When medical devices such as pacemakers are implanted in the body, they often trigger an immune response that results in the accumulation of scar tissue around the device. This scarring, known as fibrosis, can disrupt the function of the devices and may necessitate their removal. To address this issue, engineers have discovered a simple and universal method to prevent such fibrosis by coating the devices with a hydrogel adhesive. This coating binds the devices to tissue and shields them from attacks by the immune system.

The adhesive, developed by engineers at the Massachusetts Institute of Technology (MIT, Cambridge, MA, USA), is composed of cross-linked polymers known as hydrogels. It resembles a surgical tape they previously developed to seal internal wounds. The researchers have determined that other hydrogel adhesives could also guard against fibrosis, and they envision this method being applicable not only to pacemakers but to sensors and devices that administer drugs or therapeutic cells as well. Over the years, the team has engineered a range of adhesives for medical use, including tapes that are double-sided or single-sided, useful for repairing surgical incisions or internal damages. These adhesives function by quickly absorbing moisture from damp tissues through polyacrylic acid, a superabsorbent material found in diapers. Once the moisture is absorbed, chemical groups known as NHS esters within the polyacrylic acid form durable bonds with the proteins on the tissue surface in a process that is completed in about five seconds.

Several years ago, the team began to investigate if this type of adhesive could also maintain medical implants in position and stop fibrosis. To evaluate this, they coated polyurethane devices with the adhesive and implanted them into various sites such as the abdominal wall, colon, stomach, lung, or heart of rats. Upon removal weeks later, no scar tissue was evident. Further experiments with additional animal models consistently showed an absence of fibrosis where the adhesive-coated devices were implanted, persisting for up to three months. The team conducted bulk RNA sequencing and fluorescent imaging to analyze the immune response in the animals, discovering that initially, immune cells like neutrophils infiltrated the site of the implants. However, these attacks quickly subsided before any scar tissue could develop.

This adhesive has potential applications in coatings for epicardial pacemakers — devices positioned on the heart to regulate heart rate. The MIT researchers found that when they implanted wires coated with the adhesive in rats, the wires functioned effectively for at least three months without any scar tissue forming. They also experimented with a hydrogel adhesive that incorporates chitosan, a natural polysaccharide, which similarly prevented fibrosis in animal studies. In contrast, two commercially available tissue adhesives they tested did not prevent fibrosis, as they eventually detached from the tissue, allowing the immune system to resume its attack. In a different experiment, the researchers coated implants with hydrogel adhesives but then immersed them in a solution that stripped the polymers of their adhesive properties while retaining their overall chemical composition. After these were implanted and held in place by sutures, fibrotic scarring occurred, indicating that the mechanical interaction between the adhesive and the tissue plays a crucial role in preventing immune attacks, according to the researchers.

“The dream of many research groups and companies is to implant something into the body that over the long term the body will not see, and the device can provide therapeutic or diagnostic functionality. Now we have such an ‘invisibility cloak,’ and this is very general: There’s no need for a drug, no need for a special polymer,” said MIT professor Xuanhe Zhao. 

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