(28k) Microfluidic Oscillators Enable Dynamic Concentration Flow Profiles

Circadian rhythms provide cyclical cues for regulating and synchronizing physiological processes within a day. In vitro cultures broadly do not provide rhythmic stimulus for circadian synchronization similar to the physiological cues that cells and tissues normally receive in human bodies through biomolecules, such as hormones. Our current work focuses on developing the microfluidic platform to achieve a precise control of dynamic biomolecule concentration exposed to cells, replicating the rhythmic stimulus similar to the native cell microenvironment. To control dynamic concentration profiles in an automated and scalable way, we previously have microfluidic oscillators, a microfluidic platform that converts two constant flow inputs of different concentrations into an output that switches between two concentration states. However, two major barriers currently limit the ability of oscillators to apply this rhythmic stimulus of molecules of interest. The first barrier is undesired absorption of target biomolecules like hydrophobic hormones into polydimethylsiloxane (PDMS), the material traditionally used to fabricate oscillators. Therefore, we need to translate the oscillator design from PDMS to thermoplastic, a widely accepted material in biological research that has little hydrophobic molecule absorption. Fabricating the microfluidic oscillator with new materials presents the second barrier. The traditional PDMS oscillators rely on two normally closed (NC) valve (figure 1). Since the valve has a multi-layer structure, the surface of each PDMS layer is plasma treated to form a covalent bond. Critically, the valve region needs to be de-activated to avoid permanently closing. The fabrication with thermoplastic materials will form a permanently closed valve with the current NC valve design. A slightly-open-doormat (SOD) valve design circumvents the bonding issue by being normally open and therefore enables the oscillator fabrication with thermoplastics.

In order to ensure the functionality of SOD valve design in oscillator, we studied the difference between NC oscillators and SOD oscillators in PDMS with constant flow rate source provided by syringe pumps. We hypothesize that NC oscillators and SOD oscillators have similar oscillation behaviors but with slightly different properties like oscillation periods, inherent valve threshold pressure, and operational range (i.e. inlet flow rates). Firstly, we fabricated SOD oscillators and NC oscillators with soft lithography and visually verified the oscillation behaviors for both device designs. Secondly, we are collecting the inlet pressure profiles at different flow rates to analyze parameters like oscillation period, duty cycle, and valve threshold pressure. In addition, we built stand-alone single valve systems for both valve types. With the single valve systems, unlike the condition of oscillators where two valves are coupled, we will examine the generic valve threshold pressure. Lastly, since theoretical PLECS models based on electro-hydraulic analogy has been successfully built for NC oscillators, we will build PLECS models for SOD oscillators to compare the experimental and theoretical performance of SOD oscillators and also compare the performance of the different valve designs. Our theoretical model, once validated will provide a predictive tool for device performance that will provide guidance of oscillator design on thermoplastics. After the thermoplastic SOD oscillator is fabricated to provide controllable dynamic biomolecule concentration profiles, we will apply these dynamic profile to cells in vitro to mimic rhythmic physiological cues in the native cell environment. In the future, the development of this platform will have many applications like improving synchronization of cell activity or investigating of circadian rhythm related disease priming.