BIOCOMPATIBLE, REVERSIBLE PHOTO-ACTUATED HYDROGELS, OPERATIVE IN NEUTRAL ENVIRONMENTS, FOR MICRO-VALVE

APPLICATIONS IN MICROFLUIDIC DEVICES Aishling Dunne1, Wayne Francis1, Larisa Florea1*, Fernando Benito-Lopez1,2 and

Dermot Diamond1

1Insight Centre for Data Analytics, National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, IRELAND

2CIC MicroGUNE, Arrasate-Mondragón, SPAIN

ABSTRACT In this study photo-actuator hydrogels microstructures capable of operating in neutral aqueous

environments were generated using N-isopropylacrylamide-co-acrylated spiropyran-co-acrylic acid p(NIPAAM-co-SP-co-AA) copolymer, in a 100-1-5 mole ratio. In neutral aqueous environments, the acrylic acid dissociates, triggering conversion of the SP moiety to the hydrophilic protonated merocyanine form (MC-H+). MC-H+ allows water uptake, causing hydrogel expansion. When exposed to white light, the reverse sequence happens, the expanded hydrogel contracts due to the photonic-conversion of the MC-H+ moiety back to the more hydrophobic SP form. Using tetrahydrofuran (THF): water mixture (4:1 v:v) as the polymerisation solvent, resulted in the formation of hydrogels with improved porosity and remarkable contraction size of up to 50% after four minutes of white light irradiation and reswelling capacity of up to 84% of its fully hydrated size after 11 minutes in the dark. KEYWORDS: Microfluidics, photo-actuation, hydrogels, spiropyran.

INTRODUCTION

In recent years, a popular way of photo-modulating flow in microfluidic devices has been through the use of acidified hydrogel valves incorporating spiropyran (SP) moieties that needed to be externally protonated with HCl solutions. In the swollen (protonated merocyanine) form, the hydrogel blocks the channels and prevents flow. When exposed to white light, the positively charged protonated merocyanine (MCH+) is converted to the uncharged SP form, triggering shrinking of the hydrogel, and the channel opens [1]. Despite the inherent attractiveness of photo-controlled polymer valves for microfluidics, the approach has significant disadvantages such as (1) the need to expose the contracted gel to strong external acidic solutions, in order to induce re-swelling, (2) slow re-swelling times of up to several hours, making it impossible for this type of valves to be reusable and therefore overpass the barrier of being a mere scientific curiosity and being able to reach the market. To overcome this, an internal source of protons is required. For this reason, acrylic acid (AA) was copolymerised into the hydrogel polymer matrix [2]. In water, the acrylic acid comonomer spontaneously dissociates, resulting in the protonation of the photochromic spiropyran (SP) to protonated merocyanine (MC-H+). This form is hydrophilic, allowing the hydrogel to swell. Exposure to white light promotes isomerisation of the MC-H+ form to the hydrophobic SP, which triggers contraction of the hydrogel (Figure 1).

Figure 1: Self-protonation of p(NIPAAm-co-SP-co-AA) hydrogels in a neutral aqueous solution and the effect of white light irradiation.

978-0-9798064-7-6/µTAS 2014/$20©14CBMS-0001 1871 18th International Conference on MiniaturizedSystems for Chemistry and Life Sciences

October 26-30, 2014, San Antonio, Texas, USA

EXPERIMENTAL Fabrication of hydrogel microstructures For characterisation purposes, hydrogel microstructures of desired shapes were photo-polymerised through micro-patterned masks (Figure 2) using an in-house made cell which consisted of one upper 1H,1H,2H,2H- perfluorodecyl-triethoxysilane functionalised glass slide (hydrophobic substrate) and a lower bare glass slide. The spacer size was set to 55 µm and the cell was filled by capillary forces with the monomeric solution and subsequently exposed to white light irradiation, through a mask consisting of 1 mm circles, for polymerisation.

