Control of surface structure and properties of responsive hydrogel interfaces via electrostatic interactions

Deptula, Alexander Joseph

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https://hdl.handle.net/2142/132500 Copy

Description

Title

Control of surface structure and properties of responsive hydrogel interfaces via electrostatic interactions

Author(s)

Deptula, Alexander Joseph

Issue Date

2025-11-24

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

Espinosa-Marzal, Rosa M

Doctoral Committee Chair(s)

Espinosa-Marzal, Rosa M

Committee Member(s)

• Leal, Cecilia
• Rogers, Simon A
• Evans, Chris

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Coacervate hydrogels
• electrostatic interactions
• lubrication
• microstructure
• responsive
• Physical hydrogels
• charged hydrogels
• hydrogels
• Colloidal gels
• poly(methacrylamide-co-methacrylic acid)
• hydrogen bonding
• friction
• adhesion
• electrotunability

Abstract

Hydrogels are primary candidates for applications in soft robotics, biomedical devices, and tissue engineering due to their biocompatibility, extremely low coefficients of friction, and potential for a variety of responsive behaviors.[1] Their biphasic nature and viscoelastic network results in a deformable, yet lubricious material with a wide range of functionality and properties spanning multiple orders of magnitude. These properties are highly dependent on their chemical and physical structure. The optimal performance of hydrogels in these applications often relies on their interfacial properties. Hence, it is crucial to understand how to actively design and control this.[2] Among others, charge incorporation in hydrogels has been shown to influence biological attachment.[3] Furthermore, all these applications rely on the ability to control the surface morphology to target specific interfacial properties. To achieve this goal, three specific tasks (aims) were investigated: A. Understand the effects of electrostatics on interfacial microstructure and properties of chemically crosslinked charged copolymer hydrogels undergoing microphase separation B. Determine how physical (hydrogen bonding and hydrophobic interactions) crosslinks in charged copolymer hydrogel influence interfacial microstructure and properties, and specifically, how the network dynamics and responsiveness are influenced by physical (vs. chemical) crosslinks C. Elucidate the influence of composition on structure and dynamic behavior of responsive charged interfaces composed of coacervates These three tasks were accomplished by combining imaging methods by Atomic Force Microscopy with surface-sensitive spectroscopic (infrared spectroscopy) and rheological measurements to provide insight into the composition-microstructure-property relationship of the selected hydrogel interfaces, and the dynamics in response to external stimuli. First, we demonstrate the ability to modulate polyacrylamide hydrogel surface morphology, rheological properties, adhesion and frictional response by combining acrylic acid copolymerization and network confinement via grafting to a surface. Furthermore, lateral force measurements show that the microphase separations lead to speed and load-dependent lubrication regimes as well as spatial variation of friction. Next, we investigated semicrystalline colloidal gels composed of poly(methacrylamide-co-methacrylic acid) in water with storage moduli in the MPa range. A combination of SEM, X-ray scattering, and NMR reveals the evolution of the colloidal microstructure, crystallinity, and hydrogen bonding with varying monomer ratio. The dynamic and reversible nature of the involved interactions introduces a stimulus responsive behavior that enables the electrotunability of adhesion and friction. Taking a specific composition of this system reveals how salt addition alters not only swelling, but also the microstructure and dynamics, near-surface stiffness and charge, and ultimately, its lubricity. Unlike neutral hydrogels, where increased water content correlates with reduced friction, salts introduce additional dissipation mechanisms that can dominate over hydration effects, offering enhanced control of functional behavior. Further applying these principles to a more complex coacervate hydrogel system, the effects of ionic strength on a highly charged system with dynamic physical cross-linking was investigated. It found that the structure displays distinct regimes of structural properties depending on the salt concentration with large differences in viscoelastic behavior. This was found to contribute to tribological behavior until the ionic strength of the solution was high enough to dominate interactions at lower velocities. However, the specific surface polymer structure, hydration, and viscoelasticity was found to continue contributing significantly to friction at higher velocities, which was evaluated further based on theory. In summary, this work advances the knowledge necessary to design complex hydrogel interfaces that enable spatial and dynamic control of surface morphology and thereby of friction and adhesion. Further, this knowledge will inform design principles for synthetic hydrogel interfaces and advance understanding of the functional behavior of hydrogel-like materials such as biological tissues, whose lubricity–in salinity conditions similar to those studied here–is essential to their function.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132500

Copyright and License Information

Copyright 2025 Alexander Deptula

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