Hydration mechanism on a poly(methacrylic acid) film studied by in situ attenuated total reflection infrared spectroscopy

Physically Crosslinked Poly(methacrylic acid)/Gelatin Hydrogels with Excellent Fatigue Resistance and Shape Memory Properties

Hydrogels endure various dynamic stresses, demanding robust mechanical properties. Despite significant advancements, matching hydrogels' strength to biological tissues and plastics is often challenging without applying potentially harmful crosslinkers. Using hydrogen bonds as sacrificial bonds offers a promising strategy to produce tough, versatile hydrogels for biomedical and industrial applications. Poly(methacrylic acid) (PMA)/gelatin hydrogels were synthesized by thermally induced free-radical polymerization and crosslinked only by physical bonds, without adding any chemical crosslinker. The addition of gelatin increased the formation of hydrophobic domains in the structure of the hydrogels, which acted as permanent crosslinking points. The increase in PMA and gelatin contents generally led to a lower equilibrium water content (WC), higher thermal stability and better mechanical properties. The values of tensile strength and toughness reached up to 1.44 ± 0.17 MPa and 4.91 ± 0.51 MJ m-3, respectively, while the compressive modulus and strength reached up to 0.75 ± 0.06 MPa and 24.81 ± 5.85 MPa, respectively, with the WC being higher than 50 wt.%. The obtained values for compressive mechanical properties are comparable with super-strong hydrogels reported in the literature. In addition, hydrogels exhibited excellent fatigue resistance and biocompatibility, as well as great shape memory properties, which make them prominent candidates for a wide range of biomedical applications.

Self-compliant ionic skin by leveraging hierarchical hydrogen bond association

Robust interfacial compliance is essential for long-term physiological monitoring via skin-mountable ionic materials. Unfortunately, existing epidermal ionic skins are not compliant and durable enough to accommodate the time-varying deformations of convoluted skin surface, due to an imbalance in viscosity and elasticity. Here we introduce a self-compliant ionic skin that consistently works at the critical gel point state with almost equal viscosity and elasticity over a super-wide frequency range. The material is designed by leveraging hierarchical hydrogen bond association, allowing for the continuous release of polymer strands to create topological entanglements as complementary crosslinks. By embodying properties of rapid stress relaxation, softness, ionic conductivity, self-healability, flaw-insensitivity, self-adhesion, and water-resistance, this ionic skin fosters excellent interfacial compliance with cyclically deforming substrates, and facilitates the acquisition of high-fidelity electrophysiological signals with alleviated motion artifacts. The presented strategy is generalizable and could expand the applicability of epidermal ionic skins to more complex service conditions.

Role of interfacial water in determining the interactions of proteins and cells with hydrated materials

Water molecules play a crucial role in biointerfacial interactions, including protein adsorption and desorption. To understand the role of water in the interaction of proteins and cells at biological interfaces, it is important to compare particular states of hydration water with various physicochemical properties of hydrated biomaterials. In this review, we discuss the fundamental concepts for determining the interactions of proteins and cells with hydrated materials along with selected examples corresponding to our recent studies, including poly(2-methoxyethyl acrylate) (PMEA), PMEA derivatives, and other biomaterials. The states of water were analyzed by differential scanning calorimetry, in situ attenuated total reflection infrared spectroscopy, and surface force measurements. We found that intermediate water which is loosely bound to a biomaterial, is a useful indicator of the bioinertness of material surfaces. This finding on intermediate water provides novel insights and helps develop novel experimental models for understanding protein adsorption in a wide range of materials, such as those used in biomedical applications.

Interaction Forces on Polyampholytic Hydrogel Gradient Surfaces

Rational design and informed development of nontoxic antifouling coatings requires a thorough understanding of the interactions between surfaces and fouling species. With more complex antifouling materials, such as composites or zwitterionic polymers, there follows also a need for better characterization of the materials as such. To further the understanding of the antifouling properties of charge-balanced polymers, we explore the properties of layered polyelectrolytes and their interactions with charged surfaces. These polymers were prepared via self-initiated photografting and photopolymerization (SIPGP); on top of a uniform bottom layer of anionic poly(methacrylic acid) (PMAA), a cationic poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) thickness gradient was formed. Infrared microscopy and imaging spectroscopic ellipsometry were used to characterize chemical composition and swelling of the combined layer. Direct force measurements by colloidal probe atomic force microscopy were performed to investigate the forces between the polymer gradients and charged probes. The swelling of PMAA and PDMAEMA are very different, with steric and electrostatic forces varying in a nontrivial manner along the gradient. The gradients can be tuned to form a protein-resistant charge-neutral region, and we demonstrate that this region, where both electrostatic and steric forces are small, is highly compressed and the origin of the protein resistance of this region is most likely an effect of strong hydration of charged residues at the surface, rather than swelling or bulk hydration of the polymer. In the highly swollen regions far from charge-neutrality, steric forces dominate the interactions between the probe and the polymer. In these regions, the SIPGP polymer has qualitative similarities with brushes, but we were unable to quantitatively describe the polymer as a brush, supporting previous data suggesting that these polymers are cross-linked.

Chunks of charge: effects at play in the assembly of macromolecules at fluid surfaces

Large macromolecules with hydrophobic backbones are known to assemble at the interface between immiscible liquids. This assembly is often unpredictable because of the subtle interplay among hydrophobic interactions, hydrophilic solvation, structural constraints, and the thermodynamics of adsorption. In these studies, we employ vibrational sum frequency spectroscopy and interfacial tension measurements to study the assembly of a simple polyelectrolyte, poly(methacrylic acid), as it assembles at the interface between two immiscible liquids, specifically, carbon tetrachloride and water. By adjusting the polyelectrolyte charge through pH studies and the polymer size through molecular weight studies, we demonstrate that charge accumulation in segments of the polymer chains is a critical factor in macromolecular interfacial adsorption and desorption. The results have implications for related charged macromolecules whose ability to assemble between two immiscible fluid media is essential for many biological processes, water remediation efforts, and enhanced oil recovery.

Moisture plasticization for enteric Eudragit® L30D-55-coated pellets prior to compression into tablets

Enteric polymers such as cellulose esters (cellulose acetate phthalate, hydroxypropylmethylcellulose acetate succinate) and methacrylic acid-acrylate copolymers (Eudragit® L100-55 and S100) are quite brittle in the dry state and thus not suitable as pellet coatings for compression into tablets. The objective of this study was to investigate the role of humidity treatment for moisture plasticization in order to successfully compress the enterically coated pellets. The mechanical properties of Eudragit® L100-55 improved dramatically, while the properties of the other enteric polymers showed only minor changes after storage at higher humidity. The significant increase in flexibility of the Eudragit® L film was caused by hydration/plasticization; its elongation value changed from approx. 3% in the dry state to approx. 140% at the higher storage humidity. Storage at 84% relative humidity resulted in comparable release profiles of compressed and uncompressed pellets. The glass transition temperature of Eudragit® L films decreased below the compression temperature (room temperature) at storage humidities between 75% and 84%. The glass transition relative humidity leading to a change from the glassy to the rubbery state was determined by dynamic vapor sorption (DVS) to be 76.8%. Moisture resulted in superior plasticization for Eudragit® L than the conventional plasticizer triethyl citrate. The improved compressibility of high humidity treated Eudragit® L-coated pellets was also shown with single pellet compression data as indicated by an increased crushing force and deformation. In conclusion, moisture plasticization was a highly effective tool to enable the successful compression of pellets coated with the brittle enteric polymer Eudragit® L.