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A microfluidic thermal mass flow sensor based on planar micro-machining technology and a phase-change material is designed, fabricated, and characterized. The sensor configuration uses a small patch of vanadium dioxide (VO 2 ) thin film as the sensing element closely placed in the down streaming direction of the heat source. By operating the VO 2 sensor in the phase transition region, no thermal insulation structure is required due to the ultra-high thermal sensitivity in this region. The characteristic 3-order resistance change, from 290 kΩ to 290 Ω, is observed during the full heating and cooling cycles by using both substrate heating and resistive heating methods. The equivalent maximum temperature coefficient of resistance (TCR) is calculated to be −0.37 K −1 in the cooling cycle and −0.43 K −1 in the cooling and heating cycle, respectively. The sensing operation principle is determined to follow the major cooling curve to avoid falling into minor loops and to secure high TCR. The resistance of VO 2 is monitored under flow rates ranging from 0 to 37.8 μL s −1 with the maximum sensitivity of 1.383%/(μL min −1 ). The studies presented in this research may enable the application of utilizing nonlinear metamaterial in microfluidic flow sensors with orders of magnitude improvement in sensitivity. 

Abstract Our knowledge of traumatic brain injury has been fast growing with the emergence of new markers pointing to various neurological changes that the brain undergoes during an impact or any other form of concussive event. In this work, we study the modality of deformations on a biofidelic brain system when subject to blunt impacts, highlighting the importance of the time-dependent behavior of the resulting waves propagating through the brain. This study is carried out using two different approaches involving optical (Particle Image Velocimetry) and mechanical (flexible sensors) in the biofidelic brain. Results show that the system has a natural mechanical frequency of$$\sim $$ $\sim$25 oscillations per second, which was confirmed by both methods, showing a positive correlation with one another. The consistency of these results with previously reported brain pathology validates the use of either technique, and establishes a new, simpler mechanism to study brain vibrations by using flexible piezoelectric patches. The visco-elastic nature of the biofidelic brain is validated by observing the the relationship between both methods at two different time intervals, by using the information of the strain and stress inside the brain from the Particle Image Velocimetry and flexible sensor, respectively. A non-linear stress-strain relationship was observed and justified to support the same.