Microneedles offer a minimally invasive, pain-free approach to access dermal interstitial fluid (ISF). ISF concentrations of many physiological analytes of interest are comparable to well-understood blood levels, allowing clinically actionable data to be readily obtained through microneedle sensors. Optical sensing microneedles are relatively underexplored compared to electrochemical approaches but offer multiple potential advantages, including a smaller form factor when not in use, enhanced shelf stability, and ease of calibration. Incorporating luminescent analyte-sensing chromophores into or onto transparent microneedles allows for optical sensing of physiological analytes in ISF. Luminescent microneedles can then be paired with a wireless, wearable reader to allow for continuous, real-time monitoring. Two distinct optical microneedle sensor platforms were developed and are being assessed for target analytes. With the first method, microneedles are fabricated from rigid, non-degradable synthetic polymers. Then, ink jet printing is used to deposit a chromophore-containing hydrogel onto the microneedle tips in a highly controlled manner. In contrast, the second method involves microneedles made of silk, a degradable natural polymer. Analyte-sensing chromophores can first be loaded into silk nanoparticles, which are concentrated toward the microneedle tip.
Both methods were used to fabricate and validate dissolved oxygen sensing microneedles, where the presence of a Pd (II) porphyrin allows for monitoring changes in tissue oxygenation based on modulation in phosphorescence lifetime. Demonstrated sensors exhibit a dynamic range that aligns well with physiological concentrations. Such transdermal, tissue oxygen sensors could be used to give early indication of abnormal tissue oxygenation for individuals under hazardous or strenuous conditions and could also be useful for local monitoring during treatment of peripheral artery disease, various cancers, metabolic diseases, and pulmonary diseases. Moving beyond dissolved oxygen, current efforts are investigating the development of electrolyte sensors via incorporation of fluorescent molecules, particularly boron dipyrromethene derivatives, that are designed to exhibit an increase in intensity in the presence of sodium ions. By exchanging the binding moiety, the same strategy could apply to other electrolytes, such as potassium and calcium ions. Continuous monitoring of electrolyte levels could indicate hydration state and could aid in the treatment of cystic fibrosis or epilepsy. Additionally, low and high levels of electrolytes can be indicative of a number of diseases and conditions, such as cardiac arrhythmia and septic shock. Overall, this work demonstrates a unique strategy that could enable minimally invasive, health monitoring through optical methods.
