Reusable Capillary Flow-Based Wax Switch Valve for Centrifugal Microfluidics
Authors: Peshin Snehan , George Derosh , Shiri Roya , Madou Marc 
Published: July 2021

Introduction:

This paper presents a new repeatable Open-Close switch valve (SV) for the CD microfluidic platforms. The SV is designed by using a combination of controlled phase change and capillary flow of Ferrowax. The phase change of the wax is controlled using a laser. When the wax is illuminated with a laser, the energy of the laser is absorbed by the Ferro particles embedded inside the wax, causing the wax to melt. The melted wax can spontaneously wick through capillary channels due to its low contact angle. The Open-Close nature of the valve is attained by rationally designing the capillary flow path of the wax. The capillary flow path design is such a way that the melted wax is directed to the open fluidic path (Normally open configuration) to block the channel (Close configuration). The reversal of the blocking is achieved by remelting the wax and driving it to the overflow chamber downstream. A unique wax flow path is also designed to enhance the capillary flow and the response time of the valve.

Background:

Centrifugal platform-based CD fluidics has created a new paradigm of inexpensive point-of-care diagnostics[1]. It is now widely used in applications like polymerase chain reaction assays, blood plasma separation, etc.[2,3] Since CD fluidics has presented an innovation in point-of-care diagnostics, it is essential to have a proper valving system for reliable opening/closing of the channels. Currently, there are two main types of valves in the CD platform: laser valves and z-axis wax valves. Both of these valves are good for one-time use only and have complex fabrication routes. Therefore, there is a need for simple, reusable SV in CD fluidic platforms.

Theory:

The capillary pressure drives the capillary flow in a channel. The capillary pressure difference between atmosphere and the liquid meniscus inside a rectangular microchannel (Pc) is given by Young-Laplace’s equation:
-Pc = 2γ(cosθ)(1/w+1/h) (1)
where γ is the surface tension, θ is the contact angle with the channel material, w is the width of the channel, and h is the height of the channel. In this design, the resistance offered by the trailing liquid is given by the equation:
Rhyd = 12μL/(h3w*(1-0.63(h/w)) (2)
For capillary flow, the flow rate (Q) can be obtained by
Q = wh(dL/dt) = -Pc/Rhyd (3)
For a shallow channel (h<<w), we can neglect the terms containing 1/w, giving the following simplified equation for the flow velocity
dL/dt = γ(cosθ)h/(6μL) (4)
Upon integration, the fluid length at an instant t is obtained
L = √(γht(cosθ)/3μ) = W√t (5)
where W is the Washburn’s constant. We use a new channel design and a flexible adhesive film, and the modified channel shows significant improvement in the capillary flow velocity. A modified empirical model capturing this effect is currently being developed.

Materials and Method:

The channels and the chambers are milled on an acrylic sheet of thickness 3 mm using Tormac PCNC CNC milling machine (Fig 1a). The Ferro-wax is prepared by mixing Sigma Aldrich Paraffin wax (MP 53 - 37oC) with Ferrofluid (Ferrotec EFHI 60 cc) at the ratio of 2:1 and stirring the mixture at 65oC for 12 hours. The mixture is brought down to room temperature and placed inside the designated chamber. Then, the CD is laminated using a single side pressure-sensitive adhesive (Fig 1a). To achieve Close configuration (Fig 1b), the wax inside the CD is heated up for 5 minutes at 65oC. The melted wax flows to the reagent flow path and solidifies to block the channel. Remelting the wax and directing the melted wax to the overflow chamber brings the valve back to the Open configuration (Fig 1b). The unique channel edges for enhanced capillary flow are fabricated by applying two layers of tape (Fig 1a).

Experimental Results:-

Experimental characterization of SV was performed by fabricating channels having cross-section areas ranging from 0.05 to 0.625 mm2. A 2.32 W laser placed at a distance of 3 mm from the top surface of the CD is used for heating the Ferrowax. The Normally Close configuration (Fig 1b) is achieved by heating the wax inlet chamber for 5 minutes at 65oC. The wax is driven to the reagent channel by moving along the edge gaps of the wax chamber. The solidified wax prevents the fluid from moving downstream. The sequential release of the reagent is achieved using SV as shown (Fig 1c, 5). A ‘teeth’ design is used at the bottom of the wax inlet to avoid bubble formation inside the chamber. This teeth design also ensures a large number of Open-Close operations for a given amount of wax. Finally, we demonstrate the fidelity of the SV closure (Close configuration) on the CD platform on upwards of 7000 RPM. A parametric study of the air gap is being carried out to determine an optimized design for a faster response.

References

[1] Gorkin, Robert, Jiwoon Park, Jonathan Siegrist, Mary Amasia, Beom Seok Lee, Jong-Myeon Park, Jintae Kim, Hanshin Kim, Marc Madou, and Yoon-Kyoung Cho. "Centrifugal microfluidics for biomedical applications." Lab on a Chip 10, no. 14 (2010): 1758-1773. doi:10.1039/B924109D
[2] Amasia, Mary, and Marc Madou. "Large-volume centrifugal microfluidic device for blood plasma separation." Bioanalysis 2, no. 10 (2010): 1701-1710. doi:10.4155/bio.10.140
[3] Noroozi, Zahra, Horacio Kido, Régis Peytavi, Rie Nakajima-Sasaki, Algimantas Jasinskas, Miodrag Micic, Philip L. Felgner, and Marc J. Madou. "A multiplexed immunoassay system based upon reciprocating centrifugal microfluidics." Review of Scientific Instruments 82, no. 6 (2011): 064303. doi: 10.1063/1.3597578

Figure 1