# Reversible Stiffening of Biopolymeric Hybrid Networks by Dynamically Switching Cross-Links In Situ

## TL;DR

This paper presents a method to reversibly stiffen biopolymer hydrogels by dynamically switching cross-links, enabling real-time control of mechanical properties for tissue engineering.

## Contribution

A novel in situ method for reversible stiffening of biopolymer networks using dynamically switchable cross-links is introduced.

## Key findings

- Hydrogel stiffness can be reversibly adjusted using light-induced, ionic, and DNA-based cross-links.
- Mucin incorporation enhances antibacterial properties and allows a wide range of stiffness values.
- Chelating agents and DNA displacement strands effectively remove cross-links, restoring original properties.

## Abstract

Achieving reversible stiffening of biopolymer networks
in a controlled
manner remains a challenging topic in materials science, especially
when trying to assess the following changes in mechanical material
properties in real time. To address these challenges, we here utilize
a custom-made measurement setup that allows us to manipulate the cross-linking
state of alginate-based hydrogels in situ while quantifying
the achieved alterations in the viscoelastic response of the biopolymer
networks. Interpolymer connections in the biopolymer networks are
created by a combination of light-induced, covalent cross-links, ionic
cross-links, and DNA-based cross-links, where the latter two can be
successfully removed again by employing either chelating agents (e.g., ethylenediaminetetraacetic acid and
citrate) or suitable displacement DNA strands. In part, this range
of the different cross-linking options mentioned is inter
alia made possible by incorporating the glycoprotein mucin
into the alginate system, which also allows for a range of different
starting (∼0.2–400 Pa), intermediate (∼25 Pa–1.6
kPa), and final stiffnesses (∼4 Pa–1.2 kPa) of the mixed
hydrogel matrix. At the same time, the presence of mucins (1–4%
(w/v)) in the biopolymer mixture enhances the properties of the cytocompatible
hydrogel by improving its antibacterial characteristics. Such well-controllable
alginate/mucin networks with dynamically switchable mechanical properties
will likely find broad applications in cell cultivation studies or
tissue engineering applications.