Additive manufacturing of three-dimensional (3D) microfluidic-based microelectromechanical systems (MEMS) for acoustofluidic applications

Abstract

Three-dimensional (3D) printing now enables the fabrication of 3D structural electronics and microfluidics. Further, conventional subtractive manufacturing processes for microelectromechanical systems (MEMS) relatively limit device structure to two dimensions and require post-processing steps for interface with microfluidics. Thus, the objective of this work is to create an additive manufacturing approach for fabrication of 3D microfluidic-based MEMS devices that enables 3D configurations of electromechanical systems and simultaneous integration of microfluidics. Here, we demonstrate the ability to fabricate microfluidic-based acoustofluidic devices that contain orthogonal out-of-plane piezoelectric sensors and actuators using additive manufacturing. The devices were fabricated using a microextrusion 3D printing system that contained integrated pick-and-place functionality. Additively assembled materials and components included 3D printed epoxy, polydimethylsiloxane (PDMS), silver nanoparticles, and eutectic gallium-indium as well as robotically embedded piezoelectric chips (lead zirconate titanate (PZT)). Electrical impedance spectroscopy and finite element modeling studies showed the embedded PZT chips exhibited multiple resonant modes of varying mode shape over the 0-20 MHz frequency range. Flow visualization studies using neutrally buoyant particles (diameter = 0.8-70 μm) confirmed the 3D printed devices generated bulk acoustic waves (BAWs) capable of size-selective manipulation, trapping, and separation of suspended particles in droplets and microchannels. Flow visualization studies in a continuous flow format showed suspended particles could be moved toward or away from the walls of microfluidic channels based on selective actuation of in-plane or out-of-plane PZT chips. This work suggests additive manufacturing potentially provides new opportunities for the design and fabrication of acoustofluidic and microfluidic devices.

Fig 1.

Additive manufacturing concept for fabrication of 3D microfluidic MEMS devices. A combination of 3D printing and robotic embedding facilitates the integration of orthogonal in-plane and out-of-plane piezoelectric transducers, functional 3D printable materials, and microfluidic channels. In-plane and out-of-plane piezoelectric transducers facilitate the trapping of continuously flowing particles in microfluidic channels in transverse and lateral directions.

Fig. 2

a) Highlight of the seven fabrication steps (I-VII) for the 3D printed acoustofluidic device including 3D printing and embedding processes. Each step shows in vertically descending order: the assembly schematic, a photograph of the device during the fabrication step, and the height profile of the device after completion of the step. b) Cross-sectional schematic (top) and photograph (bottom) of the device.

Fig. 3

a) Experimentally measured and simulated electrical impedance response of the 3D printed acoustofluidic devices over 0 – 20 MHz and highlight of experimentally measured impedance (Z) and phase angle (φ) characteristics from four resonant modes that exhibit strong impedance-coupling. b) Photograph of the embedded PZT chip highlighting the acoustic source. c) Calculated 3D mode shape and displacement profile (dashed line) for each of the resonant modes shown in terms of the transverse displacement. d) Secondary transducer configuration for sensing of acoustic waves generated by the embedded PZT chip showing the corresponding voltage signal generated in the secondary transducer (V_s,p-p) at each mode. e) Comparison of the voltage generated in the secondary acoustic transducer with the maximum total displacement (D_max) calculated using finite element simulations for each resonant frequency (f_n). Also shown is the fast Fourier transform of the measured voltage signal shown in (d).

Fig. 4

a) Schematic of droplet-based flow visualization studies using the in-plane piezoelectric transducer. Acoustic waves generated by the robotically embedded in-plane transducer propagate into the droplet producing pressure oscillations (P) and streaming flow (v) that exert forces on suspended particles (diameter = D). b) Micrographs of the suspended particle systems under excitation at each mode of the 3D printed acoustofluidic device show distinct regimes of trapping and streaming behavior for particles ranging from 0.8 – 70 μm in size. c) Fluorescence micrographs and particle distribution plots of multi-particle systems under excitation at each mode of the 3D printed acoustofluidic device show size-selective separation of particles and mode-dependent separation profiles.

Fig. 5

a) Schematic of a 3D microfluidic MEMS device containing two orthogonal piezoelectric chips (one in-plane and one out-of-plane). b) Concept of orienting piezoelectric transducers with both the in-plane and out-of-plane components of a microchannel using additive manufacturing to facilitate manipulation of continuously flowing particles (e.g., whole cells). c) Vacuum-based robotic embedding principle associated with integration of the in-plane piezoelectric transducer. d–e) Adhesion-based robotic embedding principle associated with integration of the out-of-plane piezoelectric transducer. f) Photographs of the device before and after printing of Ag interconnects to the out-of-plane piezoelectric transducer. g) Electrical impedance spectra of the in-plane and out-of-plane piezoelectric transducers over the 0 – 20 MHz frequency range.

Fig. 6

a) Schematic of the fabrication steps associated with integration of a 3D printed microchannel in between the in-plane and out-of-plane piezoelectric transducers with corresponding photographs. b) Schematic showing the microchannel orientation with respect to the integrated piezoelectric transducers and corresponding photograph after dissolution and washout of the printed eutectic Gallium-Indium. c) Schematic showing the principle of exciting both the in-plane and out-of-plane transducer during continuous flow of suspended particles through the microchannel. Flow visualization studies showing continuously flowing 6 μm particles in the presence of stimulation from the in-plane transducer (d), presence of stimulation from the out-of-plane transducer (e), presence of simultaneous stimulation from both transducers (f), and absence of acoustic stimulation (g) (white arrow indicates the direction of flow).