Abstract
Flow sensors found in animals often feature soft and slender structures (e.g. fish neuromasts, insect hairs, mammalian stereociliary bundles, etc) that bend in response to the slightest flow disturbances in their surroundings and heighten the animal’s vigilance with respect to prey and/or predators. However, fabrication of bioinspired flow sensors that mimic the material properties (e.g. low elastic modulus) and geometries (e.g. high-aspect ratio (HAR) structures) of their biological counterparts remains a challenge. In this work, we develop a facile and low-cost method of fabricating HAR cantilever flow sensors inspired by the mechanotransductory flow sensing principles found in nature. The proposed workflow entails high-resolution 3D printing to fabricate the master mould, replica moulding to create HAR polydimethylsiloxane (PDMS) cantilevers (thickness = 0.5–1 mm, width = 3 mm, aspect ratio = 20) with microfluidic channel (150 μm wide × 90 μm deep) imprints, and finally graphene nanoplatelet ink drop-casting into the microfluidic channels to create a piezoresistive strain gauge near the cantilever’s fixed end. The piezoresistive flow sensors were tested in controlled airflow (0–9 m s−1) inside a wind tunnel where they displayed high sensitivities of up to 5.8 kΩ m s−1, low hysteresis (11% of full-scale deflection), and good repeatability. The sensor output showed a second order dependence on airflow velocity and agreed well with analytical and finite element model predictions. Further, the sensor was also excited inside a water tank using an oscillating dipole where it was able to sense oscillatory flow velocities as low as 16–30 μm s−1 at an excitation frequency of 15 Hz. The methods presented in this work can enable facile and rapid prototyping of flexible HAR structures that can find applications as functional biomimetic flow sensors and/or physical models which can be used to explain biological phenomena.
Highlights
Flow sensors found in nature are known to be sensitive to very low fluid velocities as a result of evolutionary design optimization processes operating over millions of years
The PDMS/graphene structure was annealed at a temperature of 120 °C for an hour to improve the conductivity of the graphene nanoplatelets (GNP) strain gauge and allow the GNP to form a stable thin film in the microchannel, resulting in a base resistance of approximately 100 ± 10 kΩ
The airflow impinged upon the face opposite to that containing the strain gauge so that the compressive strain was induced in the strain gauge; this was a deliberate choice since the GNP strain gauge output was found to be more stable in the compressive state than in the tensile state in preliminary tests
Summary
Flow sensors found in nature are known to be sensitive to very low fluid velocities as a result of evolutionary design optimization processes operating over millions of years. Fan et al [7] and Chen et al [8] used a sensor design comprising a fish-inspired ‘cilium’ structure located at the distal end of a piezoresistive cantilever, where the drag force-induced bending moment on the cilium was transferred onto the cantilever to generate a measurable change in resistance In both these works, the MEMS cantilever sensing structure and the vertical cilium were fabricated using surface micromachining and sacrificial etching techniques, requiring the use of cumbersome processes such as plastic deformation magnetic assembly [7, 9] and layer-by-layer spin-coating and patterning of a photosensitive polymer (SU-8) [8] to realize the HAR sensor. Fabrication processing workflow: (a) cantilever and strain gauge design, (b) SLA 3D-printed mould, (c) PDMS casting inside the mould, (d) cured and peeled PDMS structure, (e) conductive GNP ink drop-casting, (f) photograph (design C2) of five replica-moulded sensors with copper electrodes (scale bar 3 mm), (g) photograph of GNP strain gauge (scale bar 1 mm), and (h) scanning electron (SE) micrograph of graphene nanoplatelets inside microchannels (scale bar 10 μm)
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