Bioengineered in vitro models of the kidney offer unprecedented opportunities to better mimic the in vivo microenvironment. Kidney-on-a-chip technology reproduces 2D or 3D features which can replicate features of the tissue architecture, composition, and dynamic mechanical forces experienced by cells in vivo. Kidney cells are exposed to mechanical stimuli such as substrate stiffness, shear stress, compression, and stretch, which regulate multiple cellular functions. Incorporating mechanical stimuli in kidney-on-a-chip is critically important for recapitulating the physiological or pathological microenvironment. This review will explore approaches to applying mechanical stimuli to different cell types using kidney-on-a-chip models and how these systems are used to study kidney physiology, model disease, and screen for drug toxicity. We further discuss sensor integration into kidney-on-a-chip for monitoring cellular responses to mechanical or other pathological stimuli. We discuss the advantages, limitations, and challenges associated with incorporating mechanical stimuli in kidney-on-a-chip models for a variety of applications. Overall, this review aims to highlight the importance of mechanical stimuli and sensor integration in the design and implementation of kidney-on-a-chip devices.

This paper presents a performance investigation of a wearable distributed-deflection sensor in arterial pulse waveform measurement. Built on a flexible substrate, the sensor entails a polymer microstructure embedded with an electrolyte-enabled resistive transducer array. By pressing the sensor against an artery with hold-down pressure exerted by two fingers, the pulse signal from the artery deflects the microstructure and registers as a resistance change by the transducer at the artery site. The radial and carotid pulse signals of five subjects are recorded via the sensor. Related signal-processing algorithms are written in Matlab to remove motion artifacts from a recorded pulse signal, extract its key tonometric parameters, and calculate the Pulse Wave Velocity (PWV) and radial and carotid Augmentation Index (AI). Whereas the tonometric parameters of the measured radial and carotid pulse waveforms on each subject are consistent with the related findings in the literature, the difference in tonometric parameters among the subjects also reveals physiological significance. The measured PWV and radial and carotid AI of the subjects show good agreement with how they should vary with age, gender and hypertension. Finally, the effect of the hold-down pressure on a measured pulse signal and the repeatability of the sensor are examined.

Various flexible tactile sensors based on micro/nano-fabrication technology have been developed to amplify a measured pulse signal for accuracy. Yet, these sensors suffer from complicated configurations and fabrication complexity. This work is aimed to investigate the feasibility of amplifying a measured pulse signal by adding a uniform polydimethylsiloxane (PDMS) layer to a Pyrex-based microfluidic tactile sensor. The amplifying mechanism of the proposed approach is revealed by theories on sensor-artery interaction. The pulse signals at the radial artery (RA) deep under the skin and the superficial temporal artery (STA) near the skin of one subject are measured by the sensor first with no uniform layer and then with a set of uniform layers with different mixing ratios of PDMS and thickness. Arterial parameters: elasticity, viscosity and radius, are estimated from the measured pulse signals. As compared to those measured with no uniform layer, a uniform layer generates a pulse signal at transmural pressure ( $\text{P}_{\text {T}}$ ) near zero, greatly amplifies the measured pulse signal at both arteries, causes a moderate increase in estimated arterial elasticity, and has negligible effect on estimated arterial viscosity and radius. Due to their anatomical difference, pulse signal amplification is attributed to improved pulse transmission at tissue-sensor interface at the RA and alleviated suppression of the true pulse signal at the STA. The effect of overlying tissue and a uniform layer on estimated arterial parameters is further discussed. The proposed solution offers a low-cost solution to acquiring an amplified pulse signal at $\text{P}_{\text {T}}$ near zero for CV health assessment.

With remarkable progress being witnessed in recent years in the development of sensors, these advances in sensor technology provide unprecedented opportunities for (1) the early diagnosis and prevention of human diseases by detecting critical biomarkers; (2) health assessments by monitoring and analyzing human physiological signals in healthcare and biomedical applications; and (3) the efficient evaluation of human-health-relevant environmental factors by monitoring and measuring environmental determinants