Gradient-stiffness hydrogel surface layers control contact mechanics and adhesion

Hasan, Md Mahmudul

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https://hdl.handle.net/2142/127319 Copy

Description

Title

Gradient-stiffness hydrogel surface layers control contact mechanics and adhesion

Author(s)

Hasan, Md Mahmudul

Issue Date

2024-09-25

Director of Research (if dissertation) or Advisor (if thesis)

Dunn, Alison C

Doctoral Committee Chair(s)

Dunn, Alison C

Committee Member(s)

• Saif, M. Taher A
• Chasiotis, Ioannis
• Luetkemeyer, Callan

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Hydrogel
• Gradient layer
• Soft contact mechanics
• Adhesion
• Atomic Force Microscopy

Abstract

The contacting interfaces found in the human body, such as cornea or articular cartilage, feature softer outer surfaces that gradually stiffen with depth. Similar gradient-stiff surfaces of chemically cross-linked hydrogels can be synthetically fabricated, offering high potential for use as scaffolds or lubricious biomedical implants. Advances in the synthesis of gradient surface layers in hydrogels provide exciting possibilities for customized surface design for performance. State-of the-art indentation mechanics provide a foundation to identify the structure and properties of hydrogel. However, the mechanical response of gradient-stiff surfaces in hydrogel under indentation loading is complex and cumulative, complicating characterization using classical contact mechanics. These classical approaches fail to capture the complexities of gradient-stiff surfaces, leaving a critical gap in understanding indentation mechanics and load impact. Therefore, there is a pressing need to develop translatable definitions, guidelines, and applicability criteria to appropriately apply indentation mechanics to these surfaces, enabling future tuning and designs with full potential. This research explores the intricate interactions between a hard probe and the gradient layer of submerged polyacrylamide (pAAM) hydrogels, focusing on three key areas: (A) evaluating the mechanical behavior of the gradient layer and assessing the limitations of traditional mechanics models, (B) investigating the evolving adhesion behavior on hydrogels with different gradient-stiff surfaces, and (C) quantifying the dynamics of probe detachment during retraction to elucidate the network reorganization within the dangling top polymer network. Firstly, we investigated three types of gradient surfaces (continuous, laminate, and alternating) to understand their influence on indentation behavior. Simulations were validated with multiscale indentation experiments on pAAM hydrogels with softer surface layers. Results revealed that softer gradients at the top surface lead to larger contact areas and lower contact pressures than predicted by the traditional Hertz model. Secondly, we examined the depth-evolving indentation adhesion behavior of hydrogels with varying gradient layer properties. AFM results showed that hydrogels with thicker and more compliant surface layers exhibited stronger adhesion and high load-unload hysteresis when indenting within the gradient layers. Existing adhesion models were effective only for indenting thinner gradient layers or deeper indentation depths. Thirdly, we investigated the detachment of vanishing hydrogel surface layers from a hard probe during retraction at the nanoscale. Findings revealed significant stretching with minimal initial detachment, followed by a cascade of detachments. A linear spring model described the elastic stretching behavior of the polymer network, estimating single polymer chain stiffness in the range of 20-35 µN/m. Analysis indicated that detachment occurs through adhesive failure at the interface, not cohesive chain breakage, offering valuable insights for advanced hydrogel surface design. The significance of this work lies in developing guidelines and mechanisms for interpreting indentation data on gradient-stiff soft surfaces. The impact provides detailed insights into mechanics and adhesion dynamics of gradient-stiff soft surface, which could drive advancements in health science.

Graduation Semester

2024-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/127319

Copyright and License Information

Copyright 2024 Md Mahmudul Hasan

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