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Research Papers

Centrifugal Casting of Microfluidic Components With PDMS

[+] Author and Article Information
David E. Hardt

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139

This manuscript contains data from author Mazzeo's Ph.D. thesis [3].

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received February 13, 2012; final manuscript received January 9, 2013; published online April 3, 2013. Editor: Jian Cao.

J. Micro Nano-Manuf 1(2), 021001 (Apr 03, 2013) (8 pages) Paper No: JMNM-12-1016; doi: 10.1115/1.4023754 History: Received February 13, 2012; Revised January 09, 2013

This work describes the centrifugal casting and fast curing of double-sided, polydimethylsiloxane (PDMS)-based components with microfeatures. Centrifugal casting permits simultaneous patterning of multiple sides of a component and allows control of the thickness of the part in an enclosed mold without entrapment of bubbles. Spinning molds filled with PDMS at thousands of revolutions per minute for several minutes causes entrapped bubbles within the PDMS to migrate toward the axis of rotation or dissolve into solution. To cure the parts quickly (<10 min), active elements heat and cool cavities filled with PDMS after the completion of spinning. Microfluidic channels produced from the process have a low coefficient of variation (<2% for the height and width of channels measured in 20 parts). This process is also capable of molding functional channels in opposite sides of a part as demonstrated through a device with a system of valves typical to multilayer soft lithography.

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Figures

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Fig. 1

Layout for centrifugal molding of two separate parts. When stationary, gravity dominates the hydrostatic state of the PDMS. When spinning at high speeds, the PDMS has a vertical liquid-air interface near the center axis of rotation.

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Fig. 2

Centrifugal casting and fast curing process for producing multisided devices. The parameter gb is acceleration due to the gravity of the earth and gr is acceleration from spinning.

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Fig. 3

(a) Heating and cooling station for curing PDMS in a mold assembly for centrifugal casting. The total height of the system is 91 cm. (b) Ceramic heaters on a pneumatic cylinder rise to make contact with the bottom of the mold and heat the mold assembly for 4 min. (c) After heating the PDMS is complete, a set of chilled blocks come down from the top to cool the mold for 4 min.

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Fig. 4

(a) High speed video image taken while a spinning mold with a transparent top is accelerating up to 5000 rpm. (b) High speed video image taken 3.4 s later, while the centrifuge is still spinning. The PDMS solution within the disk contains fewer bubbles.

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Fig. 12

Images of pneumatically actuated channels controlling microfluidic flow. (a) Flow channels are not blocked; reservoir is evenly filled. (b) Left flow channel is block; reservoir fills with food coloring from the right (interface between fluids moves to the left). (c) Flow channels are not blocked; reservoir is evenly filled. (d) Right flow channel is blocked; reservoir fills with food coloring from the right (interface between fluids moves to the right). The ovals highlight the regions where the flow and control channels cross over each other.

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Fig. 11

(a) Schematic diagram of the device. The serpentine control channel (generally unused) was designed to block the flow channels leading to the outlets. (b) Image of food coloring being introduced to a completed microfluidic device. The flow channels on the double-sided component are facing the glass slide at the bottom of the stack, while the control channels on the double-sided component are bonded to a top PDMS blank.

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Fig. 10

(a) Rendering of measurements made with an optical profilometer on a portion of a channel micromachined into an aluminum mold insert. (b) Rendering of measurements made with an optical profilometer on a portion of a channel molded into a double-sided PDMS part. (c) Same region shown in (a) with height data depicted in gray scale. (d) Same region shown in (b) with height data depicted in gray scale

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Fig. 9

Open mold assembly sitting on a centrifuge. Two parts are centrifugally cast simultaneously. The mold inserts for the double-sided MSL architecture are shown on the left. Blank mold inserts on the right are used to produce one of the cover pieces used to complete the assembly of functional devices. The two halves of the mold are held together with a set of 12 bolts.

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Fig. 8

Run chart of the specified channel section measured on 20 different parts produced with the centrifugal casting and fast curing process

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Fig. 7

(a) Protruding channels for a Y-mixer in a mold created in bulk metallic glass. The measured height using the algorithm in appendix B of Ref. [3] was 37.9 μm. (b) Y-mixer channels in a PDMS part produced by centrifugal casting off of bulk metallic glass features shown in (a). The measured height of the channels was 36.8 μm. (c) Y-mixer micromilled in aluminum. (d) Y-mixer channels in a PDMS part produced by centrifugal casting off of the micro-machined features shown in (c). As a result of molding PDMS off of (a) to produce (b), (b) is a mirror image of (a). The images in (c) and (d) are also mirrored.

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Fig. 6

(a) PDMS part with microfluidic channels. (b) PDMS part with a diameter of 10 cm, that includes some microchannels (bottom left of image), along with some larger ones. (c) Blank PDMS part used for evaluating appropriate spin times and spin speeds. (d) Same as (c) but not spun long enough and fast enough to remove all the bubbles

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Fig. 5

Simulated time to remove bubbles from spinning PDMS (Sylgard 184) versus their initial diameter before spinning. There is a critical diameter (165 μm for this set of parameters) for which removal will require the longest amount of spinning. To the left of the peak, diffusion dominates bubble removal (i.e., small bubbles dissolve into solution). To the right of the peak, buoyancy dominates removal of the bubbles (i.e., large bubbles travel to the liquid-air interface near the center of the centrifuge). The centrifuge spins up to 4000 rpm with a slew rate of 300 rpm/s, each bubble starts 6.2 cm from the center axis of rotation, and the liquid-air interface is 3.4 cm from the center axis of rotation. See Ref. [4] for more information.

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