In Situ Monitoring of Hydrogel Polymerization Using Speckle Interferometry

One-Pot Construction of Articular Cartilage-Like Hydrogel Coating for Durable Aqueous Lubrication

Articular cartilage has an appropriate multilayer structure and superior tribological properties and provides a structural paradigm for design of lubricating materials. However, mimicking articular cartilage traits on prosthetic materials with durable lubrication remains a huge challenge. Herein, an ingenious three-in-one strategy is developed for constructing an articular cartilage-like bilayer hydrogel coating on the surface of ultra-high molecular weight polyethylene (BH-UPE), which makes full use of conceptions of interfacial interlinking, high-entanglement crosslinking, and interface-modulated polymerization. The hydrogel coating is tightly interlinked with UPE substrate through hydrogel-UPE interchain entanglement and bonding. The hydrogel chains are highly entangled with each other to form a dense tough layer with negligible hysteresis for load-bearing by reducing the amounts of crosslinker and hydrophilic initiator to p.p.m. levels. Meanwhile, the polymerization of monomers in the top surface region is suppressed via interface-modulated polymerization, thus introducing a porous surface for effective aqueous lubrication. As a result, BH-UPE exhibits an ultralow friction coefficient of 0.0048 during 10 000 cycles under a load of 0.9 MPa, demonstrating great potential as an advanced bearing material for disc prosthesis. This work may provide a new way to build stable bilayer coatings and have important implications for development of biological lubricating materials.

Soft Contact Mechanics with Gradient-Stiffness Surfaces

The stiffness in the top surface of many biological entities like cornea or articular cartilage, as well as chemically cross-linked synthetic hydrogels, can be significantly lower or more compliant than the bulk. When such a heterogeneous surface comes into contact, the contacting load is distributed differently from typical contact models. The mechanical response under indentation loading of a surface with a gradient of stiffness is a complex, integrated response that necessarily includes the heterogeneity. In this work, we identify empirical contact models between a rigid indenter and gradient elastic surfaces by numerically simulating quasi-static indentation. Three key case studies revealed the specific ways in which (I) continuous gradients, (II) laminate-layer gradients, and (III) alternating gradients generate new contact mechanics at the shallow-depth limit. Validation of the simulation-generated models was done by micro- and nanoindentation experiments on polyacrylamide samples synthesized to have a softer gradient surface layer. The field of stress and stretch in the subsurface as visualized from the simulations also reveals that the gradient layers become confined, which pushes the stretch fields closer to the surface and radially outward. Thus, contact areas are larger than expected, and average contact pressures are lower than predicted by the Hertz model. The overall findings of this work are new contact models and the mechanisms by which they change. These models allow a more accurate interpretation of the plethora of indentation data on surface gradient soft matter (biological and synthetic) as well as a better prediction of the force response to gradient soft surfaces. This work provides examples of how gradient hydrogel surfaces control the subsurface stress distribution and loading response.

Tribological Properties of Micropored Poly(2-hydroxyethyl methacrylate) Hydrogels in a Biomimetic Aqueous Environment

The applications of hydrogels in tissue engineering as implants have rapidly grown in the last decade. However, the tribological properties of hydrogels under physiologically relevant conditions, especially those of textured hydrogels, have remained largely unknown due to the complexity of their mechanical and chemical properties. In this study, we experimentally investigated the tribological properties of micopored poly(2-hydroxyethyl methacrylate) (pHEMA) with the lateral pore dimensions varied compared to untextured pHEMA, the most commonly used hydrogel in ophthalmology, under physiologically relevant conditions. The pHEMA specimens were slid against a smooth glass curve under varying loads (6-60 mN, leading to an average contact pressure of 10-21 kPa) and sliding speeds (1-10 mm/s) in phosphate-buffered saline (pH 7.4) at 33 °C to mimic the physiological conditions in human eyes. At relatively low loads and sliding speeds (e.g., 6 mN and 1 mm/s), the micopored pHEMA did not reduce the dissipated frictional energy significantly. However, at relatively high loads and sliding speeds (e.g., 60 mN and 100 mm/s), the micopored pHEMA resulted in significantly lower frictional energy (reduced by up to 68%) dissipation than the untextured pHEMA. The effect was more pronounced with the micropores with smaller dimensions. These are attributed to the greater amount and retentivity of the interfacial fluid supported by the free water squeezed out of the micropores with the smaller dimensions under the higher load and sliding speed. These results suggest that the use of micropore texturing on hydrogels in practice, such as for ocular applications, can be leveraged to reduce friction and wear under physiological conditions and hence lower the chance of inflammation near eye implants or keratoprosthesis.

