Michael Theodor

Stretchable optoelectronic circuits embedded in a polymer network.

Stretchable optoelectronic circuits, incorporating chip-level LEDs and photodiodes in a silicone membrane, are demonstrated. Due to its highly miniaturized design and tissue-like mechanical properties, such an optical circuit can be conformally applied to the epidermis and be used for measurement of photoplethysmograms. This level of optical functionality in a stretchable substrate is potentially of great interest for personal health monitoring.

Subcutaneous blood pressure monitoring with an implantable optical sensor

We introduce a minimally invasive, implantable system that uses pulse transit time to determine blood pressure. In contrast to previous approaches, the pulse wave is detected by a photoplethysmographic (PPG) signal, acquired with high quality directly on subcutaneous muscle tissue. Electrocardiograms (ECG) were measured with flexible, implantable electrodes on the same tissue. PPG detection is realized by a flat 20 mm x 6 mm optoelectronic pulse oximeter working in reflection mode. The optical sensor as well as the ECG electrodes can be implanted using minimally invasive techniques, with only a small incision into the skin, making long-term monitoring of blood pressure in day-to-day life for high-risk patients possible. The in vivo measurements presented here show that the deviation to intra-arterial reference measurements of the systolic blood pressure in a physiologically relevant range is only 5.5 mmHg, demonstrated for more than 12 000 pulses. This makes the presented sensor a grade B blood pressure monitor.

Radiative transport in large arteries

A refined model for the photon energy distribution in a living artery is established by solving the radiative transfer equation in a cylindrical geometry, using the Monte Carlo method. Combining this model with the most recent experimental values for the optical properties of flowing blood and the biomechanics of a blood-filled artery subject to a pulsatile pressure, we find that the optical intensity transmitted through large arteries decreases linearly with increasing arterial distension. This finding provides a solid theoretical foundation for measuring photoplethysmograms.

Determination of vessel wall dynamics by optical microsensors

Spectralphotometric measurement methods as, for example, pulse oximetry are established approaches for extracorporeal determination of blood constituents. We measure the dynamics of the arterial distension intracorporeally thus extending the scope of the method substantially. A miniaturized opto-electronic sensor is attached directly to larger arteries without harming the vessel. The transmitted light through the arteries shows a linear correlation with the pulsatile expansion in theory as well as in experiments. Intra-arterial blood pressure also shows a linear interrelationship with the optical signal. Measurements of blood vessel wall dynamics has great potential to quantify arteriosclerosis by this new and innovative approach.

Lock-in amplification for implantable multiwavelength pulse oximeters

Standard as well as multiwavelength pulse oximetry as established methods for measuring blood oxygen saturation or fractions of dyshemoglobins suffer from different kinds of interference and noise. Employing lock-in technique as a read-out approach for multiwavelength pulse oximetry is proposed here and strongly decreases such signal disturbance. An analog lock-in amplifier was designed to modulate multiple LEDs simultaneously and to separate the signals detected by a single photodiode. In vivo measurements show an improved signal-to-noise ratio of photoplethysmographic signals and a suppression of interference by means of the lock-in approach. This allows the detection of higher order overtones and, therefore, more detailed data for pulse wave analysis, especially for implantable sensors directly applied at arteries.

Implantable pulse oximetry on subcutaneous tissue.

Blood oxygen saturation is one of the most prominent measurement parameters in daily clinical routine. However up to now, it is not possible to continuously monitor this parameter reliably in mobile patients. High-risk patients suffering from cardiovascular diseases could benefit from long-term monitoring of blood oxygen saturation. In this paper, we present a minimally invasive, implantable patient monitor which is capable of monitoring vital signs. The capability of this multimodal sensor to subcutaneously determine blood pressure, pulse and ECG has been demonstrated earlier. This paper focuses on monitoring of blood oxygen saturation. Even though the signal amplitudes are much weaker than for standard extracorporeal measurements, photoplethysmographic signals were recorded with high quality in vivo directly on subcutaneous muscle tissue. For the first time, it has been shown that blood oxygen saturation can be measured with an implantable, but extravascular sensor. The sensor was implanted for two weeks in a sheep and did not cause any complications. This opens new perspectives for home monitoring of patients with cardiovascular diseases.

Implantable Impedance Plethysmography

We demonstrate by theory, as well as by ex vivo and in vivo measurements that impedance plethysmography, applied extravascularly directly on large arteries, is a viable method for monitoring various cardiovascular parameters, such as blood pressure, with high accuracy. The sensor is designed as an implant to monitor cardiac events and arteriosclerotic progression over the long term.

Implantable acceleration plethysmography for blood pressure determination

This paper presents an implantable accelerometer which detects plethysmograms directly at an artery. The sensor provides a new method for continuous blood pressure monitoring. In vivo measurements indicate that the accelerometer is well suited for determining the Pulse Transit Time (PTT) and the Reflected Wave Transit Time (RWTT). Both parameters show a high correlation with the systolic blood pressure. By varying the blood pressure, it was seen that RWTT more closely agrees with theory than PTT. Through several blood pressure sweeps the RWTT, as detected by the accelerometer, coincided very well with the systolic blood pressure, with a correlation coefficient of 0.96 and mean deviation of 4.3% for 1800 pulses.

Photonic sensing of arterial distension

Most cardiovascular diseases, such as arteriosclerosis and hypertension, are directly linked to pathological changes in hemodynamics, i.e. the complex coupling of blood pressure, blood flow and arterial distension. To improve the current understanding of cardiovascular diseases and pave the way for novel cardiovascular diagnostics, innovative tools are required that measure pressure, flow, and distension waveforms with yet unattained spatiotemporal resolution. In this context, miniaturized implantable solutions for continuously measuring these parameters over the long-term are of particular interest. We present here an implantable photonic sensor system capable of sensing arterial wall movements of a few hundred microns in vivo with sub-micron resolution, a precision in the micrometer range and a temporal resolution of 10 kHz. The photonic measurement principle is based on transmission photoplethysmography with stretchable optoelectronic sensors applied directly to large systemic arteries. The presented photonic sensor system expands the toolbox of cardiovascular measurement techniques and makes these key vital parameters continuously accessible over the long-term. In the near term, this new approach offers a tool for clinical research, and as a perspective, a continuous long-term monitoring system that enables novel diagnostic methods in arteriosclerosis and hypertension research that follow the trend in quantifying cardiovascular diseases by measuring arterial stiffness and more generally analyzing pulse contours.