Liquid metal electrical conductors in R2R
imprinted PDMS microfluidic channels
Olli-Heikki Huttunen,* 1,2 Johanna H. Hiitola-Keinänen, 1,2 Tuomas Happonen,1,2Sensing and Integration, Printed Electronics Processing, 1Sensing and Integration, Flexible Electronics Integration,2E-mail: olli-heikki.huttunen@vtt.fi

Polydimethyl siloxane (PDMS) has been already two decades extremely important material
for prototyping microfluidics for biomedical research. [1] Still recently, related mass
production of PDMS devices has been considered difficult and for example roll processing
even impossible.[1] Very recently, we have developed Roll-to-Roll replication process anddemonstrated functionality of such microfluidic components by performing nucleic acidamplification in roll-to roll replicated device. [2]PDMS microfluidics can be utilized in elastic electronics by applying conductive liquids, likeliquid metals in microfluidic channels [3]. We demonstrated R2R replication of microfluidic
channels in PDMS by using Momentive RTV-615 2-component silicone elastomer onMelinex ST506 PET substrate (Figure 1). We used etched metal master shim welded to asleeve which was then installed on hot embossing cylinder. We deposited pre-mixed silicone
elastomer manually and levelled it with spiral bar (nominal wet thickness of 300µm)
following the R2R imprint step on roll mold with surface temperature of 165℃. Process
sequence was as presented by Hiltunen et al 2018 [2]. Web width was 200mm and web speed
was 2 m/min which equals throughput of 24 m2/h. We lidded channels with blank cured filmof same RTV-615 PDMS using manual vacuum plasma bonding (Technics Plasma GmbHTepla 440-G, O2, 300W, 1min) channels with Eutectic Gallium – Indium alloy (eGaiIn) byusing vacuum suction. We measured electromechanical performance of this liquid metal
conductor by applying 50% strain and repeated strain 10 and 100 times. (Figures 3 and 4)
We did not observe significant hysteresis but resistance recovered perfectly. Either we did notobserve in performance with different velocity of the stretching.
Microfluidics market size was estimated for US$ 10 billion for 2018 and subject to reach
US$28billion by 2023. [4] We believe there is constantly increasing number of highly
potential PDMS based microfluidic applications developed in academy whose developers arelooking opportunities to also develop business from their research. One option is to transformand redevelop the device physics and chemistries to meet existing mass manufacturing
techniques such as injection molding. Another approach is to upscale manufacturing ofexisting prototypes. Our approach is to develop upscaled manufacturing process for
microstructures and for that we using roll to roll techniques. VTT operates open accessresearch infra structure for printed electronics, including two roll to roll pilot level machines
equipped with embossing/imprinting units capable also on PDMS processing.
VTT leads Printocent community which is based on 3 research partners, city of Oulu and over
30 industry cluster partners. Purpose of Printocent is gather whole value chain and ecosystem
together and boost industrialization and commercialization of printed electronics.

References:

Normalized resistance[1] Sackmann, E.K., Fulton, A. L, Beebe D.J., “The present and future role of microfluidics
in biomedical research”, Nature, 2014, 507, 181.
[2] Hiltunen J., Liedert C., Hiltunen M., Huttunen O.-H., Hiitola-Keinänen J., Aikio S.,Harjanne M., Kurkinen M., Hakalahti L. and Lee L. P, “Roll-to-roll fabrication ofintegrated PDMS–paper microfluidics for nucleic acid amplification,” Lab Chip, 2018,18,1552-1559[3] Shi Cheng (September 12th 2012). Elastomeric Electronics: A Microfluidic Approach,
Advanced Elastomers - Technology, Properties and Applications, IntechOpen,[4]https://www.marketsandmarkets.com/Market-Reports/microfluidics-market-1305.html

324,2 s18,4 s016,8 s0246810

Number of cycles

Figure 3 Ten cycles 50% strain resistancemeasurement with three different cycle
times

Resistance [Ω]Figure 1 Roll of R2R imprinted µfluidic
channels in PDMS. Web width 200mm.

421.5
1Rmax [Ω]0.5

Rave [Ω]0Rmin [Ω]0 20 40 60 80 100
Number of cycles

Figure 2 Resistance measurement in cycled
50% strain

Figure 4 Hundred cycles 50% strain
resistance measurement with 4,2s cycle
time