Investigating Shear-induced Damage during Hydrogel Extrusion-based Manufacturing: Rheological Insights and Mechanophore Incorporation

Hydrogels are the functional material of choice for tissue engineering applications due to their biocompatibility, high water content, flexibility and tuneable mechanical properties which can mimic natural tissues. Extrusion-based 3D printing is a versatile technique used to produce complex physical geometries from virtual designs; compatible with highly viscous hydrogels which can incorporate viable cells. During printing, shear-induced forces can be imparted to destructively effect the mechanical properties of the hydrogel, damaging the gel structure and creating undesirable changes when compared to the original hydrogel properties. This research shows, through experiment and modelling, how hydrogel formulation, the printing system and the printing parameters influence the shear stresses generated, which in turn can change the final rheological properties. It is also shown that it is feasible to include a mechanically active molecule to respond to these shear stresses and indicate through a colour change when a gel network is significantly disrupted. Initially, the “printability” of poly(acrylamide/methyl acrylate) hydrogels was investigated at a range of water content levels by linking printing assessments with characterisation of the rheological parameters, such as loss tangent (tan δ), yield stress (τ_y) and zero-shear viscosity (η_0). This led to the creation of a printability map to act as a guide to successfully print poly(AM – MA) hydrogels. The optimum parameters for a gel to be deemed printable were determined where tan δ was between 0.24 – 0.45, τ_y was between 276 – 698 Pa and η_0 was between 2.74 × 104 – 2.74 × 105 Pa.s. Secondly, the research was expanded to compare four hydrogel compositions, to study the effect of nozzle geometry, nozzle diameter and printing pressure on the final rheological properties, essentially giving new information to compare behaviour of the gel before and after printing. Key rheological parameters were measured using amplitude sweep, frequency sweep, viscosity and thixotropy tests, which are often used to determine printability. It was observed that the printing process detrimentally affected all four hydrogels, where two gel compositions had lower rheological properties after printing, one had higher rheological properties, and one had a combination, compared to the original sample. Additionally, numerical simulations were carried out to provide improved understanding of the flow history during extrusion. Simulations were directly compared with experiments for both validation and to create a novel deviation map to show the relationship between the maximum shear stress and changes in rheological properties of hydrogels caused by different printing conditions. The introduction of mechanophores into hydrogel structures was made to visually monitor and quantify shear-induced damage under different flow conditions. Hydrogels showed mechanochromic behaviour during extrusion due to the activation of spiropyran moieties, where gels with lower water content and a higher concentration of mechanophores showed a more intense colour change. This research provides a rigorous and comprehensive framework to elucidate the extent of shear-induced damage to hydrogel systems after printing and provides a path forward for incorporating mechanophores to act as real-time stress sensors. This study aims to support researchers in formulating and optimising the printability of hydrogels which maintain their structural integrity.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Watchorn-Rokutan, Erica

Advisor dc:contributor.advisor

• Daly, Ronan

• Hydrogel
• Rheology
• Additive Manufacturing
• Printability
• Mechanochemistry

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/e8a82ddc-48d0-47b7-b1a5-feca99218552/download
• http://purl.org/NET/rdflicense/allrightsreserved

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.119963

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/387017