Session: 20-17-01: Rising Stars of Mechanical Engineering

Paper Number: 172154

Mechano-Diffusion of Particles in Stretchable Hydrogels 

The movement of particles such as biomacromolecules, gold nanoparticles, and quantum dots from one location to another, broadly impacts many fundamental processes in life science and physical systems. On one hand, particle diffusion plays a critical role in maintaining the function and survival of living organisms with examples such as intracellular transport, mucus clearance, and cytoplasmic streaming. On the other hand, technologies that facilitate nanoparticle diffusion with high efficiency, specificity, and tunability have shown promising applications in drug delivery, water treatment, and in-situ biosensing.

The diffusion of particles is typically characterized by their diffusivity (D), which describes the rate of particles moving from high–concentration regions to low–concentration regions within a liquid medium. Traditionally, the particle diffusivity is governed by the liquid medium’s viscosity and temperature, following the Stokes-Einstein equation. However, controlling particle diffusion in liquid media presents two main challenges. First, the temperature variation for a specific liquid medium is often narrow, limiting the range of achievable particle diffusivities. For example, in aqueous solutions of biomedical or environmental applications, the temperature range of 273 to 373 K results in only a 2.4-fold difference in diffusivity. Second, the spatially uniform viscosity and temperature across the liquid medium do not allow for precise manipulation of specific diffusion directions or complex diffusion patterns.

In this work, we report a mechano-diffusion mechanism that harnesses mechanical deformation to control particle diffusion in stretchable hydrogels via the synergy of geometrical transformation and energy modulation, achieving a significantly enlarged tuning ratio and highly expanded tuning freedom. Using a model particle-hydrogel material system and a customized mechano-diffusion characterization platform, we systematically investigate the diffusion of gold nanoparticles (AuNPs) of various sizes in hydrogels under controlled stress states and loading rates. Our experiments demonstrate that tension loads can enhance AuNP diffusivity by up to 22 times, far surpassing that in traditional liquid medium (e.g., 2.43 times in water, 1.34 times in diluted polymer solution, 1.21 times in concentrated polymer solution) and by external fields (e.g., 3.86 times for light, 0.99 times for magnetic field, 1.15 times for electrical field). Intriguingly, torsion loads slightly suppress AuNP diffusivity by 33%. We also observe that increasing the loading rate from 0 to 4.8 s^-1 reduces AuNP diffusivity by approximately 50%. To further push the limit of mechano-diffusion, we combine our mechano-transport theory with coarse-grained simulations to study particle diffusion in biaxially stretched polymer networks, which simultaneously expand the mesh size and reduce the energy barrier. This dual effect amplifies particle diffusivity by approximately 50 times due to the synergy of the polymer network’s geometric transformation and the polymer chain’s energy modulation. This work not only provides fundamental insights into the mechano-diffusion mechanism pertinent to a broad range of biological and synthetic soft materials, but also lays a theoretical foundation for developing previously inaccessible transport-based technologies.

Presenting Author: Shaoting Lin Michigan State University

Presenting Author Biography: Dr. Shaoting Lin holds the position of Assistant Professor in the Department of Mechanical Engineering at Michigan State University. He earned his Ph.D. degree (2019) at MIT and got his M.S. degree (2013) and B.S. degree (2010) at Tsinghua University. The research in Lin Research Group at MSU, at the intersection of solid mechanics, polymer science, and advanced manufacturing, aims to understand the processing-structure-property relationships of soft materials, thereby pushing the limit of mechanical and physical properties of soft materials. Our mission is to leverage Extreme Soft Materials for developing next-generation technologies including in-situ hydrogel bioelectronics, high-precision sperm selection, and physics-empowered tactile robots. Dr. Lin has published 30 first/co-first/corresponding authored papers in leading journals such as Science, Sci. Adv., Nat. Commun., Nat. Nanotechnol., PNAS, Adv. Mater., JMPS. Dr. Lin was the recipient of the Chinese Government Award for Outstanding Self-Financed Students Abroad (2018), the co-founder of the EASF_Young Webinar (2020), the Executive Editorial Board Member of Giant (2022), the Editor of the ASME Technical Committee of Mechanics of Soft Materials (2023), the Secretary of the ASME Technical Committee of Mechanics of Soft Materials (2024), the Vice Chair of the ASME Technical Committee of Mechanics of Soft Materials (2025), and the NSF Faculty Early Career Award (2024).

Authors:

Shaoting Lin Michigan State University