For individuals with transplanted kidneys, continuous monitoring of their organ's health is vital. There is a possibility of organ rejection which can occur immediately post-transplant or years later, often without evident symptoms. Monitoring kidney health primarily involves tracking specific markers in the blood. Physicians rely on measurements of creatinine and blood urea nitrogen levels to gauge kidney function. However, fluctuations in these biomarkers, unrelated to rejection, can lead to inconclusive results, causing both false negatives and positives. The current gold standard for detecting rejection involves invasive biopsies, where physicians extract tissue samples from the transplanted organ, analyzing them for signs of impending rejection. Unfortunately, such procedures carry risks, including bleeding, infection, pain, and inadvertent damage to nearby tissues. Alternative blood biomarkers, when used alongside creatinine and blood urea nitrogen monitoring, have less than optimal predictive values.

Researchers at Northwestern University (Evanston, IL, USA) have developed the first electronic device for continuous, real-time monitoring of the health of transplanted organs. The sensor itself is remarkably compact, measuring just 0.3 centimeters in width, 0.7 centimeters in length, and 220 microns in thickness, making it smaller than a pinky fingernail and approximately the width of a single hair. To attach it to the kidney, the research team took advantage of the organ's natural biology. The kidney is enveloped by a protective fibrous layer called the renal capsule, and the sensor was designed to fit snugly beneath this layer, in direct contact with the kidney. This ultra-thin, soft implant can detect temperature irregularities associated with inflammation and other physiological responses linked to transplant rejection.

The device incorporates a highly sensitive thermometer capable of detecting minuscule temperature variations (0.004 degrees Celsius) specifically on the kidney. Although the sensor also measures blood flow, temperature emerged as a more reliable indicator of rejection. Elevated temperatures often accompany inflammation, leading the researchers to hypothesize that detecting abnormal temperature increases and variations could serve as an early warning sign of potential transplant rejection. These sensors connect to a compact electronics package, including a miniature coin cell battery for power, positioned adjacent to the kidney. Using Bluetooth technology, they continuously and wirelessly transmit data to external devices.

In a study, the device was tested on a small animal model with transplanted kidneys, and it proved capable of detecting early signs of rejection up to three weeks earlier than current monitoring methods. The researchers observed that the local temperature of the transplanted kidney increased, sometimes by as much as 0.6 degrees Celsius, before rejection events occurred. In animals not receiving immunosuppressant medications, temperatures increased two to three days before biomarkers in blood samples changed. In animals on immunosuppressant medications, temperature increases, along with additional variations, were noted as much as three weeks before creatinine and blood urea nitrogen levels rose. This extended lead time could allow physicians to intervene sooner, potentially improving patient outcomes, preserving donated organs, and addressing the growing demand for organs amidst a shortage crisis.

This innovative device not only detects rejection signs earlier than existing methods but also offers continuous, real-time monitoring. Following the success of the small animal trial, the researchers are now testing the system in a larger animal model. Additionally, they are exploring ways to recharge the coin cell battery for potential lifelong use. While initial studies focused on kidney transplants, the researchers believe this technology could extend to other organ transplants, including the liver and lungs, as well as other disease models.

“If rejection is detected early, physicians can deliver anti-rejection therapies to improve the patient’s health and prevent them from losing the donated organ,” said Northwestern’s John A. Rogers, a bioelectronics pioneer who led the device development. “In worst-case scenarios, if rejection is ignored, it could be life threatening. The earlier you can catch rejection and engage therapies, the better. We developed this device with that in mind.”

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