Kilgore, K. | bioRxiv Science

bioRxiv2025

Exploring the Effect of LED-to-Photodetector Spacing on Subcutaneous Photoplethysmography for Continuous Blood Pressure Measurement

Photoplethysmography (PPG) is widely used to measure heart rate, blood oxygenation, and more recently, blood pressure. Implanted PPG systems offer the possibility to measure similar real-time measures of cardiovascular health, however, the detection method may vary due to a lack of capillary vessels for PPG sensors to observe in muscle tissue. To improve volumetric blood detection in large muscles, without relying on the capillary density of skin, a flexible PPG sensor was developed. The sensor included multiple spacing of illuminating infrared (IR) light emitting diodes (LEDs) and a single IR photodetector. This arrangement was expected to enable detection of blood volume changes at variable distances from the sensor face, potentially at much longer depths then feasible with skin-mounted PPG devices. IR bench phantoms simulating a large blood vessel embedded in IR absorbing, tissue-mimicking rubber were developed and used to determine the sensor performance in vitro. A preliminary in vivo test used an adult rabbit to provide additional performance validation. Test results reveal an observed trend of increased SNR for deeper vessel depths for the farthest LED to detector spacing which is aligned with our initial prediction. However, ANOVA and post-hoc tests reveal that these trends did not reach statistical significance. The in vivo test showed a relationship consistent with relevant literature. Future experiments are required to improve the phantoms representation of the biological setting and to confirm a reduced SNR variation for the farthest spacing. Clinical RelevanceDevelopment of a subcutaneous, continuous blood pressure sensor may provide benefits towards monitoring autonomic dysreflexia and hypertension for people affected by spinal cord injury.

bioengineering↗

bioRxiv2024

Distinct Motor Map Characteristics for Biceps and Triceps Muscles in Persons with Chronic Tetraplegia: Implications for Arm Function

Following spinal cord injury (SCI), intact neural resources undergo widespread reorganization within the brain. Animal models reveal motor cortical representations devoted to spared muscles above injury expand at the expense of territories occupied by weaker muscles. In this study, we investigated whether motor representations are similarly reorganized between a relatively spared biceps muscle and a weakened triceps muscle in persons with chronic tetraplegia following traumatic cervical SCI in association with upper limb motor function. Twenty-four adults with cervical SCI and 15 able-bodied participants underwent motor mapping using transcranial magnetic stimulation. We determined following map characteristics: area, amplitude (maximal motor evoked potential and volume), and center of gravity. Maximal voluntary contraction (MVC) and motor function (Capabilities of the Upper Extremity Test or CUE-T) were also assessed. Findings reveal that participants with SCI had hyper-excitable biceps maps than triceps, and hyper-excitable biceps maps also compared to biceps maps in able-bodied participants. Higher amplitude of biceps and triceps maps was associated with better motor function (higher CUE-T) and more distal injury (i.e., more spared segments) in persons with SCI. Amplitudes of biceps but not the triceps maps were associated with higher muscle MVCs. In conclusion, over-excitable biceps than triceps map in SCI may represent deafferentation plasticity. For the first time, we demonstrate how map reorganization of spared and weaker muscles in persons with chronic cervical SCI is associated with upper limb motor status. Use-dependent mechanisms may shift neural balance in favor of spared muscles, supporting potential use as response biomarkers in rehabilitation studies. New & NoteworthyOur study reports evidence in humans with cervical SCI that motor representation for the relatively spared muscle becomes hyper-excitable compared to that for the weaker muscle to the extent that hyper-excitability is even higher compared to biceps maps in uninjured individuals. Use-dependent mechanisms likely favor such heightened excitability of spared maps. For the first time, we demonstrate clinical relevance of map excitability in humans with SCI, supporting potential use as a biomarker of recovery.

neuroscience↗