Despite the effort to optimize soft contact lens performance, almost half of the 140 million contact lens wearers worldwide experience symptoms of ocular dryness and discomfort, especially towards the end of the day. These symptoms are attributed to reduced compatibility between the contact lens and the ocular surface and are the main reason for contact lens discontinuation. As the interactions of the contact lens-eye interface are dynamic, the surface properties play a key role in improving ocular compatibility, comfort and overall performance of contact lenses. One promising method to reduce adverse interfacial interactions between the contact lens and the ocular surface is to modify the contact lens surface with a biomimetic layer inspired by the ocular surface and the tear film. Hyaluronic acid (HA) is a non-sulfated glycosaminoglycan naturally found in the ocular environment providing ocular hydration and lubrication. Proteoglycan 4 (PRG4), a mucin-like glycoprotein naturally produced at the ocular surface contributes to natural lubrication during blinking and to tear film stability. Surface modification with HA or PRG4 has been shown to result in improved wetting, lubricating and antifouling properties. Moreover, HA and PRG4 have been previously found to interact and synergistically reduce friction further. In the current work, novel HA and PRG4-grafted soft contact lens surfaces were prepared, and the impact of the surface tethered layer on important contact lens properties was assessed. Furthermore, the potential synergistic effect between HA and rhPRG4 on the examined properties was evaluated. Surface immobilization of HA on model conventional (pHEMA) and silicone (pHEMA-co-TRIS) hydrogel contact lenses was achieved by thiol-ene “click” chemistry, while full-length recombinant human PRG4 (rhPRG4) was surface grafted via carbonyldiimidazole (CDI) linking chemistry respectively. The chemical structure after each modification step was determined by attenuated total reflectance FTIR (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS) analyses. HA-grafted model soft contact lenses were characterized by improved surface wettability, antifouling and water retentive properties, while a decreasing trend in boundary friction was observed but only for the HA-grafted pHEMA-co-TRIS materials. Surface-tethering of rhPRG4 was found to effectively enhance the surface wettability and boundary lubricating properties of pHEMA-co-TRIS hydrogels only, whereas both rhPRG4-grafted pHEMA and pHEMA-co-TRIS materials exhibited lower protein sorption and dehydration rate. Overall, the surface immobilization processes followed herein did not alter the optical transparency of the model soft contact lenses or their in vitro compatibility with human corneal epithelial cells. Finally, there was evidence that HA and rhPRG4 synergistically interacted, further improving the contact lens properties. However, the degree of HA/rhPRG4 synergy was found to be dependent on the configuration of the formed HA/rhPRG4 complex as well as the composition of the substrate hydrogel material, with the noted improvement being more significant for the model silicone hydrogels. This is the first study to examine surface grafted full-length rhPRG4 and the effect of this modification on contact lens properties. Moreover, the study is the first to investigate the interactions between covalently tethered rhPRG4 and solutions containing HA. The results of this thesis demonstrate that HA and rhPRG4 are good candidates for the development of novel biomimetic surfaces, especially for silicone hydrogel contact lenses. The potential for using these compounds in synergy was also demonstrated, with wetting solutions of HA showing promise for modifying rhPRG4 modified materials to improve symptoms of discomfort. These naturally occurring ocular agents have the potential to improve the management of ocular dryness and discomfort, thus optimizing the overall soft contact lens performance. / Thesis / Doctor of Philosophy (PhD)

