Hydrogel: the future of 'smart Band-Aids'

What is a hydrogel?

Hydrogels feature in various everyday products, from soft contact lenses and condoms to disposable diapers. Hair gel, toothpaste and plant water crystals all make use of hydrogels. Alginate hydrogels combined with aloe vera provide a wound dressing that keeps the wound moist and allows for regeneration of cells.

Gavin Braithwaite, of the Cambridge Polymer Group in Boston, MA, notes that hydrogels are hydrophilic, with the potential to contain 80% or more water, permeable and allowing solute transport. They can also be viscoelastic and lubricious. They are also environmentally sensitive. All these properties make them multi-functional.

Already meeting a wide range of functions, hydrogels are dreaming of a bright future, including a role in spinal cord regrowth, nerve and tissue engineering, and even organ generation.

The structure of hydrogel is the key to its success. Its physical or chemical cross linkages of hydrophilic polymer chains enable it to either contain or absorb water up to 99% of its volume.

To create hydrogels, the polymer chains that form their basis are either chemically synthesized or derived from natural polymers. These may be proteins, such as collagen and gelatin, or polysaccharides, such as starch, alginate and agarose. Natural sources of hydrogels include shrimp shell and seaweed.

The high water content makes them either soft, “squishy” and flexible, like contact lenses, or highly absorbent, as in babies’ diapers. They can also be quite brittle. Their characteristics depend on their composition.

Materials scientists, who have for some time seen the potential of hydrogels for different applications, have been pushing the boundaries of this exceptional substance.

Hydrogel dressings

Hydrogel dressings are not new, with the first ones dating back to the 1950s. However, recent developments are producing some revolutionary concepts.

Familiar hydrogel dressings include free-flowing gels available in tubes and foil packets, preparations where hydrogel is saturated onto a gauze pad or strips, or a sheet of gel supported by a thin fiber mesh.

Hydrogel dressings provide moisture, promote healing and remove dead tissue from wounds. The high water content cools the wound and relieves pain relief. Hydrogels also prevent the dressing from sticking to the wound surface.

Hydrogels are strong and flexible, and they can be porous, allowing for diffusion, or dense, depending on composition. They can be adapted to meet different needs.

The University of Wollongong in Australia describe hydrogels as “some of the most biocompatible materials on the planet.” In fact, animal bodies are mainly composed of hydrogels.

Body tissues and synthetic hydrogels have a lot in common, and the newest products have properties similar to body tissues, making them a good candidate for a growing range of medical applications.

Last year, Medical News Today reported on the development of a hydrogel that could stretch like skin.

Scientists have been working to harness these properties, in the hope of creating a “smart material” that will mimic biological tissue and function.

The new hydrogel

In December 2015, the MIT team announced the creation of a set of “tough hydrogels” containing 70-95% water, with “extraordinary mechanical properties.”

The new hydrogel matrix is highly stretchable, transparent and can sense temperatures at different locations on the skin. The scientists have also incorporated a number of remarkable, additional features.

To allow for sustained drug release, the scientists made pathways in the matrix by inserting tubes or drilling tiny holes. They also created tiny drug reservoirs and added temperature sensors, regularly spaced. Other electronic components used to enhance the material included conductive titanium wires and semiconductor chips.

When tested, the system allowed mock drugs to flow through the hydrogel and be delivered on demand. The sensors enabled the dressing to monitor skin temperature and release drugs to different parts of the body as needed, even when the dressing was highly stretched. The sensors can also measure vital signs.

Embedded LED lights, which worked even when stretched across the knee and elbow, indicated when drug levels were low in the reservoirs.

A titanium wire, encapsulated in the matrix, formed a transparent, stretchable conductor that maintained constant electrical conductivity.

Prof. Zhao explains why a hydrogel matrix could be the key to using electronics in the biomedical context:

“If you want to put electronics in close contact with the human body for applications such as health care monitoring and drug delivery, it is highly desirable to make the electronic devices soft and stretchable to fit the environment of the human body. That’s the motivation for stretchable hydrogel electronics. You need to think of long-term stability of the hydrogels and interfaces.”

The texture, sensitivity and mechanical capability of the new smart wound dressing bring scientists a step further toward artificial biological tissues that truly mimic the functions of nature.

Applications: present and future

An immediate use of the dressing, suggest the researchers, would be to treat burns or other dermatological conditions.

The ability to release specific drugs from specific reservoirs on demand, in response to reactions picked up by the sensors from specific locations, and to do this consistently over time, would bring important benefits.

We asked Prof. Zhao whether the smart dressing would be customizable, or whether it would have to be purchased ready-prepared.

He told us: “The current smart bandage can be programmed by physicians or health workers, such as the type and dose of drugs delivered.”

In terms of cost, he said: “Since the materials (hydrogels) and devices (sensors) are relatively low cost, we expect that the system will be affordable.”

So far, Prof. Zhao told us, the dressing has not been tested for therapeutic capability, but the team is currently working on in vivo tests of the hydrogel-based device.

In the following video, Prof. Zhao talks about the properties of the new hydrogel dressing and what it could be used for:

Apart from its role as a superficial dressing, the scientists believe the material could be used to deliver electronics inside the body.

Prof. Zhao told MNT:

“New devices that consist of electronics integrated with biocompatible hydrogels are expected to find a broad range of applications in the biomedical area.”

The matrix could be used in implants, such as valves to control the flow of microfluids or in microlenses that would change shape. Implantation devices incorporating hydrogels could support mobile health, or mHealth, systems. MNT recently reported on the growing field of mHealth.

Existing glucose sensors tend to trigger an immune reaction, which covers the sensor with dense fibers, so that they have to be replaced often. The team believes that the new hydrogel could be used to create a more robust and long-lasting product.

Similarly, says Prof. Zhao, the new hydrogel could improve the effectiveness of neural probes.

Likening the brain to “a bowl of Jell-O,” he points out that the hydrogel has similar physiological properties. Together with its mechanical potential, it could be a suitable candidate for neural probes.

Further ahead, hydrogels are being considered as potential scaffolds for new tissue, and possibly even as a basis for new organs, in what is known as regenerative medicine.

This would involve seeding or encapsulating cells of the required tissue into the hydrogel. The hydrogel would then be injected into the body, where it would replace the damaged tissue and allow for delivery of nutrients. As the cells reproduce, the hydrogel scaffold would degrade, and the new tissue would replace the old.

MNT recently reported on a liquid crystalline hydrogel with 3D form that has properties similar to soft tissue.