Soft Lithographic Procedure for Producing Plastic Microfluidic Devices with View-ports Transparent to Visible and Infrared Light

Summary

A protocol for the fabrication of plastic microfluidic devices with transparent view-ports for visible and infrared light imaging is described.

Abstract

Infrared (IR) spectro-microscopy of living biological samples is hampered by the absorption of water in the mid-IR range and by the lack of suitable microfluidic devices. Here, a protocol for the fabrication of plastic microfluidic devices is demonstrated, where soft lithographic techniques are used to embed transparent Calcium Fluoride (CaF₂) view-ports in connection with observation chamber(s). The method is based on a replica casting approach, where a polydimethylsiloxane (PDMS) mold is produced through standard lithographic procedures and then used as the template to produce a plastic device. The plastic device features ultraviolet/visible/infrared (UV/Vis/IR) -transparent windows made of CaF₂ to allow for direct observation with visible and IR light. The advantages of the proposed method include: a reduced need for accessing a clean room micro-fabrication facility, multiple view-ports, an easy and versatile connection to an external pumping system through the plastic body, flexibility of the design, e.g., open/closed channels configuration, and the possibility to add sophisticated features such as nanoporous membranes.

Introduction

Fourier Transform Infrared Spectro-microscopy (FTIR) has been extensively utilized as a label-free and non-invasive imaging technique to provide detailed chemical information of a sample. This enables the extraction of biochemical information to study the chemistry of biological samples, with a minimum amount of preparation since the absorption spectrum of the specimen carries the intrinsic fingerprints of its chemical composition^1,2. Recently, FTIR has been increasingly applied to the study of live biological samples, e.g., cells³. However, water, which is the medium for livin....

Protocol

1. Preparation of Silicon Primary Mold

NOTE: A photomask is required for the preparation of the primary mold. The photomask can be either purchased from independent providers or fabricated in-house through standard optical mask fabrication procedures. A photomask with bright field polarity is used in this case (Figure 2a).

1. Pattern definition
   1. Spin coat a 4 inch silicon wafer with SU-8 3010 negative photoresist at 2,300 rpm for 30 s.
   2. Soft-bake the photoresist on a hot plate at 65 °C for 2 min and then at 95 °C for 8 min.
   3. Exp....

Results

Figure 6 presents the transmittance spectra of a brand new CaF₂ window, the patterned half of the device, and the complete device. All three spectra exhibit excellent transparency to mid IR with transmittance larger than 80%. The interference pattern visible in the spectrum of the full device (yellow curve in the figure) is caused by the air gap in the range of 9-10 µm between the two windows. These spectra demonstrate that the fabrication approach.......

Discussion

In order to assess and optimize the fabrication protocol, we used a simple layout for the microfluidic pattern with a large rectangular chamber (5 mm x 2.5 mm size) at the center, two small rectangular chambers (5.5 mm x 0.75 mm size) separated from the main circuit on the upper and lower sides, and 300 µm wide in-let/out-let channels. The central chamber is used for the seeding and observation of the cells, while the two separated smaller chambers are used to measure the air background during FTIR experiments as a .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Chemical
Trichloro(1H,1H,2H,2H-perfluorooctyl)silane 97%	Sigma Aldrich	448931-10G
Sylgard 184 Silicone Elastomer Kit	Dow Corning		Polydimethylsiloxane or in short, PDMS
Norland Optical Adhesive 73	Norland Products Inc.	7304
SU8 3010 photoresist	MicroChem	Y311060
SU8 developer	MicroChem	Y020100
Material
Silicon wafer, 4 inch, prime grade	Bonda Technology Pte Ltd
CaF2 IR-grade windows	Crystran, UK	CAFP10-1	10 mm diameter, 1 mm thickness
Acrylic templates			Custom made
Equipment
UV-KUB 2 (UV LED exposure system)	KLOE		Emission spectrum 365nm ± 5nm
Newport UV lamp	Newport	Model 66902	50-500 Watt Hg arc lamp
CEE Spin coater	Brewer Science	Model 200x
MJB4 mask aligner	SUSS MicroTec
Precision digital hot plate	Harry Gestigkeit GmbH	2860SR
Plasma Surface Technology	Diener Electronic GmbH + Co. KG		For O2 plasma treatment
IDP-3 Dry Scroll Vacuum Pump	Agilent Technologies		ultimate pressure 3.3 x 10-1 mbar
Bruker IFS 66v/s FTIR Spectrometer	Bruker