Twisted fiber microfluidics: a cutting-edge approach to 3D spiral devices

Microfluidics, the manipulation of small fluid volumes, has broad applications. Traditional methods like photolithography and soft lithography, often using PDMS, have limitations in material selection and complex 3D geometries. Inertial focusing in straight microchannels relies on inertial lift forces, while spiral geometries generate Dean drag forces, eliminating the need for external fields. However, fabricating 3D spiral microfluidics remains challenging. Existing techniques like 3D printing suffer from resolution limitations and surface roughness, while multi-step assembly processes are labor-intensive. This paper addresses these challenges by presenting a novel miniaturized rotational thermal drawing process (mini-rTDP) for creating twisted fiber-based microfluidics with 3D spiral channels. This approach offers advantages in terms of speed, cost-effectiveness, material versatility, and the ability to create complex geometries.

Early work on inertial focusing exploited inertial lift forces for particle manipulation. However, conventional 2D planar microchannels often exhibit multiple equilibrium points, complicating particle control. To improve focusing, additional forces (electrical or magnetic fields) were introduced, but this added complexity. Spiral microchannels, inducing Dean flow, offer an alternative, but their 2D planar structure leads to inconsistencies in Dean flow. 3D printing provides an accessible prototyping method but has resolution limitations. Multi-step assembly processes offer more control but are complex and time-consuming. Existing methods like femtosecond laser machining and abrasive jet machining are expensive and not suitable for rapid prototyping. Previous work used thermal drawing to fabricate fibers with straight microfluidics for inertial focusing, but applying this to 3D spiral structures remained unexplored.

The researchers developed a mini-rTDP system that combines thermal drawing with rotational capabilities to fabricate twisted fiber-based microfluidics. A polycarbonate (PC) preform with hollow channels (square, rectangular, or round) is created using macro-machining. The mini-rTDP system then draws and rotates the preform, creating the 3D spiral channels within the fiber. The spiral pitch is controlled by adjusting the drawing and rotation speeds. The process allows for the creation of fibers with various channel geometries and multiple channels. The impact of thermal reflow and rotational forces on channel profile is analyzed, considering factors like Laplace pressure, zero-shear viscosity, surface tension, and angular momentum. Numerical simulations explored the flow dynamics within the 3D spiral microchannels, analyzing primary and secondary flow velocities at different Reynolds numbers (Re) and Dean numbers (De). Microtomographic particle image velocimetry (µTPIV) was used to experimentally visualize the 3D velocity field and identify vortex regions using the Q criterion.

The mini-rTDP successfully fabricated fibers with diverse 3D spiral microfluidic structures, demonstrating versatility in channel geometry (rectangular, semicircular, circular) and the ability to create multi-channel fibers. The millimeter-scale channels in the preform were scaled down to 200-300 µm in the fiber. The study showed that the spiral pitch can be tuned, achieving pitches of 8.4 mm or greater. The PC fibers are highly transparent, providing good optical access. Simulations and µTPIV results confirmed the generation of Dean vortices at high flow rates, altering primary flow patterns and impacting particle positioning. Analysis of thermal reflow and rotational forces provided insights into channel profile control during the rTDP process. Equations (1) and (2) described the influence of thermal reflow and rotational forces respectively. The Reynolds number (Re) and Dean number (De) were used to characterize the flow regime and the influence of Dean vortices. µTPIV measurements, using the Q criterion to identify vortices, validated the simulation results.

The mini-rTDP method provides a simple, cost-effective, and rapid way to prototype complex 3D spiral microfluidic devices. The ability to control channel geometry and pitch offers significant advantages for optimizing particle manipulation and flow dynamics. The successful generation of Dean vortices, confirmed by both simulation and µTPIV, demonstrates the effectiveness of this approach for inertial focusing. The transparency of the PC fibers enables easy visualization of flow patterns. The ability to create multi-channel fibers allows for parallel processing, increasing throughput. The study's findings suggest that this technique could significantly advance research in various fields involving microfluidics.

This work successfully demonstrates a novel mini-rTDP method for fabricating complex 3D spiral microfluidic devices using twisted fibers. The method offers advantages in speed, cost, material flexibility, and geometric control compared to existing techniques. Future research could explore different materials, optimizing the process for specific applications, and investigating the use of this platform for advanced lab-on-a-chip devices.

While the study demonstrates the feasibility of the mini-rTDP method, further investigation is needed to fully optimize the process for different polymers and channel geometries. The current study focuses on a limited set of materials and flow conditions. More comprehensive studies exploring a wider range of parameters, including different polymer types and flow rates, would be beneficial.