surface.
Lateral (x–y, in-plane) resolution is coarser than vertical resolution, typically on the order of a few nanometres (roughly 1–10 nm for a standard tip), because it is fundamentally limited by the finite radius of curvature of the physical probe tip: the image is effectively a convolution of the true surface topography with the tip's own shape, which broadens and can distort fine lateral features (a phenomenon known as tip convolution).
AFM offers several distinct advantages for surface imaging compared with electron-beam microscopies. It requires no vacuum — imaging can be performed in ambient air or, importantly, in liquid/physiological buffer, allowing live cells and biomolecules to be imaged under near-native, even dynamic, conditions, which is not possible with SEM/TEM. It needs no conductive coating, since AFM does not rely on detecting scattered/emitted electrons, whereas non-conductive SEM samples typically must be sputter-coated (a step that can obscure fine surface detail). AFM provides genuine, quantitative three-dimensional topographic (height) data at every pixel, whereas SEM produces primarily a 2D projection image with only indirect and less quantitative height information, and TEM requires the sample to be sectioned extremely thin and only images a transmitted 2D projection through that section. AFM requires minimal sample preparation (no sectioning, no staining, no fixation/dehydration required as for TEM), and it is comparatively non-destructive, avoiding the ionizing electron-beam dose that can damage beam-sensitive (especially biological/organic) samples in SEM/TEM. Finally, because the AFM tip directly senses mechanical and other local forces, it can simultaneously map properties beyond simple topography — such as surface adhesion, stiffness/elasticity, or friction — that electron microscopy cannot access.
