Surface gel layers on commercially available contact lenses have been shown to reduce frictional shear stresses and mitigate damage during sliding contact with fragile epithelial cell layers in ...vitro. Spencer and co-workers recently demonstrated that surface gel layers could arise from oxygen-inhibited free-radical polymerization. In this study, polyacrylamide hydrogel shell probes (7.5 wt % acrylamide, 0.3 wt % N,N'-methylenebisacrylamide) were polymerized in three hemispherical molds listed in order of decreasing surface energy and increasing oxygen permeability: borosilicate glass, polyether ether ketone (PEEK), and polytetrafluoroethylene (PTFE). Hydrogel probes polymerized in PEEK and PTFE molds exhibited 100× lower elastic moduli at the surface ( E PEEK * = 80 ± 31 and E PTFE * = 106 ± 26 Pa, respectively) than those polymerized in glass molds ( E glass * = 31,560 ± 1,570 Pa), in agreement with previous investigations by Spencer and co-workers. Biotribological experiments revealed that hydrogel probes with surface gel layers reduced frictional shear stresses against cells (τ PEEK = 35 ± 15 and τ PTFE = 22 ± 16 Pa) more than those without (τ glass = 68 ± 15 Pa) and offered greater protection against cell damage when sliding against human telomerase-immortalized corneal epithelial (hTCEpi) cell monolayers. Our work demonstrates that the “mold effect” resulting in oxygen-inhibition polymerization creates hydrogels with surface gel layers that reduce shear stresses in sliding contact with cell monolayers, similar to the protection offered by gradient mucin gel networks across epithelial cell layers.
Protective mucin gel layers established by epithelial cell surfaces in biology have water contents above 90% and provide a low-shear stress nonadhesive interfacial boundary on epithelial surfaces ...throughout the body. Adhesion between gels and mucin layers, muco-adhesion, is an important aspect of drug delivery, biocompatibility, and the prevention of damage during insertion, use, and removal of medical devices in contact with moist epithelial surfaces. This manuscript develops a simple mathematical model to suggest that gel-adhesion and muco-adhesion are controlled by dehydration. For a fully swollen gel, the osmotic pressure is balanced by the elastic stress in the polymer gel, and differences in the elastic modulus are used to calculate dehydration stresses. A model based on Winkler contact mechanics gives a closed form expression for the force of adhesion that is dependent on the contact radius and gel thickness, inversely proportional to the mucin layer stiffness, and proportional to the square of the differences in elastic modulus. Submerged contact experiments conducted on Gemini gel interfaces of polyacrylamide aqueous gels showed increasing adhesion with increasing dehydration of the probe. Additionally, experiments conducted against mucinated epithelial cell monolayers found mucin transfer onto the most dehydrated gels and no transfer on swollen gels. The model and experiments reveal that high water content fully swollen gels are not intrinsically muco-adhesive, which is consistent with previous tribological experience showing increased lubricity with increasing water content and mesh size.
Polytetrafluoroethylene (PTFE) is a solid lubricant known for its low friction coefficient and high wear rate. When filled with a low volume percent of alumina particles (5 wt%), its wear rate is ...decreased over four orders of magnitude. The development of a thin, uniform and well adhered transfer film during sliding is partially responsible for this decrease in wear rate by creating a low shear interface and forming a protective layer between the PTFE/alumina sample and metal countersample. In this work, a “striped” transfer film was generated by sliding up to one million reversals over a gradually decreasing stroke length. Wear and friction experiments were performed on a microtribometer to determine the robustness of the transfer film. Interferometry and profilometry were used to measure the height and wear of the film. Microscopy was used to investigate the morphology of the transfer film over sliding distance.
