University of Connecticut researchers have developed a biodegradable pressure sensor that could help doctors monitor health ailments like chronic lung disease, swelling of the brain and other medical conditions before it dissolves in the patient’s body.
The small sensor is flexible and made of medically safe materials that have already been approved by the U.S. Food and Drug Administration for surgical sutures, bone grafts and medical implants. The sensor is designed to replace existing implantable pressure sensors that are potentially toxic to the patient.
The sensors currently in use must be removed after use. This subjects patients to an additional invasive procedure, which not only extends their recovery time but also increases their risk of infection.
The UConn sensor emits a small electrical charge when pressure is applied to it, so the device can also be used to provide electrical stimulation for tissue regeneration. Other potential applications include monitoring glaucoma patients, heart disease patients and patients with bladder cancer.
"We are very excited because this is the first time these biocompatible materials have been used in this way," said Thanh Duc Nguyen, the paper's senior author and an assistant professor of mechanical and biomedical engineering in the Institute of Regenerative Engineering at UConn Health and the Institute of Materials Science at the Storrs campus.
"Medical sensors are often implanted directly into soft tissues and organs," Nguyen noted. "Taking them out can cause additional damage. We knew that if we could develop a sensor that didn't require surgery to take it out, that would be really significant."
The lab created a prototype sensor that was made of a thin polymer film that was five millimeters long, five millimeters wide and 200 micrometers thick. The sensor was implanted in the abdomen of a mouse to monitor the mouse’s respiratory rate. It emitted reliable readings of contractions in the mouse’s diaphragm for four days before breaking down into the individual organic components.
In order to make sure the sensor was also medically safe, the researchers implanted it in the back of a mouse and then watched for a response from the mouse’s immune system. The results showed only minor inflammation after the sensor was inserted and the surrounding tissue returned to normal after four weeks.
One of the biggest challenges the researchers faced was getting the biodegradable material to produce an electrical charge when it was subjected to pressure or squeezed. This process is knowns as the piezoelectric effect. In the usual state, the medically safe polymer used for the sensor — a product called Poly(L-lactide) or PLLA — is neutral and doesn’t emit an electrical charge under pressure.
Eli Curry, a graduate student in Nguyen’s lab and the lead author, provided the project’s key breakthrough. He successfully transformed the PLLA into a piezoelectric material by carefully heating it, stretching it and cutting it at just the right angle so that the internal molecular structure was altered and it adopted piezoelectric properties. Curry then connected the sensor to electronic circuits so the material’s force-sensing capabilities could be tested.
The UConn sensor is made of two layers of piezoelectricc PLLA film sandwiched between tiny molybdenum electrodes and then encapsulated with layers of polylactic acid or PLA, a biodegradable product that is commonly used for bone screws and tissue scaffolds. Molybdenum is used for cardiovascular stents and hip implants.
The piezoelectric PLLA film emits a small electrical charge when the smallest pressure is applied. The small electrical signals can be captured and transmitted to another device for review by a doctor.
As part of their proof of concept test, the research team hardwired an implanted sensor to a signal amplifier placed outside of the mouse’s body. The amplifier then transmitted the enhanced electrical signals to an oscilloscope where the sensor’s readings could be easily viewed.
During testing, the sensor’s readings were equal to those of existing commercial devices and were just as reliable. The new sensor was able to capture a wide range of physiological pressures, like those found in the brain, behind the eye and in the abdomen. The sensitivity of the sensor can be adjusted by changing the number of layers of PLLA used and other factors.
Nguyen’s group is investigating ways to extend the senor’s functional lifetime. The lab’s goal is to develop a sensor system that is completely biodegradable within the human body.
Until then, the new sensor can be used in its current form to help patients avoid the invasive removal surgery.
"There are many applications for this sensor," said Nguyen. "Let's say the sensor is implanted in the brain. We can use biodegradable wires and put the accompanying non-degradable electronics far away from the delicate brain tissue, such as under the skin behind the ear, similar to a cochlear implant. Then it would just require a minor treatment to remove the electronics without worrying about the sensor being in direct contact with soft brain tissue."
The paper on this research was published in Proceedings of the National Academy of Sciences.