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The use of a noninvasive hemodynamic monitor to estimate arterial stiffness, by measurement of pulse transit time (PTT), was demonstrated in microgravity. The monitor's utility for space applications was shown by establishing the correlation between ground-based and microgravity-based measurements. The system consists of a scale-based ballistocardiogram (BCG) and a toe-mounted photoplethysmogram (PPG). PTT was measured from the BCG I-wave to the intersecting tangents of the first trough and maximum first derivative of the PPG waveforms of each subject. The system was tested on a recent series of parabolic flights in which the PTT of nine subjects was measured on the ground and in microgravity. An average of 60.2 ms PTT increase from ground to microgravity environments was shown, and was consistent across all test subjects (standard deviation = 32.9 ms). This increase in PTT could be explained by a number of factors associated with microgravity and reported in previous research, including elimination of hydrostatic pressure, reduction of intrathoracic pressure, and reduction of mean arterial pressure induced by vasodilation.

A novel modality for the electrical stimulation of cardiac cells is described. The technique is based on HF stimulation-burst of HF (1-25 kHz) biphasic square waves-to depolarize the cells and trigger action potentials (APs). HF stimulation was demonstrated in HL-1 cardiomyocyte cultures using microelectrode arrays, and the underlying mechanisms were investigated using single-cell model simulations. Current thresholds for HF stimulation increased at higher frequencies or shorter burst durations, and were typically higher than thresholds for single biphasic pulses. Nonetheless, owing to the decreasing impedance of metal electrodes with increasing frequencies, HF bursts resulted in reduced electrode voltages (up to four fold). Such lowered potentials might be beneficial in reducing the probability of irreversible electrochemical reactions and tissue damage, especially for long-term stimulation. More significantly, stimulation at frequencies higher than the upper limit of the AP power spectrum allows effective artifact reduction by low-pass filtering. Shaping of the burst envelope provides further reduction of the remaining artifact. This ability to decouple extracellular stimulation and recording in the frequency domain allowed detection of APs during stimulation-something previously not achievable to the best of our knowledge.

Analyzing systolic time intervals-specifically the preejection-period (PEP)-is widely accepted as one of the few methods for the noninvasive assessment of cardiac contractility. In this paper, we investigated the ballistocardiogram (BCG) as a way to noninvasively measure myocardial contractility when combined with the ECG. Specifically, we derived a parameter from the BCG and ECG that we hypothesized would be highly correlated to PEP. This is the time delay between the J-wave peak of the BCG and the R-wave of the ECG, which we refer to as the RJ interval. The RJ interval was correlated to PEP (r(2) = 0.86) for 2126 heartbeats across ten subjects, with a y-intercept of 138 ms and slope of 1.05. This suggests that the RJ interval can be reliably used as a noninvasive assessment of cardiac contractility.

Ballistocardiography (BCG) is a non-invasive technique used to measure the ejection force of blood into the aorta which can be used to estimate cardiac output and contractility change. In this work, a noise sensor was embedded in a BCG measurement system to detect excessive motion from standing subjects. For nine healthy subjects, the cross-correlation of the motion signal to the BCG noise—estimated using a simultaneously acquired electrocardiogram and statistics of the BCG signal—was found to be 0.94 and 0.87, during periods of standing still and with induced motion artifacts, respectively. In a separate study, where 35 recordings were taken from seven subjects, a threshold-based algorithm was used to flag motion-corrupted segments of the BCG signal using only the auxiliary motion sensor. Removing these flagged segments enhanced the BCG signal-to-noise ratio (SNR) by an average of 14 dB (P < 0.001). This integrated motion-sensing technique addresses a gap in methods available to identify and remove noise in standing BCG recordings due to movement, in a practical manner that does not require user intervention or obtrusive sensing.

We describe a walking microrobot that is propelled by cilialike thermal bimorph actuator arrays. The robot consists of two actuator array chips, each having an 8 × 8 array of “motion pixels,” which are composed of four orthogonally oriented cilia. Each group of unidirectional cilia is controlled independently for each chip, which provides planar motion with three degrees of freedom (x, y, θ). The robot is approximately 3 cm in length, 1 cm in width, and 0.9 mm in height and has a mass of 0.5 g. By varying the actuation frequency and motion gait strategy, the direction and velocity of the motion can be controlled. In this paper, we present the system architecture, control mechanism, and modeling of the robot, as well as experimental results, during linear and rotary motion. The robot can carry loads up to seven times its own mass, and it can operate at speeds up to 250 μm/s with step sizes from 1 to 4 μm.

An adaptive noise canceller was used to reduce the effect of floor vibrations on ballistocardiogram (BCG) measurements from a modified electronic bathroom scale. A seismic sensor was placed next to the scale on the floor and used as the noise reference input to the noise canceller. BCG recordings were acquired from a healthy subject while another person stomped around the scale, thus causing increased floor vibrations. The noise canceller substantially eliminated the artifacts in the BCG signal due to these vibrations without distorting the morphology of the measured BCG. Additionally, recordings were obtained from another subject standing inside a parked bus while the engine was running. The artifacts due to the vibrations of the engine, and the other vehicles moving on the road next to the bus, were also effectively eliminated by the noise canceller. The system with automatic floor vibration cancellation could be used to increase BCG measurement robustness in home monitoring applications. Additionally, the noise cancellation approach may enable BCG recording in ambulances-or other transport vehicles-where noninvasive hemodynamic monitoring may otherwise not be feasible.