Goosebumps‐Inspired Microgel Patterns with Switchable Adhesion and Friction

Abstract

directly links to the underlying mechanism of skin deformation and implies a useful function. Goosebumps represent an array of tiny bumps generated from a smooth surface through the contraction of miniature muscles that are attached to each hair, and cause the surrounding area to protrude to form bumped structures. The occurrence of goosebumps helps to regulate body temperature and takes part in heat transfer balance, but also delivers important emotional information by altering touch adhesion and friction. In the skins covered with thick hairs, goosebumps can significantly increase moving drag by tilting up the long hairs, for example, the feather of flying birds. These phenomena suggest that the reversible morphology change of the goosebumpslike structure on a surface constitutes a novel strategy to develop "smart" coatings such as biosensors and actuation devices.Herein, we attempt to mimic the reversible goosebump surface by using responsive hydrogels. Hydrogels can undergo topographical changes between swollen and shrunken states through switching the solvation state of the polymer network by external stimuli such as pH or temperature. [14][15][16] Hydrogels have been widely used to fabricate smart materials, like artificial muscles. [3,17] During the shrinking or swelling, water molecules are expelled from or sorbed in the polymer network, which is a typical slow diffusion process. Several strategies have been developed to reduce diffusion pathway to speed up the response, including introducing porous structures in the hydrogel or decreasing the size of the hydrogel to increase the response speed. Microgels represent one of the most promising materials to achieve fast A substrate mimicking the surface topography and temperature sensitivity of skin goosebumps is fabricated. Close-packed arrays of thermoresponsive microgel particles undergo topographical changes in response to temperature changes between 25 and 37 °C, resembling the goosebump structure that human skin develops in response to temperature changes or other circumstances. Specifically, positively charged poly[2-(methacryloyloxy) ethyltrimethylammonium chloride] (PMETAC) brushes serve as an anchoring substrate for negatively charged poly(NIPAm-co-AA) microgels. The packing density and particle morphology can be tuned by brush layer thickness and pH of the microgel suspension. For brush layer thickness below 50 nm, particle monolayers are observed, with slightly flattened particle morphology at pH 3 and highly collapsed particles at pH above 7. Polymer brush films with thickness above 50 nm lead to the formation of particle multilayers. The temperature responsiveness of the monolayer assemblies allows reversible changes in the film morphology, which in turn affects underwater adhesion and friction at 25 and 37 °C. These results are promising for the design of new functional materials and may also serve as a model for biological structures and processes.Goosebumps [+] Present address:

Hydrogel: Diversity of Structures and Applications in Food Science

“…However, microgels possess unique properties of both hydrogels and colloidal particles, such as structural integrity, compartmentalization, orthogonal functionalization, softness, deformability, permeability, stimuli-responsiveness, reversible swelling, and adaptivity, and have been found to be promising for a wide range of applications in various fields including controlled drug release, separation technology, bio-, and chemical sensors, etc. [312][313][314][315][316][317][318][319][320][321][322][323][324][325][326] Microgels can be fabricated mainly via emulsion polymerization with an added surfactant, surfactant-free emulsion polymerization (SFEP), inverse mini-, and microemulsion polymerization, as well as the crosslinking of prepolymer chains, etc. [327,328] Typically, the techniques used to break-up and gel the particles for microgel formation will determine the final microgel properties, including the structure, size, and strength [329] and the presence and amount of crosslinks determine the "colloidal" or "macromolecular" character of the microgel.…”

Surface‐Bound Microgels for Separation, Sensing, and Biomedical Applications

“…Li et al corroborated these findings using poly(NIPAm-co-acrylic acid) (P(NIPAm-co-AA) microgels bound to silicon wafers, showing that microgels switched reversibly between tall conical structures in the contracted state (i.e., above 37 °C) and hemispherical structures in the swollen state (i.e., below 25 °C). [80] Silicon wafers also exhibit the innate ability to adsorb a wide array of hydrophilic polymers, such as polyethylene imine (PEI) and poly(diallyldimethylammonium chloride) (PDADMAC), which provide a route to study the effects of electrostatic interactions on microgel adsorption. For example, Schmidt et al utilized AFM and ellipsometry to examine the impact of the charge on the packing density of microgels deposited on PEI-coated silicon wafers at different pH values.…”

“…morphologies. [80] The timescale of the coating process allowed the microgels to diffuse across the entire substrate surface area for adsorption at the base and along the walls of the wells.…”

Exploration on Aqueous Lubrication of Polymeric Microgels between Titanium Alloy Contacts

“…Microgels are intramolecularly cross-linked micro/nanoscale polymeric colloids, whose network structure can be filled with free-flowing water. , Based on the advantage of colloidal stability, convenient preparation, facile functionalization, and good biocompatibility, microgels have evolved to be a good candidate as lubricating additives for hydration lubrication . In particular, stimuli-responsive microgels can be employed as smart lubricants for modulating the interfacial friction under the environmental stimuli (e.g., temperature, light, and pH). ,, As a representative, thermosensitive poly­( N -isopropylacrylamide) (PNIPAAm) based microgels have been widely studied for aqueous lubrication. , In 2013, Liu et al investigated the tribological behavior of poly­( N -isopropylacrylamide)- graft -poly­(ethylene glycol) (PNIPAAm- g -PEG) microgels and stated that these microgels exhibited a good lubricating effect between steel/steel contacts. In 2019, Xu et al further confirmed the aqueous lubricating performance of PNIPAAm microgels between steel and ultrahigh-molecular-weight polyethylene (UHMWPE).…”

“…22 In particular, stimuli-responsive microgels can be employed as smart lubricants for modulating the interfacial friction under the environmental stimuli (e.g., temperature, light, and pH). 20,23,24 As a representative, thermosensitive poly(Nisopropylacrylamide) (PNIPAAm) based microgels have been widely studied for aqueous lubrication. 25,26 In 2013, Liu et al 22 investigated the tribological behavior of poly(Nisopropylacrylamide)-graf t-poly(ethylene glycol) (PNIPAAmg-PEG) microgels and stated that these microgels exhibited a good lubricating effect between steel/steel contacts.…”