Zwitterionic polymer networks have shown promise in reducing the short- and long-term inflammatory foreign body response to implanted biomaterials by combining the antifouling properties of zwitterionic polymers with the mechanical stability provided by cross-linking. Cross-link density directly modulates mechanical properties (i.e., swelling behavior, resistance to stress and strain, and lubricity) but theoretically could reduce desirable biological properties (i.e., antifouling) of zwitterionic materials. This work examined the effect of varying poly(ethylene glycol) dimethacrylate cross-linker concentration on protein adsorption, cell adhesion, equilibrium swelling, compressive modulus, and lubricity of zwitterionic thin films. Furthermore, this work aimed to determine the appropriate balance among each of these mechanical and biologic properties to produce thin films that are strong, durable, and lubricious, yet also able to resist biofouling. The results demonstrated nearly a 20-fold reduction in fibrinogen adsorption on zwitterionic thin films photografted on polydimethylsiloxane (PDMS) across a wide range of cross-link densities. Interestingly, either at high or low cross-link densities, increased levels of protein adsorption were observed. In addition to fibrinogen, macrophage and fibroblast cell adhesion was reduced significantly on zwitterionic thin films, with a large range of cross-link densities, resulting in low cell counts. The macrophage count was reduced by 30-fold, while the fibroblast count was reduced nearly 10-fold on grafted zwitterionic films relative to uncoated films. Increasing degrees of cell adhesion were noted as the cross-linker concentration exceeded 50%. As expected, increased cross-link density resulted in a reduced swelling but greater compressive modulus. Notably, the coefficient of friction was dramatically reduced for zwitterionic thin films compared to uncoated PDMS across a broad range of cross-link densities, an attractive property for insertional implants. This work identified a broad range of cross-link densities that provide desirable antifouling effects while also maintaining the mechanical functionality of the thin films.

Hypothesis: Application of photografted zwitterionic coatings to cochlear implant (CI) biomaterials will reduce friction and insertion forces. Background: Strategies to minimize intracochlear trauma during implantation of an electrode array are critical to optimize outcomes including preservation of residual hearing. To this end, advances in thin-film zwitterionic hydrogel coatings on relevant biomaterials may show promise, in addition to the potential of these materials for decreasing the intracochlear foreign body response. Methods: Using a recently designed one-step process, thinfilm coatings derived from zwitterionic sulfobetaine methacrylate (SBMA) were photopolymerized and photografted to the surface of polydimethylsiloxane (PDMS, silastic) samples and also to CI arrays from two manufacturers. Fluorescein staining and scanning electron microscopy with energydispersive X-ray spectroscopy verified and characterized the coatings. Tribometry was used to measure the coefficient of friction between uncoated and coated PDMS and synthetic and biological tissues. Force transducer measurements were obtained during insertion of uncoated (n ¼ 9) and coated (n ¼ 9) CI electrode arrays into human cadaveric cochleae.Results: SBMA thin-film coating of PDMS resulted in >90% reduction in frictional coefficients with steel, ceramic, and dermal tissue from guinea pigs ( p < 0.0001). We employed a novel method for applying covalently bonded, durable, and uniform coating in geographically selective areas at the electrode array portion of the implant. Image analysis confirmed uniform coating of PDMS systems and the CI electrode arrays with SBMA polymer films. During insertion of electrode arrays into human cadaveric cochleae, SBMA coatings reduced maximum force by $40% during insertion ( p < 0.001), as well as decreasing force variability and the overall work of insertion. Conclusion: Thin-film SBMA photografted coatings on PDMS and electrode arrays significantly reduce frictional coefficients and insertional forces in cadaveric cochleae. These encouraging findings support that thin-film zwitterionic coating of CI electrode arrays may potentially reduce insertional trauma and thereby promote improved hearing and other longterm outcomes.

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Objective: Durability of photografted zwitterionic hydrogel coatings on cochlear implants was examined to determine viability of these anti-fouling surfaces during insertion and long-term implant usage. Approach: Tribometry was used to determine the effect of zwitterionic coatings on lubricity of surfaces, varying hydration level, applied normal force, and timeframe. Additionally, flexural resistance was investigated using mandrel bending. Ex vivo durability was assessed by determination of friction between tissues and treated surfaces, as well as cochlear implantation force measurements using cadaveric human cochleae. Main results: Hydrated zwitterionic hydrogel coatings maintained decreased friction for 20 hours under normal force. For loosely crosslinked systems, films remained stable and retained lubricity after complete drying. All films were able to remain hydrated and functional for at least 30 minutes under normal force, with lower crosslink densities showing the greatest longevity. Under flexural force, zwitterionic films underwent dehydration for up to 60 minutes before failure. Furthermore, photografted zwitterionic hydrogel samples showed nearly identical lubricity before and after implantation. Importantly, zwitterion-coated cochlear implants experienced a significantly lower mean force during insertion than uncoated implants. Significance: This work demonstrates that photografted zwitterionic hydrogel coatings are sufficiently durable to maintain viability before, during, and after implantation. Mechanical properties, including greatly increased lubricity, are preserved after complete drying and rehydration for various applied forces. Additionally, the results show retention of up to 90% increase in lubricity relative to uncoated samples which translates to decreased force and overall less trauma during insertion of implants.