Natural composites like wood are scale-integrated structures that range from molecular to the macroscopic scale. Inspired by this design, layer-by-layer (LbL) deposition technique was used to create lignocellulosic composites from isolated wood polymers namely cellulose and lignin, with a lamellar architecture. In the first phase of the study, adsorption of alkali lignin onto cationic surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). Complete coverage of the cationic surface with alkali lignin occured at low solution concentration; large affinity coefficients were calculated for this system at differing pH levels. Adsorption studies with organosolv lignin in an organic solvent, and spectroscopic analysis of mixtures of cationic polymer with alkali lignin revealed a non-covalent interaction. The work demonstrated how noncovalent interactions could be exploited to molecular organize thin polyphenolic biopolymers on cationic surfaces. The second phase of the study examined the adsorption steps during the LbL assembly process to create novel lignocellulosic composites. LbL assembly was carried out using oxidized nanocellulose (NC) and lignin, along with a cationic polymer poly(diallyldimethylammonium chloride) (PDDA). QCM-D was used to follow the sequential adsorption process of the three different polymers. Two viscoelastic models, namely Johannsmann and Voigt, were respectively used to calculate the areal mass and thickness of the adsorbed layers. Atomic force microscopy studies showed a complete coverage of the surface with lignin in all the disposition cycles, however, surface coverage with NC was seen to increase with the number of layers. Free-standing composite films were obtained when the LbL process was carried out for 250 deposition cycles (500 bilayers) on a cellulose acetate substrate, following the dissolution of the substrate in acetone. Scanning electron microscopy of the cryo-fractured cross-sections showed a lamellar structure, and the thickness per adsorption cycle was estimated to be 17 nm. The third phase of the study investigated the effect of LbL ordering of the polymers versus a cast film composed of a blended mixture of the polymers, using dynamic mechanical analysis. A tan ï ¤ peak was observed in the 30 – 40 ºC region for both films, which was observed in the neat NC film. Heating of the samples under a compressive force produced opposite effects in the films, as the LbL films exhibited swelling, whereas the cast films showed densification. The apparent activation energy of this transition (65 – 80 kJ mol-1) in cast films, calculated based on the Arrhenius equation was found to be coincident to those reported for the ï ¢ transition of amorphous cellulose. The peak was seen to disappear in case of LbL films in the second heat, whereas it was recurring in case of cast films of the blended mixture, and neat NC films. Altogether, the together the work details a novel path to integrate an organized lignin and cellulose molecular structure, albeit modified from their native form, into a three-dimensional composite material. / Ph. D.

Biomimetic scale-covered systems offer immense potential and applications, particularly in soft robotics, protective armors, wearable materials, and multifunctional aerospace structures. A typical system consists of stiff rectangular plate like scales embedded in a softer media and arranged periodically. Experimentally, these systems indicate pronounced nonlinear strain stiffening behavior even when the underlying substrate strains are small. However, capturing these behaviors using commercial finite element (FE) codes has proved difficult due to multiple sliding contacts between the scales after engagement. Therefore, accurate and reliable analytical models of architecture-property-relationships are needed for analysis and design. This thesis investigates the contact kinematics and mechanics of biomimetic scale-covered plates subjected to bi-directional bending. Both synclastic and anti-clastic deformations of the plate are considered. The mechanical moment-curvature relationships are derived using the work-energy balance principle. The results show that when a plate is bent to a certain curvature, a quasi-rigid locked emerges for both synclastic and anticlastic curvature. Interestingly, while for anticlastic bending, the curvature at locking is nearly the same curvature as a beam with equivalent geometry and configuration, for synclastic bending, locking occurs significantly earlier due to cross-curvature effects. The moment-curvature relationships indicate strongly anisotropic behavior of the plate. The anisotropy itself was not constant, being strongly influenced by the state of deformation. The effect of scale arrangement parameters (lattice geometry) directly influenced the nonlinear behavior including the locked state. The analytical models developed are compared with equivalent FE analysis for validation for select cases and excellent agreements have been found. The outcome of this work would enhance the understanding of the nonlinear and anisotropic behavior of scale-covered plate systems, paving the way for systematic design and integration tailored for specific applications.

