BME 401 Senior Design – Nerve Heating Cuff

Neha Damaraju and Charlotte Weixel

For our senior design project, we are working with the MacEwan Lab at the Washington University School of Medicine to build a sciatic nerve cuff that blocks pain signals through heating technology.

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Background

What is Sciatica?

Sciatica, pain experienced due to injury or irritation of the sciatic nerve, is a common condition. Sciatica affects between 1% and 5% of the general population annually with over 40% of people in the United States experiencing sciatica at least once in their lifetime (Cleveland Clinic, 2016). The sciatic nerve originates in the gluteal area and runs through the hips, buttocks, and down a leg on both sides of the body. At the knee, the nerve branches out into other nerves and continues down the leg towards the foot and toes. Additionally, the sciatic nerve is the longest and widest nerve in the entire body (Cleveland Clinic, 2016). The most common form of Sciatica arises when a bulging or herniated disc from the lower back presses on the sciatic nerve roots, pinching the nerve. When the sciatic nerve is compressed or inflamed, patients experience mild to severe pain along the path of the sciatic nerve including the back, legs, foot, and toes (Lagerbäck, 2018).

Spinal surgery is a common treatment for Sciatica, especially when non-surgical alternatives such as medication and physical therapy have not eased a patient’s symptoms (Cleveland Clinic, 2016). Additionally, surgery is the primary procedure for patients experiencing severe pain and symptoms such as bowel and bladder dysfunction, spinal stenosis, and neurological dysfunctions. Surgery can be performed in order to relieve the pressure felt on the sciatic nerve, thereby easing pain. There are two prevailing types of spinal surgery to treat Sciatica. The first surgical option, Microdiscectomy, removes part of or an entire herniated disk pushing on the sciatic nerve to relieve nerve compression. The second surgical option, a Laminectomy, removes both sides of the laminae, the back part of the vertebrae, in order to open up space for the nerves and relieve pain. For both surgical methods, the typical recovery time following the procedure ranges from six weeks to three months (Lagerbäck, 2018).

Opioids – What is the Issue?

Opioids are commonly prescribed postoperatively after spinal surgery to control patient pain (Hah et al., 2017). Of all surgical procedures, orthopedic and neurosurgery procedures tend to have the highest rates of opioid use, with sciatic nerve surgery falling at the cross-section of both subspecialties (Berardino, 2021). Approximately 51 million Americans undergo inpatient surgery annually, with opioids remaining the primary mode of pain management. Opioid prescriptions for medical purposes have quadrupled since 1999, paralleling the rise in the number of overdoses from the most common prescription opioids, which has tripled since 1999 and continues to grow. In 2008, opioid overdose-related deaths surpassed the number of deaths from automobile status. As such, the Centers for Disease Control have regarded the opioid epidemic as a public health emergency and Congress has even passed an eight-billion-dollar SUPPORT for Patients and Communities Act to address the national opioid epidemic (Hah et al., 2017). In 2017, it was shown that nearly 70% of drug overdoses were from physician-prescribed opioids, such as those given postoperatively following spinal surgery (Berardino, 2021).

The practice of using opioids to relieve pain following surgery presents a challenging issue that requires physicians and public health workers to contemplate two competing interests. The ability of opioids to help patients manage pain must be balanced with minimizing the risks of opioid misuse following surgery. The economic cost of prescription opioid-related overdose and abuse is over 78.5 billion dollars with the majority of the sum relating to the cost of healthcare, substance abuse treatment, and lost productivity. As such, finding ways to reduce the risk of opioid addiction and lessen the economic cost is of high importance in both the medical and legislative fields (Berardino, 2021).

The Opportunity for a Novel Solution

The increasing societal burden caused by opiate misuse motivates the development of localized, nonopioid, and nonaddictive pain management techniques. An avenue of prominent research has investigated various technologies that may mitigate postoperative pain without the use of opioids. However, existing solutions aside from addictive substances are significantly more expensive while also requiring higher physician and patient intervention (Berardino, 2021). As such, there is an open market for alternative pain-relieving technologies that are both cost-effective and user-friendly. Specifically, with regards to sciatic nerve surgery, miniature implantable devices with the ability to eliminate pain signals along the peripheral nerve would be able to provide pain relief while avoiding the side effects associated with opioids and other analgesics. The client, the MacEwan/Ray Lab at the Center for Neural Engineering at the Washington University School of Medicine, conducts research that develops electrode-tissue interfaces for neuro-rehabilitative purposes. Further, one area of study within the lab is the development of resorbable implants that alter peripheral nerve activity, with a specific focus on how nerve heating and cooling can block pain signals in peripheral nerves. The client’s prior experience creating miniature implantable devices that alter neuronal activity ensures that the client is appropriate for the creation of a non-addictive post-operative pain relief alternative. 

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Our Solution: The Heating Nerve Cuff

The chosen solution, a bio-resorbable, fully implantable, wirelessly powered nerve heating cuff has three main components: the bio-resorabable cuff base, the heating microcircuit piece, and the wireless powering coil. Below is an initial mock-up of the device.

The microcircuitry component is sourced from a collaborator of the MacEwan lab. The project will involve the optimization of the microcircuitry to fit within the overall nerve cuff design. Additionally, validation testing to ensure that the microcircuitry within the cuff heats the nerve to safe temperature levels and does not damage the sciatic nerve will be required. Further, outside of the microcircuitry, multiple parameters of the cuff itself need optimization and development. Namely, its block length, cuff structure, bioresorbability, and heating pulse sequences all need development to best fit the sciatic nerve and optimally block compound action potentials of the pain signals in the nerve. 

Block length, which is determined by the length of the microcircuitry component, is an important parameter to optimize nerve block and minimize potential nerve damage. The design, ideally, should optimize block length such that a lower temperature threshold is needed to initiate nerve block as this will prevent axon damage. A 2021 study on the optimal block length for laser nerve heating found that a block length of 1.13 mm was ideal to achieve sufficient nerve block and also minimize damage (Ford et al., 2021). Based on these results, the current design will feature microcircuitry of approximately this length, however, tests will be performed against a 2 mm and 3 mm block length to confirm this is ideal. 

The bioresorbable base of the cuff will be made of poly(1,8-octanediol-co-citric acid) (POC). This polymer material was chosen as prior study by the MacEwan lab has shown that POC can effectively bioresorb in 45 days within the body (Reeder et al., 2022). POC is a stiff material and will need to be shaped to create a cuff-like structure with the proper circumference of the patient’s sciatic nerve. In addition, another design requirement is that this device remains in a cuff-like structure as it slowly resorbs into the body. This will ensure that nerve heating is localized and sciatic nerve specific. 

A final design requirement involves programming the microcircuitry component of the heating nerve cuff to optimize the heating pulse sequence. Primarily, the heating by the nerve cuff should effectively halt compound action potentials corresponding to pain signals within the sciatic nerve without creating damage to the nerve or regular neuronal activity. Constant heating of the nerve may rapidly damage the nerve. Therefore, instead of constant heating, the nerve cuff will deliver pulses of heat to the nerve. Many iterations of pulse sequences will need to be tried and tested to find the safest and most effective pain relief heating sequence for the sciatic nerve utilizing the heating nerve cuff.