Superlubricity of pH-responsive hydrogels in extreme environments

Abstract

Poly(acrylamide-co-acrylic acid) (P(AAm-co-AA)) hydrogels are highly tunable and pH-responsive materials frequently used in biomedical applications. The swelling behavior and mechanical properties of these gels have been extensively characterized and are thought to be controlled by the protonation state of the acrylic acid (AA) through the regulation of solution pH. However, their tribological properties have been underexplored. Here, we hypothesized that electrostatics and the protonation state of AA would drive the tribological properties of these polyelectrolyte gels. P(AAm-co-AA) hydrogels were prepared with constant acrylamide (AAm) concentration (33 wt%) and varying AA concentration to control the amount of ionizable groups in the gel. The monomer:crosslinker molar ratio (200:1) was kept constant. Hydrogel swelling, stiffness, and friction behavior were studied by systematically varying the acrylic acid (AA) concentration from 0–12 wt% and controlling solution pH (0.35, 7, 13.8) and ionic strength (I = 0 or 0.25 M). The stiffness and friction coefficient of bulk hydrogels were evaluated using a microtribometer and borosilicate glass probes as countersurfaces. The swelling behavior and elastic modulus of these polyelectrolyte hydrogels were highly sensitive to solution pH and poorly predicted the friction coefficient (µ), which decreased with increasing AA concentration. P(AAm-co-AA) hydrogels with the greatest AA concentrations (12 wt%) exhibited superlubricity (µ = 0.005 ± 0.001) when swollen in unbuffered, deionized water (pH = 7, I = 0 M) and 0.5 M NaOH (pH = 13.8, I = 0.25 M) (µ = 0.005 ± 0.002). Friction coefficients generally decreased with increasing AA and increasing solution pH. We postulate that tunable lubricity in P(AAm-co-AA) gels arises from changes in the protonation state of acrylic acid and electrostatic interactions between the probe and hydrogel surface.

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 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 . Here, a custom-built microtribometer − 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.…”

Advances in NIR-Responsive Natural Macromolecular Hydrogel Assembly Drugs for Cancer Treatment

“…The high photothermal efficiency maintains the high concentration of metformin in the tumor, which provides an excellent synergy between PDT and IT [117] Melanoma Combination of targeted drug therapy and PTT [118] MXenes Osteosarcoma Thermal ablation of residual tumor cells can effectively promote bone tissue regeneration and has antibacterial properties [119] HTA Osteosarcoma Induction of apoptosis in osteosarcoma-related cells and thermal ablation of tumors [120] SPIIN Breast cancer…”

“…The high photothermal efficiency maintains the high concentration of metformin in the tumor, which provides an excellent synergy between PDT and IT [117] Melanoma Combination of targeted drug therapy and PTT [118] MXenes Osteosarcoma Thermal ablation of residual tumor cells can effectively promote bone tissue regeneration and has antibacterial properties [119] HTA Osteosarcoma Induction of apoptosis in osteosarcoma-related cells and thermal ablation of tumors [120] SPIIN Breast cancer Tumor growth and metastasis to the lung were inhibited by 1064 nm light irradiation [121] PDA NPS Melanoma Dual regulation of pH and NIR can improve the tumor's hypoxic environment and maintain drug concentration [122] Breast cancer Cooperates with PPT and IT [123] Osteosarcoma High photothermal effect and good biocompatibility provide a good carrier for drug delivery [124] DOX/DOPA-rGO Breast cancer Combined photothermal and CT effectively reducing the viability to 21% [125] Au Melanoma Gene-targeted therapy for ocular melanoma under photothermal synergy [126] Breast cancer The gold coating was attached to enhance the fluorescence signal for easy diagnosis, and the photothermal conversion efficiency reached 65% [127] 4.…”

Superlubricity of Materials: Progress, Potential, and Challenges

“…Researchers have explored the use of self-lubricating materials, such as hydrogels and polymer composites, to achieve superlubric conditions in orthopedic applications. These materials can provide lubrication and reduce friction, thereby reducing wear debris and the potential for inflammation or implant failure [ 74 , 75 , 76 ].…”