Geckos are famous for the skill of switchable adhesion that they use to stick on various surface while keep their fingers super clean. In the dissertation, a unique mechanism was discovered to explain gecko self-cleaning phenomena. Using atomic force microscopy (AFM), we managed to compare the microparticle-substrate adhesion and the microparticle-seta adhesion with a single seta bonded to the AFM cantilever. A dynamic effect was approved that high pulling-off speed could increase the microparticle-substrate adhesion and thus the self-cleaning appears at high moving speed. Based on the self-cleaning theory, a gecko-inspired N-doped graphene surface with switchable adhesion was achieved, which was designed into a bio-inspired micromanipulator with a success rate over 90%. When electrical bias was applied on this biomimetic surface, the charge concentration induced an electrical double layer (ELD) on the convex surfaces, which attracts polar water molecules to form a water bridge on it, significantly enhancing the adhesion on the wrinkled graphene surface, mimicking the capillary force on beetle feet. Therefore, the bio-inspired adhesive surface can be controlled with speed, electrical bias, humidity and different material surfaces. The water attraction phenomenon on the polarized surface was further tested for the potential application of water collection and evaporation in microsystems.

Low noise, directional microphones are critical for hearing aid applications. This thesis is focused on further development of a biomimetic micromachined directional microphone based on the ear structure of the Ormia Ochracea, a parasitic fly able to locate sound sources in the audio frequency range with high accuracy. The development efforts have been on implementing a version of the microphone for a behind the ear (BTE) package while improving the overall optical efficiency and noise level, demonstrating pulsed laser operation for reduced power consumption, and electrostatic control of the microphone diaphragm position for stable operation over a long time. The new packaging method for the microphone addressed the need for tighter placement tolerances along with a redesigned diaphragm and integration of a microscale optical lens array to improve the optical efficiency of the device. The completed packages were characterized for sensitivity improvement and optical efficiency. The overall optical efficiency was significantly increased from less than 1% to the photo diode array collecting 50% of the emitted optical power from the Vertical Cavity Surface Emitting Laser (VCSEL). This, coupled with the new diaphragm design, improved the acoustic performance of the microphones. Consequently, the noise levels recorded on the devices were about 31 dBA SPL, more than 15dB better than conventional directional microphones with nearly 10 times larger port spacing. Since the application for this technology is hearing aids, the power consumed by the working device needs to be at an acceptable level. The majority of the power used by the microphone is from continuously operating the VCSEL with 2mW optical output power. To reduce this power requirement, it was suggested to pulse the VCSEL at high enough frequency with low duty cycle so that the acoustic signals can be recovered from its samples. In this study, it was found that the VCSEL can be pulsed with little to no degradation in signal to noise ratio as long as the thermal mechanical noise dominated the noise spectrum. The results also indicated that a pulse train with a duty cycle of around 20% can be used without a major loss of performance in the device, meaning the device can effectively run at 1/5 of its original power under pulsed operation mode. Finally, a control technique to overcome some inherent problems of the microphone was demonstrated. Since the optical sensitivity of the microphone depends on the gap between the diaphragm grating and the integrated mirror, it is important to keep that bias gap constant during long term operation against environmental variations and charging effects. Using a simple electrostatic bias controller scheme, the sensitivity variation of the microphone was improved by a factor of 7.68 with bias control. Overall, this thesis has addressed several important aspects of a micromachined biomimetic microphone and further demonstrated its feasibility for hearing aid applications.

