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The unexpected use of silk to prevent preterm delivery

Silk.  It’s a wonderful fabric to wear during pregnancy.

Michael House, MD is a maternal-fetal medicine physician at Tufts Medical Center.Dr. Michael House, an obstetrician and researcher at Tufts Medical Center’s Mother Infant Research Institute, has a strong interest in the use of silk for pregnancy.  Pure silk, that is, for medical purposes.  

He is investigating the use of an injectable silk gel to strengthen the cervices of pregnant women. The cervix is the lower part of the uterus (womb). It can be measured with an ultrasound examination. If it is short, the pregnant woman will have a higher risk to deliver prematurely.  Since no new therapy for preterm birth prevention has been developed for 25 years, Dr. House’s work is potentially groundbreaking.

Dr. House’s research is important in the battle against preterm birth in the United States, where 12% of pregnant women delivered prematurely in 2012. This added a staggering $26.2 billion to healthcare costs.  And the human costs are immeasurable: delivery before 32 weeks is the major cause of infant mortality, and can lead to long-term problems with emotion control, learning ability and memory,  as well as cerebral palsy.

To understand the danger of a short cervix during pregnancy, Dr. House compares normal cervical anatomical development to that of a dysfunctional cervix.  “The cervix normally is closed during the nine months of the baby’s growth.  Then, when the uterus starts contracting, it opens up and allows the baby to come out.  In some women, the cervix starts to shorten and open too early. It’s controversial why that happens, but it’s clear that when the cervix shortens and opens too early, the woman is at risk for having a premature birth.”

The current treatments are progesterone supplementation and cerclage (placing a surgical suture, or stitch, in and around the weakened cervix).  These treatments are helpful to women who have a history of preterm birth in a prior pregnancy or a short cervix in the current pregnancy.  But “these treatments are not effective for many women,” according to Dr. House.  Although bedrest is commonly recommended, new research suggests that it does not prevent pre-term birth in women with cervical insufficiency.  Yet, bed rest is “such an ingrained practice in obstetrics that it’s hard to stop recommending it,” Dr. House says, noting that there are risks to bed rest, such as blood clots and muscle wasting.

Taking a bioengineering approach to improve cervical function for women with cervical insufficiency

An engineer turned clinician, Dr. House first worked with colleagues in his lab to create a bioengineering model for studying the biochemical changes that occur in the cervix during pregnancy.  

 Overall, the goal is to develop a biomaterial to stiffen the cervical tissue that will be strong, biocompatible (not rejected by the body’s immune system) and biodegradable (stiff but gradually degrading to open for cervical dilation and birth).  “It’s a challenging design problem to make tissue in a wet body environment. The unique challenge of pregnancy is that you need something that’s strong initially but gradually degrades over time,” he explains.

“While these are key design features in an ideal biomaterial the thing that we’re focusing on, in the short term, is functionality,” says Dr. House. “Can you stiffen the tissue with an injected material?”

While there are other biomaterials, Dr. House chose to evaluate the feasibility of silk.  Why silk?

Purified silk from silkworms certainly met the strength considerations.  It is three times as strong as Kevlar, the material used to make bulletproof vests.  To process it for insertion into cervical tissue, Dr. House says, “We take the silk thread and make it into a liquid and purify it to create a gel.” The silk cocoons from which the silk is derived are inexpensive.  Collaborating with Dr. David Kaplan, Chair of Biomedical Engineering at Tufts University, who is a world expert on using silk as a biomaterial, Dr. House made rapid progress in experiments on the silk-based injectable gel.

 Silk has been used in the body for hundreds of years, and continues to be studied.  Some examples: silk sutures, silk mesh material to repair the abdominal wall, tissue engineering (the creation of tissue to improve tissue function), and silk implants for time-release drug treatment.  

Animal experiments with silk-based injectable gel demonstrate its feasibility

In a proof of concept experiment, Dr. House’s team used six rabbits to test whether it’s possible to perform a cervical injection to stiffen the cervix during pregnancy, with the long-term goal of coming up with an alternative to the standard cerclage therapy.

Four pregnant rabbits received a small amount of silk into their cervices at day 12 of the 30-day rabbit pregnancy, about halfway. The two others received salt water as controls (for comparison).  Liquid during the injection, the silk gelled once it was inside the cervix, approximately six hours later.  After two weeks in the rabbits’ cervices, “the silk was still present in the cervix and had caused the tissue to stiffen significantly.”

While researchers don’t know if a short cervix causes preterm birth or is a consequence of a different cause, Dr. House notes, “…at the end of the day, one way to address that question of what happens first is to try and prevent cervical shortening, and if you have an impact on preterm birth, you must be on the right track.”  Silk-based injectable gel is an exciting starting point in the search for a treatment.