Rheological characterization of stiffness and photodegradation rate of Fe(III)-alginate hydrogels

Hydrogels (HGs) are a versatile class of materials that have been used in a wide range of applications. Among them, stimuli-responsive HGs stand out due to their ability to undergo structural transformations in response to environmental changes. In particular, photodegradable HGs are of high interest for applications demanding precise spatial and temporal control over material degradation, including but not limited to drug delivery, tissue engineering and dynamic cell culture systems. Fe(III)-alginate HGs have emerged as promising photodegradable materials that utilize a metal oxidation state dependent bond strength, yet their properties remain underexplored. In this study, we conducted a response surface experiment using rheology to assess how the Fe³⁺ concentration, alginate concentration, and manufacturer-defined alginate type influence the mechanical (e.g. stiffness) and photodegradation properties (e.g. degradation rate) of Fe(III)-alginate HGs. A design of experiment (DOE) approach revealed the following key insights in the studied range: (i) increasing the Fe³⁺ concentration enhances the HG stiffness and prolongs degradation time, (ii) alginate concentration alone has minimal impact on stiffness and degradation, and (iii) alginate structure, particularly molecular weight and block composition, strongly influences HG properties, with manufacturer-defined low-viscosity alginate yielding softer, faster-degrading HGs, and medium-viscosity alginate forming the stiffest, most resilient networks. The developed statistical models accurately predicted the HG performance, with experimental values consistently falling within the 95% prediction intervals. These findings provide a valuable framework for fine-tuning Fe(III)-alginate HG properties, enabling their design for specific applications and expanding their potential across various biomedical and engineering fields.