Designing Superlubricious Hydrogels from Spontaneous Peroxidation Gradients

Abstract

Hydrogels are hydrated three-dimensional networks of hydrophilic polymers that are commonly used in the biomedical industry due to their mechanical and structural tunability, biocompatibility, and similar water content to biological tissues. The surface structure of hydrogels polymerized through freeradical polymerization can be modified by controlling environmental oxygen concentrations, leading to the formation of a polymer concentration gradient. In this work, 17.5 wt % polyacrylamide hydrogels are polymerized in low (0.01 mol % O 2 ) and high (20 mol % O 2 ) oxygen environments, and their mechanical and tribological properties are characterized through microindentation, nanoindentation, and tribological sliding experiments. Without significantly reducing the elastic modulus of the hydrogel (E* ≈ 200 kPa), we demonstrate an order of magnitude reduction in friction coefficient (from μ = 0.021 ± 0.006 to μ = 0.002 ± 0.001) by adjusting polymerization conditions (e.g., oxygen concentration). A quantitative analytical model based on polyacrylamide chemistry and kinetics was developed to estimate the thickness and structure of the monomer conversion gradient, termed the "surface gel layer". We find that polymerizing hydrogels at high oxygen concentrations leads to the formation of a preswollen surface gel layer that is approximately five times thicker (t ≈ 50 μm) and four times less concentrated (≈ 6% monomer conversion) at the surface prior to swelling compared to low oxygen environments (t ≈ 10 μm, ≈ 20% monomer conversion). Our model could be readily modified to predict the preswollen concentration profile of the polyacrylamide gel surface layer for any reaction conditions�monomer and initiator concentration, oxygen concentration, reaction time, and reaction media depth�or used to select conditions that correspond to a certain desired surface gel layer profile.

Bioinspired Lubricity from Surface Gel Layers

“…The Winkler foundation model also depends on t , which is the approximate thickness of the surface gel layer. In this work, t was estimated to be 100 μm, which was informed by previous work by our group and others. , F normaln , normalHertz = 4 3 E * R 1 / 2 d 3 / 2 F normaln , normalWinkler = π E * R d 2 t − 1 …”

“…In this work, t was estimated to be 100 μm, which was informed by previous work by our group and others. 44,48…”

“…44−47 Reaction-kinetics models of peroxidation gradients at air−liquid interfaces agreed with these findings and further proposed surface gel layer thickness and their friction-reducing capabilities could be controlled with oxygen concentration and polymerization time. 48 In vitro investigations modeling device-tissue sliding interfaces demonstrated that surface gel layers on commercially available contact lenses reduce frictional shear stress and damage to human telomerase-immortalized corneal epithelial (hTCEpi) cell layers. 14 Sawyer and co-workers used microrheology to estimate the water content of contact lens surface gel layers to be above 90% and their elastic modulus to be below 1 kPa.…”

“…Furthermore, Spencer and co-workers demonstrated that these surface gel layers reduce both elastic modulus and the coefficient of friction over a wide range of sliding conditions (e.g., contact pressure, sliding velocity) and configurations (e.g., migrating/stationary/Gemini contact). Contact mechanics approaches developed by Dunn and co-workers enabled estimations of the thickness of oxygen-inhibited surface gel layers ranging from single to tens of micrometers. − Reaction-kinetics models of peroxidation gradients at air–liquid interfaces agreed with these findings and further proposed surface gel layer thickness and their friction-reducing capabilities could be controlled with oxygen concentration and polymerization time …”

Survival-Associated Cellular Response Maintained in Pancreatic Ductal Adenocarcinoma (PDAC) Switched Between Soft and Stiff 3D Microgel Culture

“…Previous nanomechanical measurements of microscale polyacrylamide hydrogel spheres 37 resulted in similar values of elastic modulus (between 1 and 10 kPa) as those determined from micromechanical measurements of millimeter-scale polyacrylamide hydrogels with similar water content and cross-link density. 38 Here, a custom-built microtribometer 39 − 43 was used for microindentation measurements of bulk hydrogels prior to fragmentation. Hydrogel samples were equilibrated in ultrapure water for at least 24 h prior to micromechanical testing.…”