Droplet-based interfacial capacitive sensing

Abstract

This paper presented a novel droplet-based pressure sensor using elastic and capacitive electrode-electrolyte interfaces to achieve ultrahigh mechanical-to-electrical sensitivity (1.58 μF kPa(-1)) and resolution (1.8 Pa) with a simple device architecture. The miniature transparent droplet sensors, fabricated by one-step laser micromachining, consisted of two flexible polymer membranes with conductive coating and a separation layer hosting a sensing chamber for an electrolyte droplet. The sensing principle primarily relied on high elasticity of the sensing droplet and large capacitance presented at the electrode-electrolyte interface. A simple surface modification scheme was introduced to the conductive coating, which reduced hysteresis of the droplet deformation without substantially compromising the interfacial capacitance. Moreover, the major concern of liquid evaporation was addressed by a mixture of glycerol and electrolyte with long-term stability in a laboratory environment. Theoretical analyses and experimental investigations on several design parameters (i.e., the dimensions of the sensing chamber and the droplet size) were thoroughly conducted to characterize and optimize the overall sensitivity of the device. Moreover, the environmental influences (e.g., temperature and humidity) on the capacitive measurement were further investigated. Finally, the simply constructed and mechanically flexible droplet sensor was successfully applied to detect minute blood pressure variations on the skin surface (with the maximum value less than 100 Pa) throughout cardiovascular cycles.

Monitoring of Vital Signs with Flexible and Wearable Medical Devices

“…The same principle applies for pressure sensors-the systolic peaks can be detected using pressure sensor placed on the radial artery [ 39 ] or the carotid artery. [ 40 ] Schwartz et al used a pressure sensitive polymer transistor and picked up the pulse signal from the radial artery ( Figure 6 a-c). [ 39 ] Using the same pressure sensing technique, Nie et al picked up the pulse signal from the carotid artery ( Figure 6 d,e).…”

“…However, in this case, the sensor was a dropletbased capacitive pressure sensor. [ 40 ] Using the three sensing techniques: electrical, optical, and pressure, the HR can be measured accurately. The sensing method should be chosen according to the sensing location.…”

“…[ 38 ] Plethysmography is another powerful method for measuring the HR. The pulse signal can be obtained from the plethysmogram by using optical [ 6 ] and pressure [ 39,40 ] sensors. With every heartbeat, the heart pumps oxygenated blood to the body and pulls back the deoxygenated blood; these actions distend the arteries and arterioles in the subcutaneous tissue.…”

Nanomaterial‐Enabled Wearable Sensors for Healthcare

“…For wearable pressure sensors that are used for healthcare applications, the sensors should be highly compliant to accommodate the skin deformations and highly sensitive to capture small pressure associated with biosignals such as blood pulse or respiration, where pressure down to tens of Pa can be involved. [6,138] Numerous materials and configurations have been reported for resistive pressure sensing. Pressure sensitive pathways can be formed using interlocked structures, [139][140][141][142][143][144] percolative networks of nanomaterials, [57,145,146] microfabricated structures (e.g., micropyramids, micropillars), [147][148][149] porous structures (e.g., sponges, foams, porous rubbers) [26,150,151] and so forth.…”

Entry effects of droplet in a micro confinement: Implications for deformation-based circulating tumor cell microfiltration

“…16 The pressure signature 33 and possible deformation evolution under the influence of both surface tension of the cancer cell and viscous shear stresses acting on the cell at different device operating conditions remain elusive. Transitional operating conditions between surface tension-dominant and viscous shear stress-dominant flow behaviors are also characterized.…”