Figure 2. Scanning electron microscopy image of surface bound circular microstructures photo-polymerised using a micro-patterned mask. Channel fabrication The fabrication of the master mold was carried out using a laser ablation system-excimer/CO2 laser (Optec Laser Micro- machining Systems, Belgium) by cutting the microfluidic structures in a 175 µm poly(methylmethacrylate) sheet attached to a 70 µm double sided pressure sensitive adhesive film, PSA, (AR8890, Adhesives Research, Ireland) and pasting one of the PSA sides to a petri dish. The channels were 350 µm high, 1600 µm wide and contained circular pillars of 550 µm in diameter. For polydimethylsiloxane (PDMS) casting, the precursor was prepared by mixing PDMS elastomer with the curing agent from Sylgard 184 kit at a weight ratio of 10 : 1, poured onto the master mould, and cured in an oven at 60 °C for 2 h. Following curing, the PDMS layer is peeled from the master. The inlets and outlets (800 mm in diameter) were made using a manual puncher (Technical Innovations, Inc., Brazoria, TX). The PDMS replica (~2 mm height) was thoroughly washed with isopropanol and exposed to oxygen plasma to seal the chip to a clean glass slide (35 mm x 64 mm, Agar Scientific Limited, England). Silicon tubes were employed to further connect the main inlets with a syringe pump (PHD 2000 Syringe, Harvard Apparatus) for sample delivery and washing. Micro-valve fabrication For valve applications, the hydrogel microstructures were photo-polymerised in-situ inside PDMS/glass microfluidic channels around 550 µm diameter pillars using photo-masks. RESULTS AND DISCUSSION

The shrinking and re-swelling mechanism was studied under white light irradiation and in the dark, respectively (Figure 3). A remarkable contraction in hydrogel size of up to 50 % of its fully hydrated size was obtained after four minutes of white light irradiation. After removal of the white light source, the hydrogel re-swelled to ~84 % of its fully hydrated size after 11 minutes in the dark. This represents, to our best knowledge, the greatest reversible photo-induced size change of a hydrogel in the absence of any other stimuli. Moreover, this shrinking and re-swelling process can be repeated successfully, with great reproducibility, indicating that these hydrogels, when integrated as micro-valves, are reusable (Figure 3b).

1872

Figure 3. Shrinking and swelling behavior of the hydrogel microstructures when exposed to white light. (a) First cycle of photo-actuation. (b) Repeatable photo-actuation over 3 cycles in real time.

When the hydrogel micro-valves that were photo-polymerised in-situ inside PDMS/glass microfluidic

channels were exposed to white light, the valves contracted thus opening the channel and allowing fluid to flow. The opposite was seen when the valve was kept in the dark (Figure 4).

Figure 4. Microchip for photo-induced fluid manipulation showing the photo-responsive micro-valves obtained by in-situ polymerisation. The channel is opened after white light irradiation (4 min) and reclosed in the dark (after additional 11 min). CONCLUSION

In conclusion, the optimisation of the polymerisation solvent mixture has resulted in the fabrication of hydrogels with faster and reproducible shrinking and reswelling cycles. This study demonstrates these hydrogels can be successfully used as photo-controlled valves in microfluidic systems for repeatable ON/OFF flow control in neutral environments.

ACKNOWLEDGEMENTS

The project has been carried out with the support of the Science Foundation Ireland under the Insight initiative, grant SFI/12/RC/2289. FBL acknowledges the Ramón y Cajal Programme (Ministerio de Economía y Competitividad) and the Gobierno Vasco, Dpto. Industria, Innovación, Comercio y Turismo under ETORTEK 2013 with grant no. IE13-360.

REFERENCES 1. F. Benito-Lopez, R. Byrne, A. M. Raduta, N. E. Vrana, G. McGuinness, D. Diamond, "Ionogel-based light-

actuated valves for controlling liquid flow in micro-fluidic manifolds," Lab Chip 10, 195, 2010. 2. Z. Bartosz, L. Florea, T. Jannick, F. Benito-Lopez, D. Diamond, "Self-protonating spiropyran-co-NIPAM-

co-acrylic acid hydrogel photoactuators," Soft Matter 9, 8754-8760, 2013. CONTACT

* Larisa Florea, tel: +353-1-7006009; larisa.florea2@mail.dcu.ie

1873