Waterloo engineers built a hydrogel whose stiffness can be dialled from brain-soft to tendon-firm by soaking it in salt, and it was absorbed by the body in about eight weeks.

A sea cucumber can stiffen or soften its body wall within seconds, turning from something nearly liquid into something rigid enough to wedge itself into a crevice. The trick is in the connective tissue, which rearranges its own mechanical properties on demand.

Engineers at the University of Waterloo have spent nearly four years borrowing the principle. Writing in Advanced Functional Materials, they describe a hydrogel whose stiffness can be tuned across a wide range, and report that rat tendons treated with it healed better than tendons closed with stitches alone.

“Soft tissues don’t all require the same scaffold,” said Dr Yimin Wu, a mechanical engineering professor at Waterloo who spearheaded the study. “A material placed near the soft brain must behave differently than one used to repair a hard tendon.”

Why one stiffness does not fit all

That point is the reason the work exists. Tissue engineering has long relied on scaffolds: porous materials implanted at an injury that give cells somewhere to attach and organise themselves while new tissue forms, then degrade and disappear.

The difficulty is that cells read the mechanical properties of what they sit on. A cell on a stiff surface behaves differently from the same cell on a soft one, in how it moves, what it secretes and what it becomes. The phenomenon has a name, mechanotransduction, and it means a scaffold’s stiffness is not a detail of engineering but an instruction to the tissue.

Human soft tissues span an enormous range. Brain tissue is among the softest material in the body; tendon is rope-like and carries the force of a muscle. A scaffold suited to one is wrong for the other, and most materials come with the stiffness they come with.

Freeze, then soak in salt

The Waterloo gel is a hydrogel, a water-rich polymer network, and the tuning method is unusually simple. The researchers freeze the material to set its internal structure, then soak it in salt solutions to make the result softer or firmer.

Freezing works as a structuring step because ice crystals growing through the material push the polymer aside, leaving a network of pores where the ice was. Salt then alters how the polymer chains interact with water and with each other, which changes how the network resists deformation.

What makes the approach attractive is that both steps are applied after the material is made, rather than requiring a different chemical recipe for each target stiffness. The same gel can be taken in either direction.

Tendons in rats

The preclinical test was tendon repair in rats. Tendons treated with the gel healed better than those closed with stitches alone, and the new tissue was better organised.

“We were extremely encouraged by the results,” said Dr Youchao Teng, who led the project under Wu’s supervision. “The material did more than perform well. The tendon was regenerated with more organized new tissue and comparable function to a healthy tendon.”

Organisation matters more than it might sound. A tendon’s strength comes from collagen fibres aligned along the direction of pull. Healing tissue that forms as disordered scar is weaker and more prone to re-injury, which is why tendon repairs often leave lasting deficits. A scaffold that guides fibres into alignment addresses the actual failure mode.

The gel was absorbed over roughly eight weeks, which is the other half of what a scaffold has to do. A material that persists becomes a foreign body, and one that disappears too quickly withdraws support before tissue can bear load.

The release does not say how many animals were treated, which is a meaningful gap: a rat study’s weight depends heavily on the number of animals and whether the comparison was properly controlled.

What the team hopes to reach

The tissues the researchers name as possible future targets are muscle, cartilage, skin, and the delicate tissue of the brain and other organs. Those claims are prospective, and the release is explicit that further research is needed to confirm safety and effectiveness for any of them.

The brain is the most speculative and the most interesting. Neural tissue is soft enough that conventional implants cause chronic irritation simply by being stiffer than their surroundings, and a scaffold that could be matched to it would be useful well beyond repair after injury.

Cartilage is the opposite case and no easier. It bears load, has almost no blood supply and heals poorly on its own, which is why cartilage damage so often becomes permanent.

Patents and a company

Wu and Teng have applied for two patents and formed a company to commercialise the work. The next steps are testing in larger animals and, eventually, clinical trials.

That sequence is the standard path and a long one. Larger animals are required because rat tendons are small, heal faster than human tissue and carry different loads, so a result in rats says little about how a material behaves in a knee or shoulder. Regulatory approval for an implanted, degradable material then demands evidence about what it breaks down into and where those products go.

The commercial involvement is worth noting plainly, since the researchers reporting the result also stand to benefit from it. That is ordinary in materials science and not a reason to discount the work, but it belongs in the record alongside the findings.

The study involved collaborators including Dr Song Li at the University of California, Los Angeles, Dr Badri Narayanan at the University of Louisville, and the Waterloo engineering professors Dr Zhao Pan and Michael K.C. Tham, along with others at facilities and universities in the United States, Taiwan, China and Canada. The release does not state the project’s funding sources.

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Notes

1. “Bioinspired Hydrogels with Broadly Programmable Mechanics for Soft-Tissue Interfaces.” Advanced Functional Materials, 2026. DOI: 10.1002/adfm.78596.
2. University of Waterloo. “Sea cucumber-inspired gel could help heal damaged tissue in humans.” EurekAlert!, 20 October 2026. https://www.eurekalert.org/news-releases/1148398.