<p>We explore the effect of size on the phase stability of nanosystems by comparing calculated trends with the annealing behavior of nanoparticles (NPs) initially in a core-shell configuration. The NPs are characterized using a variety of transmission electron microscopy (TEM) techniques.</p> <p>We first theoretically consider the equilibrium within a Au-Pt NP of a given size. When considering the contribution of surface and interface energies, we note the appearance of a restricted composition range of the phase diagram over which the liquid and solid phases cannot coexist in a core-shell configuration. A critical radius of ",42 nm is identified below which the NP is single-phased for any composition. It is demonstrated that both branches of the miscibility gap of the Au-Pt phase diagram shift towards the Au-rich composition with increasing curvature. The magnitude of the shift is found to be strongly correlated with the coupling of nonlinear terms entering the Gibbs energy. The main contribution to the shift arises from the composition-dependent surface energy, calculated by considering the selective adsorption of Au to the surface, evaluated using the available thermodynamic properties of the Au-Pt system.</p> <p>An array of TEM-related analytical methods were developed or adapted for the characterization of individual NPs. In particular, chemical maps with quantitative information from a NP with a spatial resolution of '" 1.2 nm could be achieved, with their corresponding error analysis. We introduce an algorithm to retrieve the radial elemental composition from the projected chemical map of a NP if a spherical symmetry can be assumed and test it with NPs of known structures. We also present a technique to determine the composition of a NP having one of the elements depleting during analysis, and test it experimentally with 5-20 nm Au-Ag NPs. Typically, for every Ag characteristic X-ray detected, one Ag atom is lost to knock-on damage. We discuss the detection limit of the method as a function of NP size and composition.</p> <p>We follow the structural evolution of a ",20 nm Au(core) Pt(shell) NP during annealing at various temperatures between 300 and 800 °e. At low temperatures, interdiffusion occurs between the core and the shell, while at temperatures abovt: ",600 °e, the configuration evolves towards one composed of Au- and Pt-rich spherical caps, separated by a relatively fiat interface. We could measure a 5-10% shift in the composition of each phase with respect to the bulk phase diagram that we assigned to capillarity effect. The shift agrees qualitatively with the calculated trends. The ratio of the surface to the interface energy is measured directly from a TEM micrograph of a segregated NP and is in close agreement with the calculated ones.</p> <p>This work contributes to the understanding of the phase stability of binary NPs. The prospect of extending these studies to NPs of other bimetallic systems while probing their properties seems promising, especially in view of their catalytic, magnetic and optical potential.</p> / Doctor of Philosophy (PhD)

Bioabsorbable orthopedic fixation devices are conceptually more attractive than metallic devices in repairing damaged tissues or in fastening implants. Our study focuses on investigating bioabsorbable composites for potential use as materials for orthopedic appliances. The study focuses on Poly(l-lactic acid) (PLLA), Polyglycolic acid (PGA), Poly-e-caprolactone (PCL), matrices with Carbon fibers (AS4), Nylon fibers and PLLA fibers. Fiber coating effects have also been investigated, with compliant polymers (1%, 50% and 100% of matrix properties) and with hydroxyapatite (HA). Unidirectional, continuous fiber plies, and multi-directional, random and quasi-random short-fiber composites were considered in our study. NDSANDS a concentric cylinder model computer software, was used to evaluate the stiffness and strength of the bioabsorbable composites with unidirectional fiber orientation. To achieve a better physical understanding, the NDSANDS predictions were also compared with those given by a simple, mechanics of materials approach. The theory for multidirectional short fiber composites, recently developed by Giurgiutiu and Reifsnider was employed with three fiber-orientation distribution functions and three failure mechanisms. Stiffness and strength of bioabsorbable composites were predicted over a range of fiber volume fraction. It was found that AS4/PLLA with 16% fiber volume fraction can have properties close to the bone when used in short fiber composite. Similar results are obtained using AS4/PLLA with hydroxyapatite coating. PLLA/PGA and PLLA/PLLA also demonstrated properties close to those of the bone in the range of 25% and 64%. / Master of Science

The main objective of this work is to give contribution in both additive manufacturing (AM) and tribometry derived from the application and study of materials available with the use of biomimetic designs. Additional contributions are determining what effects treatments for flooring surfaces may have on the dynamic coefficient of friction and the effects of these products on common surfaces. The validity of the proposed methodology for a proof of concept was demonstrated by comparing measured dynamic coefficient of friction for designs using standardized equipment and comparing these values to plantar skin tested using an accepted and standardized testing method that has been extensively researched and validated. Initial biomimetic designs and characteristics unique to each design were researched and compared. Eleven designs were selected to be fabricated, tested, and compared to select the most desirable applications for further investigation. Research into potential treatments commercially available for use was done to determine the efficacy of these products. Prototype sensor designs were selected and fabricated using direct light processing (DLP) technology. Examination of the measured values was done through an analysis of the variances in the response variable and comparisons using Fisher and Tukey pairwise comparison method. Future work recommendations are provided for further development and improvement of the topics presented in this thesis.

