Modulus adaptive lubricating prototype inspired by instant muscle hardening mechanism of catfish skin

In nature, living organisms evolve unique functional components with mechanically adaptive compatibility to cater dynamic change of interface friction/lubrication. This mechanism can be used for developing intelligent artificial lubrication-regulation systems. Inspired by the muscle hardening-triggered lubrication of longsnout catfish, here we report a modulus adaptive lubricating hydrogel prototype consisting of top mucus-like hydrophilic lubricating layer and muscle-like bottom hydrogel that can stiffen via thermal-triggered phase separation. It exhibits instant switch from soft/high frictional state (~0.3 MPa, μ~0.37) to stiff/lubricating state (~120 MPa, μ~0.027) in water upon heating up. Such switchable lubrication is effective for wide range of normal loads and attributed to the modulus-dominated adaptive contact mechanism. As a proof-of-concept, switchable lubricating hydrogel bullets and patches are engineered for realizing controllable interface movements. These important results demonstrate potential applications in the fields of intelligent motion devices and soft robots.

controlling the input signal of the artificial arm. The testing velocity was kept at 50 mm/min. The lubrication sample was pasted on both sides of the glass sheet for experiment at 20 °C and 80 °C. Toy submarine was commercially purchased. The driving force of toy submarine was 16mN. The lubrication sample was pasted on the submarine for experiment at 20 °C and 80 °C. A certain grasping force (200mN per arm) was obtained by controlling the input signal of the artificial arm. Toy tanks (40 g) was commercially purchased. The tilting Angle of the tilting platform is 45 °. The sample was attached to a sheet of glass. Andthe experimental temperature of the demonstration experiment is 20 °C and 80 °C.

Hertz contact analysis and Finite Element Analysis.
The contact stress and deformation analysis was based on Hertz contact theory. 1,2 Paraments in Hertz contact model were used from the experimental results in this work. Mechanical property paraments of hydrogels was obtained from related data in mechanical test and nano-indentation test. The modulus of MALH substrate (P(AAc-CaAc-co-HEMA-Br) hydrogel) at soft and rigid state was 0.3 MPa and 120 MPa, respectively. The surface modulus of lubricating layer of MALH at soft and rigid state was 0.06 MPa and 0.07 MPa, respectively. However, due to the substrates effect and mechanical trapping effect of MALH, the overall modulus of the lubrication layer was gradient. The modulus of this layer should be much higher when close to the substrates. Due to the limitation of experimental conditions, exact modulus gradient of lubricating layer was unknow. So, before a Finite Element Analysis, a simple analysis in Hertz contact model was used to identify the effect of the lubricating layer in the whole MALH contact properties. And these experiment data and analysis data were used to help the Finite Element Analysis predicting the deformation, contact stress, contact radius more accurately. Finite Element Analysis was performed by Ansys 2020R1.
First, in order to simplify the analysis, we carried out a macrosopcis indentation test (indentation radius (R): 3 mm, load (F):1 N) and analyses MALH as a whole. Displacement (d) of Blank-S (Soft) in macroscopic sphere indentation was ~901 μm, and that of MALH-S was ~1017 μm. Displacement of Blank-R (Rigid) in macroscopic sphere indentation was ~23 μm, and that of MALH-R was ~55 μm. It was found that the displacement of MALH in soft state was ~100 μm deeper than that of blank in soft state after grafting polymer brush, while the displacement of MALH in rigid state was only ~22 μm more than that of blank in rigid state. This difference resulted from the substrates effect and mechanical trapping effect. The stiffening of the substrate resulted in an increase in the modulus gradient of the lubricating layer.
This displacement data can be transformed into contact radius (a) by geometry condition in Hertz model.
The effective modulus (Eeff) of the whole MALH can be calculated by, And the contact stress distribution (P) was calculated by,  have poor load-bearing capacity in soft state, or require complex molecular design. [9][10][11][12][13][14][15][16][17] In addition, the modulus change range of these materials also can't match the modulus of the biological tissue. 27 MALH can match the biological tissue modulus well, have a large change range and a good load bearing capacity.
Dynamic covalent polymer networks such as reversible Diels-Alder reaction can also achieve dynamic modulus changes, but also require complex molecular design. 18,19 Modulus change range of material (PAA hydrogel) based on cooperative effects of hydrophobic interaction and ionic interaction can match the modulus range of biological soft tissue. 20 However, PAA materials are not wear-resistant. MALH system is simple and has good lubrication performance.
Furthermore, most of the mechanically adaptive systems based on temperature control only stiffen in the absence of these stimulus, while soften in the presence of a stimulus. The systems in Supplementary Figure 4, red area, achieve a dramatic stiffness change thanks to these physical phenomena: the crystallization-melting transition and glass transition. This leads to most smart polymer materials that tend to soften rather than harden when thermal stimuli are applied. This is totally opposite of the stimulus input stiffening mechanism in fish skin or other stress reaction in biological system. Thermal stiffening mechanism of MALH can adapt to this bio-mechanism. Furthermore, most of soft robots are usually in soft state at room temperature and stiffen when needed. Therefore, MALH exhibited extraordinary potential application in soft robots.