Artificial cartilage. Image Xu et al, HKU

Making stronger hydrogels

10 July 2023

By mimicking how the body structures soft tissues, researchers are making hydrogels that could repair cartilage inside the body, act as electronic skins and even be used as water purifiers.

Hydrogels are materials made of up to 90% water kept together by polymer networks, so they can act like a solid. This makes them attractive for biocompatible applications – those that work with the body – since they have similar properties to biological materials.

But few hydrogels have made it into practical use because it has been difficult to get the right mechanical properties. The water content and porosity of the material needed for biocompatibility tend to make it weak.

A research team led by Dr Lizhi Xu of the Department of Mechanical Engineering in the Faculty of Engineering at the University of Hong Kong (HKU) has developed new types of hydrogels that have outstanding mechanical properties and can be functionalised – made to carry out different tasks – for example by adding electronic components.

Super-strength nanofibers

The team overcame the mechanical weakness of previous hydrogels by introducing special nanofibers during the fabrication process. The nanofibers were derived from Kevlar, a polymer material used in bullet-proof vests and helmets. These provide the strength of natural tendons while polyvinyl alcohol polymers provide the required flexibility.

The first application they put the resulting hydrogel to was mimicking tendons and ligaments in the body. When these are damaged, such as the common sporting injury of tearing the ACL (anterior cruciate ligament) in the knee, they are often repaired by taking a piece of tendon from elsewhere in the body and grafting it onto the injured site.

Instead, hydrogels could be used as synthetic replacements if they have similar mechanical properties. The team’s new nanofiber-infused hydrogel is stretched in one direction during fabrication, aligning the nanofibers in a way that mimics collagen fibres in natural tendons and produces closely matching properties; with a water content of 60%, the hydrogel has a Young’s modulus (a measure of elasticity) of ~1 GPa and strength of ~80 MPa, outperforming other synthetic hydrogels by orders of magnitude.

It’s not just the close match to the properties of natural tendons that’s exciting though. The hydrogel can create biophysical cues – messages based on the environment – that can direct the differentiation, migration, and other activities of cells on the surface, helping integrate the hydrogels.

The hydrogels can also be imbued with other properties that make them useful. For example, adding sensors could help track the progress of healing, and embedded electronics could communicate this information wirelessly to external readers. Xu and his team published their findings in a paper in Science. The work was also covered by Nature Communications.

Electronic skin

The ability to add electronic components has been exploited by the team for another use: electronic skin. The idea is to transform electronics from hard, inflexible devices to thin, flexible, soft forms that can be fitted to the 3D contour of a body surface: a sort of temporary electronic tattoo.

For this application, the team combined the Kevlar-derived nanofibers and polyvinyl alcohol with polypyrrole, a polymer that has high electrical conductivity. Here, the nanofibers provided a structure around which the polypyrrole self-organised, providing high electrical conductivity with low density. This mean adding the conductivity was possible without sacrificing the other important properties, like porosity and strength.

Imagine a fitness tracker, but instead of wearing a bulky device on your wrist, you instead have a thin, flexible patch on your skin. This is what this kind of hydrogel could enable: it would even have wireless capabilities, so your phone could read data about your heart rate, oxygen level, and more.

Besides appealing to the fitness-interested, these hydrogels could also be used in the body, such as for implantable pacemakers that sit more comfortably beside the heart. The functional possibilities stretch far: the hydrogels could, for example, be implanted with drugs when certain conditions are detected.

M. Sun and L. Xu. Image: HKU

Beyond biology

With so many possibilities, where do you go next? Xu says the idea is to design materials for specific clinical problems: using the toolkit of possible functions to create a hydrogel with just the right properties for the application.

But there are also tantalising applications beyond the biological. The team are currently exploiting similar conductive hydrogels for solar water purification. The components can absorb solar light strongly enough to can generate a steam vapour, creating a simple setup to for the purification of water, including the desalination of seawater. Another application the team are pursuing is energy harvesting. If you load the hydrogel with mobile ions then you compress it, electricity can be generated by mechanical motion.

Xu credits the success of the team’s approach to the principle of biomimicry. He says: “Currently there are lots of people working on hydrogels, with different people looking at the problem from different perspectives. But our effort has focused on how to use biomimetic concepts, such as mimicking not just the molecular properties but also the microstructure of natural tissues, allowing us to engineer various biomedical devices.”