We demonstrate the utility of block polyelectrolyte (bPE) additives to enhance viscosity and resolve challenges with the three-dimensional (3D) printability of extrusion-based biopolymer inks. The ...addition of oppositely charged bPEs to solutions of photocurable gelatin methacryloyl (GelMA) results in complexation-driven self-assembly of the bPEs, leading to GelMA/bPE inks that are printable at physiological temperatures, representing stark improvements over GelMA inks that suffer from low viscosity at 37 °C, leading to low printability and poor structural stability. The hierarchical microstructure of the self-assemblies (either jammed micelles or 3D networks) formed by the oppositely charged bPEs, confirmed by small-angle X-ray scattering, is attributed to the enhancements in the shear strength and printability of the GelMA/bPE inks. Varying bPE concentration in the inks is shown to enable tunability of the rheological properties to meet the criteria of pre- and postextrusion flow characteristics for 3D printing, including prominent yielding behavior, strong shear thinning, and rapid recovery upon flow cessation. Moreover, the bPE self-assemblies also contribute to the robustness of the photo-cross-linked hydrogels; photo-cross-linked GelMA/bPE hydrogels are shown to exhibit higher shear strength than photo-cross-linked GelMA hydrogels. Last, the assessment of the printability of GelMA/bPE inks indicates excellent printing performance, including minimal swelling postextrusion, satisfactory retention of the filament shape upon deposition, and satisfactory shape fidelity of the various printed constructs. We envision this study to serve as a practical guide for the printing of bespoke extrusion inks where bPEs are used as scaffolds and viscosity enhancers that can be emulated in a range of photocurable precursors.
Spatiotemporally functionalized hydrogels have exciting applications in tissue engineering, but their preparation often relies on radical‐based strategies that can be deleterious in biological ...settings. Herein, the computationally guided design, synthesis, and application of a water‐soluble cyclopentadienone‐norbornadiene (CPD‐NBD) adduct is disclosed as a diene photocage for radical‐free Diels‐Alder photopatterning. We show that this scalable CPD‐NBD derivative is readily incorporated into hydrogel formulations, providing gels that can be patterned with dienophiles upon 365 nm uncaging of cyclopentadiene. Patterning is first visualized through conjugation of cyanine dyes, then biological utility is highlighted by patterning peptides to direct cellular adhesion. Finally, the ease of use and versatility of this CPD‐NBD derivative is demonstrated by direct incorporation into a commercial 3D printing resin to enable the photopatterning of structurally complex, printed hydrogels.
A water‐soluble cyclopentadienone‐norbornadiene adduct is disclosed as a photocage for cyclopentadiene‐based Diels‐Alder photoclick chemistry. We show that this scalable CPD–NBD derivative can be incorporated into hydrogel formulations and commercial 3D printing resins, providing biocompatible hydrogels that can be patterned with maleimide bearing species upon 365 nm uncaging of cyclopentadiene.
Polyacrylamide (PAAm) hydrogels are excellent synthetic materials for in vitro biotribology studies. Recent work with hydrogels sliding in a Gemini contact has revealed unique friction behavior at ...low speed that is contrary to the classic Stribeck curve. In these interfaces the friction coefficients are minimum at low speeds and appear to be speed-independent. In this report, we investigate the role of mesh size, ξ, on the low friction regime, termed thermal fluctuation lubrication, and we also explore the origins of a transition from this behavior at higher speeds to polymer relaxation lubrication. PAAm hydrogels of varying concentration were prepared and tested in a Gemini configuration using a pin-on-disk microtribometer with an applied load of 2mN and over a range of sliding speeds from 0.03mm/s to 100mm/s. We found that increasing mesh size or decreasing polymer concentration promotes lower friction coefficients. Many samples underwent a transition from a low friction behavior to an increasing friction coefficient with increasing sliding speed that scaled with speed to the 1/2 power. This transition speed was found to correlate with the mesh size and relaxation time of the polymer network.
When the eyelid blinks down over a soft hydrogel contact lens, the tear film is partitioned or even consumed by the contact lens, introducing relative sliding on both sides against the corneal ...epithelium and the eyelid wiper. This work presents a numerical fluid model of the resulting pressures and sliding speeds in both pairs of sliding. Between the eyelid wiper and front curve surface, contact pressures ranged 12–18kPa for initial eyelid wiper sliding speeds of 10–100mm/s, with corresponding aqueous film thicknesses of 260–820nm. Maximum contact lens deflection was 0.5%. Sliding with those conditions points to a hydrodynamic regime, while the base curve/cornea sliding more likely falls in the boundary regime. A lubrication curve is presented for hydrated contacts under ocular sliding and loading conditions.
