The adhesion and friction of synthetic, textured surfaces on soft and wet substrates is a fundamental problem underlying a wide range of applications in biomedical engineering.  A representative example is the locomotion of in vivo robotic devices on soft tissues to enable future robotic surgery. This project aimed to advance the fundamental knowledge on the three-dimensional (3D) contact mechanics between micro-pillars and soft hydrogel substrates upon normal and shear indentation.

Specifically, the main goals of this project were to: 1) acquire direct experimental data on the 3D large deformation of soft gel substrates in contact with micro-pillar arrays, 2) understand the coupling between shear and normal loadings of the micro-pillar array caused by nonlinear deformation of the gel substrate, and the role of pillar array geometry in such coupling, 3) establish a connection between micromechanical events in the soft gel substrate and the global adhesion and friction forces, 4) investigate the surface interactions along the normal and shear directions between soft fluid-coated gel substrates and relatively stiff elastomers, 5) develop and implement computational tools for designing geometry of micro-pillar arrays to achieve desired adhesion and friction forces on soft gel substrates, and 6) advance the area of nonlinear contact mechanics between architected surface structures and soft substrates. These goals were met by integrating experimental and modeling investigations.

On the experimental side, we have established and validated a new approach to directly measure 3D deformation fields in soft hydrogel substrate indented by micro-pillars with arbitrary geometries by tracking randomly distributed fluorescent particles embedded in the hydrogel substrate. The hardware component of this approach included a Micro-Indentation and Visualization (MIV) system for testing normal and shear contact forces between micro-pillars and soft gel substrates while performing in situ confocal imaging of the gel substrate. The software component of this approach was mainly reflected in a particle tracking algorithm for tracking 3D displacements of fluorescent particles and interpolating the discrete displacement data into continuous displacement and strain fields. Using the MIV system and the particle tracking algorithm, we discovered that the dramatically enhanced friction between micro-pillars and a wet gel substrate, in comparison to that of a flat surface on a wet gel substrate, is the local normal contact between the lateral surface of the micro-pillar and the deformed surface of the gel substrate. Building upon this discovery, we acquired direct microscopic imaging data revealing how the pillar geometry and indentation depth govern the friction performance by affecting the lateral contact. These microscopic data were corroborated by systematic macroscopic experiments on the adhesion and friction between micro-pillar arrays with different geometries and soft substrates with different stiffness and surface conditions (i.e., wet or dry). Motivated by the observed effects of micro-pillar geometry on adhesion and friction performance, we achieved tunable adhesion and friction by applying biaxial stretch to vary pillar spacing and shape in micro-pillar arrays made of stretchable silicone elastomers.

On the modeling side, significant results have included a series of 2D and 3D finite element models for simulating the adhesive and frictional contact between micro-pillars and soft substrates. Appropriate interface models to capture adhesive and frictional interactions between the micro-pillars and different types of substrates (e.g., wet or dry) were identified. These computational models were used to interpret experimental findings and to guide the mechanical design (e.g., geometry and stiffness) of the micro-pillars.

The fundamental knowledge established in this project has impacted technological applications in biomedical engineering, including the design of patterned enteroscopy balloon for enhanced tissue anchoring and robotic endoscope with in vivo mobility.

Last Modified: 01/11/2021
Modified by: Rong Long