20-Bio-A1 Biomaterials and Biocompatibility · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2014 — 04-Bio-A1 Biomaterials and Biocompatibility. Three hours, open book, any non-communicating calculator. Seven questions of equal value (20 marks each, 100 marks total); five constitute a complete paper and only the first five appearing in the answer book are marked. All seven are solved here, because this set is a study resource rather than an examination script. Every question is qualitative/descriptive (materials selection, surface science, host response) rather than numerical, except Question 7, which asks for an engineering interpretation of a small stress–strain data set — that question therefore quotes and reasons from the given numbers while still answering in flowing prose, as the question itself calls for discussion rather than a computed final answer.
Reference texts (the books an open-book candidate should have on the desk 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.
XPS is based on the photoelectric effect: the sample is irradiated under ultra-high vacuum with monochromatic soft X-rays (typically Al Kα or Mg Kα), and photons of known energy $h\nu$ eject core-level electrons from atoms within the sampling depth. An electron-energy analyzer measures the kinetic energy $E_K$ of the emitted photoelectrons, from which the electron's original binding energy is recovered by conservation of energy,
$$E_B = h\nu - E_K - \phi$$
where $\phi$ is the spectrometer work-function correction. Binding energy is characteristic of both the element (each element has a distinctive set of core-level binding energies, enabling elemental identification and, from peak-area ratios corrected by sensitivity factors, quantification of surface atomic composition) and of that atom's local chemical environment: electron-withdrawing or -donating neighbouring atoms shift the binding energy of a given core level by a small but resolvable amount — the "chemical shift" — so a high-resolution scan of a single core level (here, carbon 1s) can be deconvolved into several component peaks, each corresponding to a distinct carbon bonding environment (e.g. C–C/C–H, C–O, C=O, O–C=O). Because the ejected photoelectrons lose energy rapidly to inelastic scattering as they travel through the solid, only those originating within roughly the outermost 5–10 nm can escape without energy loss and be detected; XPS is therefore an intrinsically surface-sensitive technique, probing essentially the same few-nanometre-thick layer that governs a biomaterial's interaction with tissue and blood — exactly the layer that bulk techniques cannot see and that a surface modification (such as the fouling-resistant treatment in Question 1 or the coatings discussed here) is designed to change.
[Figure not reproduced: Three high-resolution C1s XPS spectra A, B and C as printed on the December 2014 paper. See the official exam paper or the cited reference text.]
Read directly from the printed fits in Figure 5.1, the spectra are consistent with the PEO modification (B) but not convincingly with the protein modification (C).
Spectrum A (plain silicone). One dominant, symmetric component at 285 eV carries essentially all of the carbon signal; the only other fitted components are trace features barely above the baseline on the high-binding-energy side. This is what poly(dimethylsiloxane) should give: its only carbon is the Si–CH3 methyl carbon, which has no oxygen or nitrogen neighbour and therefore falls in the hydrocarbon (C–C/C–H) region (Si–CH3 actually sits a few tenths of an eV below 285 eV, and adventitious hydrocarbon adds to the same peak). The trace higher-energy features are typical of slightly oxidized adventitious carbon. Consistent with unmodified silicone.
Spectrum B (silicone + PEO). A large new component appears about 1.5 eV above the main peak (≈286.5 eV), at roughly 40% of the height of the 285 eV peak, together with two small components further out (≈288–289 eV). 286.5 eV is the ether carbon C–O–C, which is the only carbon environment in the PEO repeat unit (–CH2–CH2–O–), so its appearance confirms PEO at the surface; the small 288–289 eV components are consistent with the carbonyl/ester chemistry typically used to couple PEO to the substrate. However, a complete PEO layer thicker than the 5–10 nm sampling depth would put nearly all of the carbon signal at 286.5 eV. Because the 285 eV peak still dominates, the PEO coverage is partial, or the layer is thin enough that the underlying silicone (and adventitious carbon) still contributes. Consistent with a PEO modification of limited coverage or thickness.
Spectrum C (B + protein). C is almost indistinguishable from B: the same components at the same positions, the 286.5 eV component still at roughly 40% of the main peak, and only a marginal growth of the small ≈288 eV component. A protein layer should change the spectrum clearly: every peptide bond contributes an amide carbon (N–C=O) at ≈288.0 eV — typically about a fifth to a quarter of all protein carbon — plus C–N carbon near 286 eV and side-chain hydrocarbon at 285 eV. No distinct, substantial 288 eV amide component has appeared, so spectrum C is not consistent with a significant protein layer; at most a sparse, sub-monolayer amount of protein is present. That outcome is chemically unsurprising: PEO is grafted onto biomaterials precisely because its highly hydrated, flexible chains sterically repel proteins, so a PEO-modified surface is expected to resist protein adsorption (and if the protein was meant to be covalently coupled to PEO end groups, the coupling yield was evidently very low). The C1s data alone cannot settle the matter completely, because C–N carbon overlaps C–O carbon near 286 eV. The decisive check is a survey or high-resolution N1s scan (≈400 eV): neither silicone nor PEO contains nitrogen, so any N signal must come from protein. Angle-resolved XPS would then show whether that protein sits on top of the PEO layer.
This exact analysis — confirming a claimed surface treatment is really present, at the surface, by its expected chemical shift — is precisely the verification step the Question 1 due-diligence assessment would need before accepting the acquisition target's fouling-resistance claim at face value.
