The mucus gel covers the wet epithelia that forms the inner lining of the body. It constitutes our first line of defense protecting the body from infections and other deleterious molecules. Failure of the mucus barrier can lead to the inflammation of the mucosa such as in inflammatory bowel diseases. Unfortunately, there are no effective strategies that reinforce the mucus barrier properties to recover or enhance its ability to protect the epithelium. Herein, we describe a mucus engineering approach that addresses this issue where we physically crosslink the mucus gel with low molar mass chitosan variants to reinforce its barrier functions. We tested the effect of these chitosans on mucus using in-lab purified porcine gastric mucins, which mimic the native properties of mucus, and on mucus-secreting HT29-MTX epithelial cell cultures. We found that the lowest molar mass chitosan variant (degree of polymerisation of 8) diffuses deep into the mucus gels while physically crosslinking the mucin polymers, whereas the higher molar mass chitosan variants (degree of polymerisation of 52 and 100) interact only superficially. The complexation resulted in a tighter mucin polymer mesh that slowed the diffusion of dextran polymers and of the cholera toxin B subunit protein through the mucus gels. These results uncover a new use for low molar mass mucoadhesive polymers such a chitosan as non-cytotoxic mucosal barrier enhancers that could be valuable in the prevention and treatment of mucosal diseases.

Silicon nitride (SiNx) coatings have recently been introduced as a potential material for joint implant bearing surfaces, but there is no data on wear debris morphology nor their dissolution rate, something that could play a central role to implant longevity. In this study, wear debris was generated in a ball-on-disc setup in simulated body fluid. After serum digestion the debris was analysed with scanning electron microscopy and energy-dispersive x-ray spectroscopy. The particle dissolution rate was evaluated using inductively coupled plasma techniques, on model SiNx particles. The wear debris from SiNx coatings was found to be round, in the nm range and formed agglomerates in the sub-micron to micron range. Model particles dissolved in simulated body fluid at a rate of: c(t)=39.45[1-exp(-1.11·10^-6t)], where [c(t)]=mg/l and [t]=s. This study can be used as a preliminary prediction of size, shape and dissolution rate of wear debris from SiNx coatings.

Silicon nitride (SiN_x) coatings are currently under investigation as bearing surfaces for joint implants, due to their low wear rate and the good biocompatibility of both coatings and their potential wear debris. The aim of this study was to move further towards functional SiN_x coatings by evaluating coatings deposited onto CoCrMo surfaces with a CrN interlayer, using different bias voltages and substrate rotations. Reactive direct current magnetron sputtering was used to coat CoCrMo discs with a CrN interlayer, followed by a SiN_x top layer, which was deposited by reactive high-power impulse magnetron sputtering. The interlayer was deposited using negative bias voltages ranging between 100 and 900 V, and 1-fold or 3-fold substrate rotation. Scanning electron microscopy showed a dependence of coating morphology on substrate rotation. The N/Si ratio ranged from 1.10 to 1.25, as evaluated by X-ray photoelectron spectroscopy. Vertical scanning interferometry revealed that the coated, unpolished samples had a low average surface roughness between 16 and 33 nm. Rockwell indentations showed improved coating adhesion when a low bias voltage of 100 V was used to deposit the CrN interlayer. Wear tests performed in a reciprocating manner against Si₃N₄ balls showed specific wear rates lower than, or similar to that of CoCrMo. The study suggests that low negative bias voltages may contribute to a better performance of SiN_x coatings in terms of adhesion. The low wear rates found in the current study support further development of silicon nitride-based coatings towards clinical application.

Ceramic coatings deposited on orthopedic implants are an alternative to achieve and maintain high wear resistance of the metallic device, and simultaneously allow for a reduction in metal ion release. Silicon nitride based (SiN_x) coatings deposited by high power impulse magnetron sputtering (HiPIMS) have shown potential for use in joint replacements, as a result of an improved chemical stability in combination with a good adhesion. This study investigated the effect of N, C, Cr, and Nb content on the tribocorrosive performance of 3.7 to 8.8 µm thick SiN_x coatings deposited by HiPIMS onto CoCrMo discs. The coating composition was assessed from X-ray photoelectron spectroscopy and the surface roughness by vertical scanning interferometry. Hardness and Young’s modulus were measured by nanoindentation and coating adhesion was investigated by scratch tests. Multidirectional wear tests against ultrahigh molecular weight polyethylene pins were performed for 2 million cycles in bovine serum solution (25%) at 37 °C, at an estimated contact pressure of 2.1 MPa. Coatings with a relatively low hardness tended to fail earlier in the wear test, due to chemical reactions and eventually dissolution, accelerated by the tribological contact. In fact, while no definite correlation could be observed between coating composition (N: 42.6–55.5 at %, C: 0–25.7 at %, Cr: 0 or 12.8 at %, and Nb: 0–24.5 at %) and wear performance, it was apparent that high-purity and/or -density coatings (i.e., low oxygen content and high nitrogen content) were desirable to prevent coating and/or counter surface wear or failure. Coatings deposited with a higher energy fulfilled the target profile in terms of low surface roughness (Ra < 20 nm), adequate adhesion (L_c2 > 30 N), chemical stability over time in the tribocorrosive environment, as well as low polymer wear, presenting potential for a future application in joint bearings.

Ceramic coatings may be applied onto metallic components of joint replacements for improved wear and corrosion resistance as well as enhanced biocompatibility, especially for metal-sensitive patients. Silicon nitride (SiN_x) coatings have recently been developed for this purpose. To achieve a high coating density, necessary to secure a long-term performance, is however challenging, especially for sputter deposited SiN_x coatings, since these coatings are insulating. This study investigates the time-dependent performance of sputter-deposited SiN_x based coatings for joint applications. SiN_x coatings with a thickness in the range of 4.3–6.0 µm were deposited by reactive high power impulse magnetron sputtering onto flat discs as well as hip heads made of CoCrMo. SiN_x compositional analysis by X-ray photoelectron spectroscopy showed N/Si ratios between 0.8 and 1.0. Immersion of the flat disks in fetal bovine serum solution over time as well as short-term wear tests against ultra-high molecular weight polyethylene (UHMWPE) discs showed that a high coating density is required to inhibit tribocorrosion. Coatings that performed best in terms of chemical stability were deposited using a higher target power and process heating.