Aqueous surface gels as low friction interfaces to mitigate implant-associated inflammation

Aqueous surface gels are fragile yet resilient biopolymer-based networks capable of sustaining extremely low friction coefficients despite tribologically-challenging environments. These superficial networks are ubiquitous in natural sliding interfaces and protect mechanosensitive cells from excessive contact pressures and frictional shear stresses from cell-fluid, cell-cell, or cell-solid interactions. Understanding these complex lubrication mechanisms may aid in the development of materials-based strategies for increasing biocompatibility in medical devices and implants. Equally as important is characterizing the interplay between soft and passive yet mobile implant materials and cellular reactions in response to direct contact and frictional shear stresses. Physically interrogating living biological systems without rupturing them in the process is nontrivial. To this end, custom biotribometers have been designed to precisely modulate contact pressures against living human telomerase-immortalized corneal epithelial (hTCEpi) cell layers using soft polyacrylamide membrane probes. Reverse-transcription quantitative polymerase chain-reaction (RT-qPCR) indicated that increased duration and, to a much greater extent, the magnitude of frictional shear stress lead to increased production of pro-inflammatory (IL-1β, IL-6, MMP9) and pro-apoptotic (DDIT3, FAS) genes, which in clinical studies are linked to pathological pain. The hierarchical structure often found in biological systems has also been investigated through the fabrication of high-water content (polyacrylamide) hydrogels through free-radical polymerization inhibition. Nanoindentation experiments and friction coefficient measurements indicate that these "gradient surface gels" reduce contact pressures and frictional shear stresses at the surface of the material while still maintaining stiffness within the bulk. Reducing frictional shear stresses through informed materials and surface design may concomitantly increase lubricity and quiet the immune response, and thus provide bio-inspired routes to improve patient outcomes and quality of life.

Miniature probe for the delivery and monitoring of a photopolymerizable material

Photopolymerization is a common method to cure materials initially in a liquid state, such as dental implants or bone or tissue fillers. Recent advances in the development of biocompatible gel- and cement-systems open up an avenue for in situ photopolymerization. For minimally invasive surgery, such procedures require miniaturized surgical endoscopic probes to activate and control photopolymerization in situ. We present a miniaturized light probe in which a photoactive material can be (1) mixed, pressurized, and injected, (2) photopolymerized/photoactivated, and (3) monitored during the chemical reaction. The device is used to implant and cure poly(ethylene glycol) dimethacrylate-hydrogel-precursor in situ with ultraviolet A (UVA) light (365 nm) while the polymerization reaction is monitored in real time by collecting the fluorescence and Raman signals generated by the 532-nm excitation light source. Hydrogels could be delivered, photopolymerized, and monitored by the probe up to a curing depth of 4 cm. The size of the photopolymerized samples could be correlated to the fluorescent signal collected by the probe, and the reproducibility of the procedure could be demonstrated. The position of the probe tip inside a bovine caudal intervertebral disc could be estimated in vitro based on the collected fluorescence and Raman signal.

Structure of surfaces and interfaces of poly(N,N-dimethylacrylamide) hydrogels

We investigated the surface structure of hydrogels of poly(N,N-dimethylacrylamide) (PDMA) hydrogels synthesized and cross-linked simultaneously by redox free radical polymerization. We demonstrate the existence of a less cross-linked layer at the surface of the gel at least at two different length scales characterized by shear rheology and by neutron reflectivity, suggesting the existence of a gradient in cross-linking. The composition of the layer is shown to depend on the degree of hydrophobicity of the mold surface and is weaker for more hydrophobic molds. While the macroscopic tests proved the existence of a relatively thick under-cross-linked layer, we also demonstrated by neutron reflectivity that the gel surface at the submicrometric scale (500 nm) was also affected by the surface treatment of the mold. These results should have important implications for the measurement of macroscopic surface properties of these hydrogels such as friction or adhesion.

