Room to Grow for an Implant

Inspired by paper finger-trap toys and an aquatic creature, researchers have developed a pediatric surgical implant that is meant to grow along with the child, potentially decreasing the number of surgeries that young patients with heart valve defects must endure.

Researchers from Harvard Medical School, Boston Children’s Hospital and Brigham and Women’s Hospital described their proof-of-concept design—intended for use in a procedure called a valve annuloplasty, which repairs leaking mitral and tricuspid valves in the heart—Oct. 10 in Nature Biomedical Engineering.

Artist’s rendering of how a braided, tubular implant could grow along with a child’s heart valve. Image credit: Randal McKenzie.

Although medical implants can save lives by correcting structural defects in the heart and other organs, their use in children has been complicated by the fact that fixed-size implants cannot expand in tune with a child’s natural growth. As a result, children who undergo life-saving cardiac surgeries, including mitral and tricuspid valve repairs, sometimes require several additional surgeries over the course of their childhood to re-repair or replace leaking heart valves.

“Medical implants and devices are rarely designed with children in mind, and as a result, they almost never accommodate growth,” said Pedro del Nido, co-senior author of the study, the William E. Ladd Professor of Child Surgery at HMS and chief of cardiac surgery at Boston Children’s. “We’ve created an environment here where individuals with expertise and interest in medical devices can come together and collaborate towards developing materials for pediatric surgery.”

Beyond cardiac repair, the research team says the tubular, expanding implant design could be adapted for a variety of other growth-accommodating implants throughout the body.

Joining forces

Del Nido partnered with bioengineer Jeffrey Karp, associate professor of medicine at HMS and principal investigator at Brigham and Women’s, embracing the Karp laboratory’s expertise in chemical engineering and biopolymer materials.

After vetting many different concepts for a growth-accommodating implant, the team took its inspiration from the braided, expanding design of a finger trap toy, selecting their first proof-of-concept to be a tricuspid valve annuloplasty ring implant.

“The implant design consists of two components: a degrading, biopolymer core and a braided, tubular sleeve that elongates over time in response to the tensile forces exerted by the surrounding growing tissue,” said Eric Feins, co-first author on the paper, a former research fellow in del Nido’s lab and currently an HMS clinical fellow in cardiothoracic surgery at Massachusetts General Hospital. “As the inner biopolymer degrades, the tubular sleeve becomes thinner and elongates in response to native tissue growth.”

To create the degrading core, Karp’s team recommended the use of an extra-stiff, biocompatible polymer that begins to erode on its surface following implantation. The polymer itself is made of components that already exist in the human body.

“By adjusting the polymer’s composition, we can tune the core to degrade predictably over a pre-determined amount of time,” said Karp, who is co-senior author of the study.

Growing interest in a growing implant

Based on promising in vivo experimental data from the team’s animal studies, the biomedical device company CryoLife is developing the concept into a surgical implant.

The proprietary design of the braided sleeve resembles not only a finger trap but also an organic structure engineered by nature itself.

“We solved this problem of growth accommodation with a concept that already exists in nature: the octopus has a special ability to stretch its arms into confined cracks and spaces between rocks in search of its prey,” said materials researcher Yuhan Lee, co-first author of the study and HMS instructor in medicine at Brigham and Women’s.

“It can do this because of unique, braid-like crossfibers of connective tissue that enable the simultaneous elongation and shrinking diameter of its arms, allowing it to extend its reach two to three times beyond the original arm length,” said Lee.

“This concept could be adapted for many different clinical applications, with exciting potential to be converted into an actively—rather than a passively—elongating structure that could act as a tissue scaffold encouraging growth,” said Feins.

Source: HMS

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