November 22, 2024•Physics 17, 167
A new recipe for producing hydrogels provides a material with both flexibility and semiconductivity, desirable properties for implantable medical device interfaces.
J. Zitch/University of Chicago
Bioelectronic interfaces form connections between living tissues and implanted devices, such as blood glucose monitors for diabetic patients or pacemakers for cardiac patients. One of the main challenges in creating such an interface is finding materials that are well-adapted to the soft, water-rich environment within the body while still providing accurate data collection. Scientists have previously tried to build these interfaces using semiconducting polymers, but these electronic materials are typically too hard. There are softer options, such as certain plastics and gels, but they often do not have the electrical performance of semiconductors. Now, Sihong Wang and colleagues at the University of Chicago have developed a hydrogel semiconductor that provides desirable electronic properties for biocompatible materials (1).
Hydrogels are polymeric materials made by combining hydrophilic polymers such as cellulose and polyacrylic acid. In water, this linked network expands and H2O molecules fill the spaces between the polymer fibers. Hydrogels are widely used in biotechnology due to their flexibility and permeability to oxygen and other chemical nutrients. However, its water content poses a barrier to the production of semiconducting hydrogels. “Almost all semiconducting polymers are insoluble in water,” Wang says. This means that they are not easily integrated within the hydrogel. Wang et al. solved this quagmire by separating the crosslinking and water swelling steps in hydrogel preparation.
Rather than using water as a base liquid, the researchers started with an organic solvent in which they dissolved a semiconducting polymer with hydrophilic side chains along with the hydrogel components (hydrogel monomers and a crosslinker that initiates hydrogelation). I did. Upon exposure to UV light, the hydrogel components assembled into an interconnected network to form a gel, while the semiconducting polymer was trapped within the hydrogel network. When a sheet of this gel is immersed in water, the water rushes in and pushes out the organic solvent. After solvent exchange, the gel transformed into a highly stretchable bluish hydrogel with the consistency of gelatin. The water-insoluble semiconducting polymer quickly precipitated and formed a permeable network within the hydrogel matrix. Although the resulting material was semiconducting, it retained the flexibility of traditional hydrogels.
The high interconnectivity of semiconductor clusters in new materials explains how charges can easily move through them. Although the clusters showed order over short distances, they did not form strong long-range structures, as is often seen in conventional manufacturing processes. The researchers suggest that this lack of long-range order is a result of rapid precipitation and resistance from the already existing hydrogel network.
The researchers measured the charge carrier mobility in the hydrogel and found a value of 1.4 cm2 V-1 s-1. This is low compared to common solid-state semiconductors, which have a mobility of approximately 1000 cm2 V-1 s-1. “[Although]it’s not as high-performance as silicon, it’s still good enough for many biointerface applications,” Wang says. Indeed, hydrogel semiconductors have electronic performance comparable to other polymeric semiconductors, but the softness of hydrogels is similar to that of biological tissue. Hydrogels are also porous, which makes it easy to modify their chemistry for specific applications.
The researchers tested the new material’s biocompatibility by implanting hydrogel semiconductor implants in mice and monitoring the immune response over four weeks. One of the main concerns with implantable devices is a foreign body reaction, where the immune system attempts to isolate the implant behind a collagen wall. Wang et al. measured collagen density around the implant and used it to assess biocompatibility. They found that hydrogel semiconductor implants elicited lower immune responses than other implants made with pure hydrogels or pure polymer semiconductors.
The researchers also evaluated the hydrogel semiconductor’s biosensitivity, a measure of how well biomolecules can move through the material. The biosensitivity of common semiconductors (including polymeric semiconductors) is limited by their high density, which prevents the movement of molecules into the interior to be detected in sensing applications. Wang and colleagues found that hydrogel semiconductors outperformed polymer semiconductors, allowing biomolecules as large as proteins and nucleic acids to efficiently penetrate their internal volumes.
Ximin He, a biomaterials researcher at UCLA who was not involved in the study, is impressed with the new approach’s ability to produce hydrogels with both low stiffness and good electrical conductivity. This combination is particularly difficult to achieve, she says, because softening the gel typically disrupts the molecular order needed for good electrical performance. With further development, she imagines that soft material-based bioelectronics could be used for a variety of applications.
Hydrogel semiconductors have shown promise, but more research will be needed before clinical trials, Wang said. One challenge is preparing hydrogel semiconductors in small packets that fit into sensor arrays and other devices. Wang added that further stability evaluation is needed to see if the hydrogel’s performance changes over time when implanted in the human body.
–Sachin Rawat
Sachin Rawat is a freelance science writer based in Bangalore, India.
References
Y. Dai et al., “Soft hydrogel semiconductor with enhanced biointeraction function” science 386, 431 (2024).
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