Stiffness and viscoelasticity of soft tissue-mimicking hydrogels regulate human monocyte-derived macrophage polarisation
Publication Date
July 29, 2025
Creator
Coser, Consuelo
University of Nottingham
Abstract
The implantation of medical devices often triggers a foreign body response (FBR), a major clinical challenge and leading cause of implant failure. Macrophages are central to this process, as they sense and respond to the mechanical and chemical properties of biomaterials, thereby influencing downstream inflammatory and fibrotic pathways. Existing studies on macrophage-biomaterial interactions are often limited by reliance on non-human or immortalised cell models, the use of stiffness ranges that exceed those of native soft tissues, and the presence of confounding variables. This thesis addresses these gaps by investigating how primary human monocyte-derived macrophages respond to biomaterials with tuneable mechanical properties, with particular emphasis on stiffness and stress relaxation. In the first part of this work, alginate methacrylate (ALMA) hydrogels were optimised to cover a soft tissue-relevant stiffness range (0.25–4.5 kPa) and engineered with EDC/NHS chemistry to introduce surface modifications. While GRGDSP modification had only modest effects on macrophage polarisation, stiffness emerged as the dominant regulator of macrophage phenotype. Increasing stiffness promoted pro-inflammatory activation, with the stiffest hydrogel (ALMA 6% w/v) inducing elevated TNF-α secretion, a higher calprotectin-to-mannose surface marker ratio and pronounced morphological changes that include elongation and spreading. These results demonstrate that even modest variations within the soft tissue–relevant stiffness range are sufficient to modulate macrophage behaviour, priming them toward a pro-inflammatory phenotype. The second part of this work examined viscoelasticity, a mechanical property that has only recently received attention as an independent regulator of cell behaviour. Through a systematic optimisation process in which multiple polyacrylamide (PAAm) formulations were screened by varying both the total concentration of acrylamide and bisacrylamide and their relative ratios, we identified two formulations - PAAm 15% (acrylamide-to-bisacrylamide ratio 80:1) and PAAm 20% (acrylamide-to-bisacrylamide ratio 300:1) - that exhibited comparable stiffness, porosity, and surface chemistry, but distinct stress relaxation profiles. This enabled the decoupling of viscoelasticity from other factors that could influence macrophage polarisation. PAAm 15% (80:1) exhibited predominantly elastic behaviour, showing minimal stress relaxation, whereas PAAm 20% (300:1) demonstrated pronounced viscoelasticity, with a ~70% reduction in its relaxation modulus, occurring within a biologically relevant timeframe. Macrophages cultured on viscoelastic substrates showed increased spreading and higher eccentricity compared to those on the elastic hydrogel. Further characterisation revealed that the elastic substrate amplified pro-inflammatory responses, with higher calprotectin expression and enhanced secretion of TNF-α and IL-6, whereas anti-inflammatory markers remained unaffected by viscoelasticity, establishing stress relaxation as a key regulator of macrophage phenotype.
Overall, this work demonstrates that macrophage responses are finely tuned by multiple mechanical cues, with stiffness and viscoelasticity exerting distinct and independent effects. By integrating human primary macrophages with biomaterials engineered for specific mechanical properties, this study clarifies conflicting literature results and advances the understanding of immune–biomaterial interactions. Importantly, the findings highlight the need to consider mechanical properties beyond stiffness when designing immunomodulatory biomaterials, offering new strategies to mitigate FBR and improve long-term implant performance.
Item Type
ethesis
Thesis Type
PhD
Supervisors
Yang, Jing
University of Nottingham
Ghaemmaghami, Amir
University of Nottigham
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