A Flexible Hair-Like Laser Induced Graphitic Sensor for Low Flow Rate Sensing Applications

Direct low flow sensing is of interest to many applications in medical and biochemical industries. Low flow rate measurement is still challenging, and conventional flow sensors such as thermal or Pelton wheel flow sensors do not give accurate flow measurement to very low flow rates. In some applications that require flow measurement in a small diameter tubing (e.g. intravenous (IV) infusion), using such sensors also become mechanically impractical. MEMS flow sensors, on the other hand, are capable of low flow rate sensing. However, their sophisticated fabrication methods result in a high cost and the lack of repeatability and sensitivity, which leads to reduced adoption in practical applications. Other low flow sensing technologies such as ultrasonic and optical sensors not only need extensive measurement setups but also are not in direct contact with the liquid itself and are not so reliant on the dynamics of the fluid to make the measurement.

Herein, a flexible laser-induced graphitic (LIG) piezoresistive sensor has been fabricated in a cost-effective single processing step. A set of coupled electrical and mechanical tests have been conducted to investigate the piezoresistivity of the LIG sensor. Using a thin layer coating of polydimethylsiloxane (PDMS) capacitates LIG flow sensors to be functional in liquid as well as gaseous media. By the use of nature as an inspiration to achieve the maximum deflection and sensitivity, the LIG piezoresistive sensor has packaged like cilia (perpendicular to the flow direction) using a 3D printed housing. The capability of the LIG sensor in very low flow rate measurement has been investigated by embedding the sensor within the intravenous (IV) line. A load cell has been used to calibrate the LIG flow sensor output and measure the mass flow rate. The drip bag hung from a load cell, and the tension pulled by the drip bag was measured and transformed by the load cell a to an electrical signal recorded by a national instrument NI-6234 USB DAQ. The embedded LIG hair-like sensor was tested at ambient temperature within the IV line at flow rates ranging from 0 m/s to 0.3 m/s (IV infusion free-flow rate). The LIG hair-like sensor presented in this work detects live flow rates of IV infusions with a threshold detection limit as low as 0.02 m/s (38.6 ml/min). Moreover, the deformation of the LIG hair-like sensor in response to different flow velocities that lead to resistance change in the sensor is simulated using COMSOL Multiphysics.