The exceptional flying characteristics of airborne insects motivates the design of biomimetic wing structures that can exhibit a similar structural dynamic behavior. For this purpose, this investigation describes a method for both manufacturing a biomimetic insect-sized wing using the photolithography technique and analyzing its structural dynamic response. The geometry of a crane fly forewing (family Tipulidae) is acquired using a micro-computed tomography scanner. A computer-aided design model is generated from the measurements of the reconstructed scanned model of the insect wing to design the photomasks of the membrane and the venation network required for the photolithography procedure. A composite material wing is manufactured by patterning the venation network using photoresist SU-8 on a Kapton film for the assembling of the wing. A single material artificial wing is fabricated using the photoresist SU-8 for both the membrane and the network of veins. Experiments are conducted using a modal shaker and a digital image correlation (DIC) system to determine the natural frequencies and the mode shapes of the artificial wing from the fast Fourier transform of the displacement response of the wing. The experimental results are compared with those from a finite element (FE) model of the wing. A numerical simulation of the fluid-structure interaction is conducted by coupling the FE model of the artificial wing with a computational fluid dynamics model of the surrounding airflow. From these simulations, the deformation response and the coefficients of drag and lift of the artificial wing are predicted for different freestream velocities and angles of attack. Wind-tunnel experiments are conducted using the DIC system to determine the structural deformation response of the artificial wing under different freestream velocities and angles of attack. The vibration modes are dominated by a bending and torsional deformation response. The deformation along the span of the wing increases nonlinearly from the root of the wing to the tip of the wing with Reynolds number. The aerodynamic performance, defined as the ratio of the coefficient of lift to the coefficient of drag, of the artificial wing increases with Reynolds number and angle of attack up to the critical angle of attack.

Device associated infections are a growing problem in the field of orthopaedics and dentistry. Bacteria adhering to implant surfaces and subsequent biofilm formation are challenging to treat with systemic administered antibiotics. Functionalization of implant surfaces with therapeutic coatings that are capable of inhibiting bacterial adhesion are therefore considered as a straight forward strategy to treat and prevent implant related infections. In this thesis, the use of crystalline, arc deposited TiO2 and biomimetic hydroxyapatite (HA) coatings were evaluated with respect to their potential as antibacterial surface modifications for bone-anchored implants. UV light induced photocatalysis of anatase dominated TiO2 coated surfaces was shown to provide a bactericidal effect against S. epidermidis under clinically relevant illumination times and doses. Major parts of the drug release work carried out was based on biomimetic HA (HA-B) coated fixation pins. The analysis of the coating characteristics revealed that the nanoporous structure of HA-B coatings in addition to the chemical composition and surface charge are essential parameters that influence the drug carrier performance. Loading by adsorption was demonstrated to be a feasible approach to quickly incorporate antibiotics. The controlled release of antibiotics was shown to facilitate bactericidal effects against S. aureus over application-relevant time periods, even when exposed to biomechanical forces during insertion into bone model materials. Antibiotic incorporation during coating growth was shown to promote somewhat longer drug release time periods than those obtained using adsorption loading. In summary, functionalization of implant surfaces with bioactive and biocompatible coatings is a promising concept to impact the clinical success for bone-anchored applications. The additional feature of optional, on-demand antibacterial properties of these coatings through either on-site drug release or photocatalytic antibacterial treatment is advantageous for the prevention and effective treatment of devices-associated infections. Both strategies provide an immediate response to the implant contamination by bacteria and are believed to contribute towards minimizing the origin of post-surgical infections, while at the same time improving the interfacial stability between implant and bone.