► A numerical model of lubrication on both sides of the contact lens was introduced. ► The lubrication model predicted hydrodynamic sliding at the maximum blink speeds. ► The lubrication model predicted boundary lubrication on the contact lens base curve surface and during slower ocular movements. ► The boundary regime is relevant and complex for contact lens friction.
Shear‐recoverable hydrogels based on block copolypeptides with rapid self‐recovery hold potential in extrudable and injectable 3D‐printing applications. In this work, a series of 3‐arm star‐shaped ...block copolypeptides composed of an inner hydrophilic poly(l‐glutamate) domain and an outer β‐sheet forming domain is synthesized with varying side chains and block lengths. By changing the β‐sheet forming domains, hydrogels with diverse microstructures and mechanical properties are prepared and structure–function relationships are determined using scattering and rheological techniques. Differences in the properties of these materials are amplified during direct‐ink writing with a strong correlation observed between printability and material chemistry. Significantly, it is observed that non‐canonical β‐sheet blocks based on phenyl glycine form more stable networks with superior mechanical properties and writability compared to widely used natural amino acid counterparts. The versatile design available through block copolypeptide materials provides a robust platform to access tunable material properties based solely on molecular design. These systems can be exploited in extrusion‐based applications such as 3D‐printing without the need for additives.
A robust strategy to create additive‐free 3D‐printable biomaterials based on β‐sheet–driven hydrogelation of 3‐arm star block copolypeptides that are readily synthesized using N‐carboxyanhydride ring‐opening polymerization is described. The yielding and strain recovery trends correlate with microstructure, filament uniformity, and overall shape fidelity, providing biocompatible polymers using amino‐acid building blocks in tunable hydrogels.
This manuscript presents an experimental effort to directly measure contact areas and the details behind these scaled experiments on a randomly rough model surface used in the “Contact Mechanics ...Challenge” (2017). For these experiments, the randomly rough surface model was scaled up by a factor of 1000× to give a 100 mm square sample that was 3D printed from opaque polymethylmethacrylate (PMMA). This sample was loaded against various optically smooth and transparent samples of PDMS that were approximately 15 mm thick and had a range in elastic modulus from 14 kPa to 2.1 MPa. During loading, a digital camera recorded contact locations by imaging the scattering of light that occurs off of the PMMA rough surface when it was in contact with the PDMS substrate. This method of illuminating contact areas is called frustrated total internal reflection and is performed by creating a condition of total internal reflection within the unperturbed PDMS samples. Contact or deformation of the surface results in light being diffusely transmitted from the PDMS and detected by the camera. For these experiments, a range of reduced pressure (nominal pressure/elastic modulus) from below 0.001 to over 1.0 was examined, and the resulting relative contact area (real area of contact/apparent area of contact) was found to increase from below 0.1% to over 60% at the highest pressures. The experimental uncertainties associated with experiments are discussed, and the results are compared to the numerical results from the simulation solution to the “Contact Mechanics Challenge.” The simulation results and experimental results of the relative contact areas as a function of reduced pressure are in agreement (within experimental uncertainties).
The ability of cells to reorganize in response to external stimuli is important in areas ranging from morphogenesis to tissue engineering. While nematic order is common in biological tissues, it ...typically only extends to small regions of cells interacting via steric repulsion. On isotropic substrates, elongated cells can co-align due to steric effects, forming ordered but randomly oriented finite-size domains. However, we have discovered that flat substrates with nematic order can induce global nematic alignment of dense, spindle-like cells, thereby influencing cell organization and collective motion and driving alignment on the scale of the entire tissue. Remarkably, single cells are not sensitive to the substrate's anisotropy. Rather, the emergence of global nematic order is a collective phenomenon that requires both steric effects and molecular-scale anisotropy of the substrate. To quantify the rich set of behaviours afforded by this system, we analyse velocity, positional and orientational correlations for several thousand cells over days. The establishment of global order is facilitated by enhanced cell division along the substrate's nematic axis, and associated extensile stresses that restructure the cells' actomyosin networks. Our work provides a new understanding of the dynamics of cellular remodelling and organization among weakly interacting cells.
