Wireless microfluidic control with integrated shape-memory-alloy actuators operated by field frequency modulation

Abstract

This paper reports wireless microfluidic control enabled by the selective operation of multiple bulk-micromachined shape-memory-alloy actuators using external radiofrequency magnetic fields. Each shape-memory-alloy actuator is driven by a wireless resonant heater which generates heat only when the field frequency is tuned to the resonant frequency of the heater. Multiple actuators coupled with the heater circuits that are designed to have different resonant frequencies in the range of 135–295 MHz are selectively and simultaneously controlled by modulating the field frequency to the resonant frequencies of the corresponding heaters. A wireless microsyringe device that has three actuator–heater components and a flexible parylene reservoir is developed. The 5 µl reservoir is squeezed by the 5 mm long cantilever-type actuators to eject controlled amount of liquid from the reservoir. Using the device with an acidic solution loaded in the reservoir, sequential modifications of the pH level in the liquid are experimentally demonstrated through the selective control of the three actuators. The thermal characterization of the actuator using infrared imaging shows a temperature increase of 50 °C in 4 s and the full activation of the actuator in 8 s with 300 mW field output power.

High‐power MEMS switch enabled by carbon‐nanotube contact and shape‐memory‐alloy actuator

“…Another possible cause of the mismatch could be related to the geometry of the individual CNTs that have somewhat wavy shapes; their tips can adhere to the cantilever surface due to the van der Waals force 51, potentially elongating the nanotube structures and maintaining the contacts during an early stage of the upward displacement of the cantilever, and then eventually detached from the surface. This type of the SMA cantilever used can exert very high actuation forces (∼840 mN 44) that are larger than those available with typical MEMS switches based on electrostatic designs by three to four orders of magnitude 52. This experiment verifies that the high SMA force contributes to achieving a high ON/OFF resistance ratio and that the restoring force generated by the SiO 2 reset layer is sufficient to overcome the surface force induced at the contact interface, releasing the cantilever from the forest to fully open the switch.…”

“…Figure 1 shows the layout of the developed switch device and the SMA actuator component used in the device. The SMA has a cantilever structure as shown, being fabricated to bend upward in its martensitic (cold) state at room temperature and vertically actuated downward when it is heated and enters its austenite (hot) state 44. Thus, the signal terminal 1 (CNT forest) and terminal 2 (bonding pad for SMA) indicated in Fig.…”

“…The SMA cantilever (Fig. 1) was shaped with chemical etching of the Nitinol sheet for thinning and then shaped with micro‐electro‐discharge machining (µEDM) using a commercial µEDM system (EM203, SmalTec International Inc., IL, USA) 44. (This µEDM technique may also be used to planarize the top surface of the CNT forest and adjust its height 49, 50 to address the thickness variation of the forest discussed earlier.)…”

“…The former two features are advantageous in making stable, low‐resistance contacts in the ON state and high isolation in the OFF state, respectively 38. Shape‐memory‐alloy (SMA) actuators have been widely investigated in MEMS 42–44. As one type of thermal (often electrothermal) actuators, they offer similar characteristics together with high fatigue resistance to cyclic operations, which is also advantageous for switching applications 45, 46.…”

Wireless Displacement Sensing of Micromachined Spiral-Coil Actuator Using Resonant Frequency Tracking

“…When the device is exposed to the external field, an electromotive force is induced in the LC circuit. The electromotive force is most effectively converted to Joule heat when the field frequency, f m , matches f r ; thus, by tuning f m with respect to f r , the actuation of the SMA coil can be controlled [ 28 , 29 ]. The inductance of the spiral SMA coil, L , is dependent on the out-of-plane displacement of the coil due to the change in coil's mutual inductance caused by the variation in the gap between the coil turns [ 30 ], while the capacitance value remains constant (10 pF in this design).…”