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Rapid Fabrication of Low-Cost Thermal Bubble-Driven Micro-Pumps


Thermal bubble-driven micro-pumps are an upcoming actuation technology that can be directly integrated into micro/mesofluidic channels to displace fluid without any moving parts. These pumps consist of high power micro-resistors, which we term thermal micro-pump (TMP) resistors, that locally boil fluid at the resistor surface in microseconds creating a vapor bubble to perform mechanical work. Conventional fabrication approaches of thermal bubble-driven micro-pumps and associated microfluidics have utilized semiconductor micro-fabrication techniques requiring expensive tooling with long turn around times on the order of weeks to months. In this study, we present a low-cost approach to rapidly fabricate and test thermal bubble-driven micro-pumps with associated microfluidics utilizing commercial substrates (indium tin oxide, ITO, and fluorine doped tin oxide, FTO, coated glass) and tooling (laser cutter). The presented fabrication approach greatly reduces the turn around time from weeks/months for conventional micro-fabrication to a matter of hours/days allowing acceleration of thermal bubble-driven micro-pump research and development (R&D) learning cycles.

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Main Learnings

  • Fabrication time of TIJ resistors was reduced from weeks/months to hours/days

  • Low-cost (<$6,000), custom, open-source control and imaging system was developed to image at up to 10 Mfps

  • Laser ablation of thin films is a low-cost, rapid way to make thermal bubble-driven micro-pumps

  • In a 515x315 μm2 channel of length 13.268 mm, a 300x700 μm2 resistor can generated a saturated flow rate of 3.34 nL/pulse

Resistor Fabrication through Laser Cutting

This work demonstrates that high power micro-resistors used for explosive metastable boiling can be fabricated through laser ablation processes. Specifically, a fiber laser and UV femtosecond laser cutter was used in this study to fabricate working thermal bubble-driven micro-pumps. In this study, ITO/FTO coated glass was used as the substrate upon which a vector cut operation defined the resistive region. Unlike conventional micro-fabrication approaches, this technique is single-step which vastly reduces the fabrication time for such devices.

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Microfluidic Fabrication through Laser Cutting

Once the high power micro-resistors are fabricated, the associated microfluidics must be fabricated to confine the resistors in a micro-channel. This work utilizes both laminate processing and controllable micro-milling to rapidly fabricate 2D and 2.5D microfluidic channels. The micro-channels were then manually aligned and placed on the FTO/ITO glass substrate to create a fully functional thermal bubble-driven micro-pump device. 

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Experimental Setup: Driving Electronics and Imaging

Thermal bubble-driven micro-pumps locally vaporize a thin interfacial layer of liquid above the resistor's surface during a microsecond heating pulse to create a high pressure vapor bubble that performs mechanical work. For a 300x700 um2 resistor, expansion and collapse takes approximately 70 us. As such, high speed imaging is needed to study these devices; in this work, we develop a custom experimental setup to both (a) deliver high power, microseconds heating pulses and(b) image the resulting bubble dynamics. Stroboscopic imaging was used as a cost effective solution enabling imaging at up to 10 Mfps. A custom GUI controller was used to synch electrical signals to enable stroboscopic imaging.

Experimental Setup Image.png

Flow Rate Analysis

Thermal bubble-driven micro-pumps can be characterized using standard particle tracking approaches. In this work, we characterized the flow rate from an example thermal bubble-driven micro-pump device in a U-shaped channel by seeding the flow with 27-32 um particles and imaging the particle displacement per pulse of the resistor to reconstruct the flow profile as shown below. Once enough particle statistics are acquired, the flow profile across the channel cross-section can be computed and matched to theory in order to extract the per pulse flow rate as shown in the last figure.

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