Laser Engraving Technique for Fabricating Micropneumatic Circuits to Facilitate the Development of Autonomous Microfluidics
High-throughput manufacturing techniques for microfluidic devices facilitate the implementation of denser and complex control logic on-chip, thereby reducing the requirement for external control equipment. Laser engraving is one such technology that has been used to prototype microfluidic devices and is emerging as an efficient alternative to time-consuming traditional approaches such as soft-lithography. Processing thermoplastics such as polymethylmethacrylate (PMMA) used commonly in microfluidic devices is also less challenging with laser engraving.
One of the elemental blocks in control logic is a clock generator or timer circuit. Using laser engraving, the PMMA flow and control layers of a ring oscillator are fabricated and surface activation of PMMA by treatment with 3-aminopropyltriethoxysilane(APTES) enables assembly of the device with a PDMS membrane layer. The performance of the oscillator is analyzed and its application to drive a microfluidic line is demonstrated.
Microfluidic devices contain large arrays of valves and pumps performing multiple analyses for diagnostic assays and biological applications. The increasing number of external connections for control of these components has motivated the development of techniques to facilitate implemention of control circuitry on-chip. Simplification of the fabrication process would contribute to reducing the prototyping and subsequent mass-manufacturing time. Laser-engraving has emerged specifically as a very promising technology due to factors such as fastest turnaround time, comparatively low operational costs and ease of design processing.
A wide range of polymers can be processed by a laser cutter, including thermoplastics such as polymethylmethacrylate (PMMA) that are extensively used for microfluidics. Valves and pumps have been demonstrated by sandwiching polydimethylsiloxane (PDMS) between channel structures created on PMMA . One of the important requirements for on-chip digital logic is a timing reference or oscillator. Pneumatic oscillators fabricated with glass and PDMS and PMMA/PDMS circuits manufactured by use of CNC milling have been shown by Duncan et al.
A ring oscillator chip with features manufactured on the PMMA flow and control layers using laser engraving and PDMS as the membrane layer was driven with a syringe to demonstrate portability. The device was assembled by permanently bonding the three layers using surface activation of PMMA by treatment with 3-aminopropyltriethoxysilane(APTES). Oscillation frequency is dependent on two main design parameters: (1) inverter stage resistance Rs—the sum of valve, pull-up, and parasitic interconnect resistances, and (2) valve chamber volume Vc (which directly impacts the pneumatic capacitance of the valve). Theoretically, the time period is proportional to Rs x Vc/Ps where Ps is the pressure supply. Rs is calculated using the formula:
where h is the height of the channel, w is its width, L is the length and µ is the viscosity of the medium. f is the geometric form factor related to the rectangular shape. When h << w, f~1 does not influence the value of R.
The frequency of the fabricated oscillators was in the range of 3–7.5 Hz with a maximum of 14 min constant frequency syringe-powered operation. The control of a fluidic channel with valves driven by the individual oscillator stages was shown:
The fabrication process, based on laser engraving and APTES treatment, has proven to be reliable for the relatively fast, efficient prototyping of these circuits during the design and development phase. It took approximately 10–20 min to complete the laser engraving of PMMA and then four hours for the assembly. Higher-resolution laser engravers should allow for fabricating higher-density, smaller valve devices driven at higher frequencies.
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