Nanoscopic mechanical anisotropy in hydrogel surfaces

The bulk mechanical properties of soft materials have been studied widely, but it is unclear to what extent macroscopic behavior is reflected in nanomechanics. Using an atomic force microscopy (AFM) imaging method called force spectroscopy mapping (FSM), it is possible to map the nanoscopic spatial distribution of Young's modulus, i.e. "stiffness," and determine if soft or stiff polymer domains exist to correlate nano- and macro-mechanics. Two model hydrogel systems typically used in cell culture and polymerized by a free radical polymerization process, i.e. poly (vinyl pyrrolidone) (PVP) and poly(acrylamide) (PAam) hydrogels, were found to have significantly different nanomechanical behavior despite relatively similar bulk stiffness and roughness. PVP gels contained a large number of soft and stiff nanodomains, and their size was inversely related to crosslinking density and changes in crosslinking efficiency within the hydrogel. In contrast, PAam gels displayed small nanodomains occuring at low frequency, indicating relatively uniform polymerization. Given the responsiveness of cells to changes in gel stiffness, inhomogeneities found in the PVP network indicate that careful nanomechanical characterization of polymer substrates is necessary to appreciate complex cell behavior.

Novel differential refractometry study of the enzymatic degradation kinetics of poly(ethylene oxide)-b-poly(epsilon-caprolactone) particles dispersed in water

A poly(ethylene oxide)-b-poly(epsilon-caprolactone) (PEO-b-PCL) diblock copolymer was micronized into small micelle-like particles (approximately 80 nm) via dialysis-induced microphase inversion. The enzymatic biodegradation of the PCL portion of these particles in water was in situ investigated inside a recently developed novel differential refractometer. Using this refractometry method, we were able to monitor the real-time biodegradation via the refractive index change (Deltan) of the dispersion because Deltan is directly proportional to the particle mass concentration. We found that the degradation rate is proportional to either the polymer or enzyme concentration. Our results directly support previous speculation on the basis of the light-scattering data that the biodegradation follows the first-order kinetics for a given enzyme concentration. This study not only leads to a better understanding of the enzymatic biodegradation of PCL, but also demonstrates a novel, rapid, noninvasive, and convenient way to test the degradability of polymers.

Substrate effect on the formation of hydrogels with heterogeneous network structure

It was found that when an aqueous solution of vinyl monomers is polymerized on a hydrophobic substrate, obvious heterogeneity occurs in the region of the interface. This substrate effect was observed on polytetrafluroethylene (Teflon), polypropylene (PP), polyethylene (PE), polystyrene (PS), and polyvinylchloride (PVC), but not on hydrophilic substrates. Compared with synthesis on hydrophilic surfaces, the surfaces of hydrogels synthesized on a hydrophobic substrate exhibit a larger degree of swelling, a lower surface coefficient of friction and elastic modulus, weaker interfacial adhesion, and reduced interaction with biological cells. This substrate effect has been observed for many types of aqueous monomer solutions. It was found that the above properties are related to the loosely cross-linked architecture, containing some graft-like polymer chains, that is formed on the gel surface when the gel is prepared on a hydrophobic substrate. To understand the mechanism of the substrate effect, two novel optical methods, electric speckle pattern interferometry (ESPI) and real-time laser sheet refraction (RT-LSR), were developed. It was found that oxygen trapped in the composite interface between the monomer solution and rough hydrophobic substrates played an important role in the substrate effect.

Substrate effects of gel surfaces on cell adhesion and disruption

Substrate effects of hydrogel surfaces prepared on hydrophilic and hydrophobic substrates on the cell adhesion and disruption were studied. The adhesion of tobacco protoplasts onto anionic hydrogels was strongly influenced by the substrates on which the gels were synthesized. In the case of anionic poly(2-acrylamido-2-methylpropanesulfonic acid) gel, more cells adhered on the gel surface prepared on hydrophobic substrates than that prepared on hydrophilic substrates. On the other hand, in the case of cationic quaternized poly-(dimethylaminopropylacrylamide) gel, cell disruption occurred in a few seconds accompanied with an intensive release of cellular contents on the gel surface prepared on the hydrophilic substrates, while the cationic gel synthesized on hydrophobic substrates induced no cell disruption. These different behaviors of the cell have been made in terms of different structures of gel surfaces associated with the presence of flexible dangling chains.