- Email: [email protected]
On-chip optical components and microfluidic systems
Microelectronic Engineering 73–74 (2004) 876–880 www.elsevier.com/locate/mee
On-chip optical components and microfluidic systems Q. Kou a
a,*
, I. Yesilyurt a, V. Studer a, M. Belotti a, E. Cambril a, Y. Chen
a
Laboratoire de Photonique et de Nanostructures, CNRS Route de Nozay, 91460 Marcoussis, France Available online 15 April 2004
Abstract We present the concept and the fabrication of on-chip optical components based on new microfluidic functionalities. Soft lithography has been used for the fabrication of microfluidic devices made of poly-dimethylsiloxane (PDMS). By filling the microchannels with a liquid of high refractive index, waveguides can be obtained. Other types of microoptical components such as beamsplitters, couplers, lenses and prisms are also fabricated. By inserting optical fibres into the microchannels, optical measurements are performed to show the expected effects. In addition, the dynamic spectroscopic detection of micro-beads of different colours has been demonstrated, and this should be applicable in various chemical and biological applications. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Microfluidic optical component; Liquid waveguide; Soft lithography; Fluorescent particle detection
1. Introduction Microfluidic devices have been developed for advanced applications in biotechnology, analytical and clinical chemistry. Basically, a microfluidic system is a miniaturization of standard analytical laboratory equipment which is able to perform variable transport and sensing operations on liquids. With microfluidic systems, it is possible to work with volumes of the picolitres range and thus much smaller sample quantities as well as shorter analysis time are needed. Microfluidic components such as microchannels, valves, mixers, switches * Corresponding author. Permanent address: Laboratoire de Photophysique Moleculaire, CNRS 91405, Orsay cedex, France. E-mail address: [email protected] (Q. Kou).
and pumps are being actively developed for different integrated microfluidic devices. In contrast, much less efforts have been devoted to the fabrication of optical components and their integration into microfluidic devices [1]. Moreover, most fabricated components consist of semiconductors or solid state devices [2,3]. To create new functionalities of microfluidic systems, it is highly desirable to fabricate microfluidic on-chip components for optical sensing and analysis in combining multitudes of other building blocks such as mechanical and electronic structures. In this work, we report on the design and the fabrication of microfluidic optical structures such as waveguides, beamsplitters, lenses and prisms by using soft lithography. For waveguides and beamsplitters, the cladding is solid and the core is liquid. The refractive index of the core is higher
0167-9317/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2004.03.068
Q. Kou et al. / Microelectronic Engineering 73–74 (2004) 876–880
877
than that of the cladding, so that total internal reflection occurs. For the fabricated lenses and prisms, the expected performances of light focusing and deflection are obtained. Because of the flexibility of the method, optical fibres can be coupled directly into the chip, allowing testing of the different optical functionalities. In particular, spectroscopic analysis can be performed in an efficient way for fluorescent particle detection.
guides and beamsplitters have been designed, but only multimode optical components were studied in this work since we were primarily interested in channel dimensions which are compatible with our cell-biology applications [6]. To make microfluidic devices for spectroscopic analysis, light has not necessarily to be guided. Low index liquids such as ethanol (n ¼ 1:36) and water (n ¼ 1:33) are then used to fill microchannels.
2. Fabrication of microfluidic components
3. Results and discussions
Elastomeric poly-dimethylsiloxane (PDMS) was used as basic material to fabricate microstructures of different optical components by using soft lithography [4,5]. For fast prototyping of optical components, we have used a high-resolution (3600 pixel per square millimetre) printer to pattern a transparent plastic film as a mask for contact printing optical lithography. The designed optical circuits were then replicated into a thin layer of AZ100 photoresist deposited on a silicon wafer by using a UV light exposure. Typically, the spin-coated resist thickness, which defines the channel depth, was chosen to be between 10 and 50 lm. Before elastomer (PDMS) casting, the resist patterns were exposed to trimethylchlorosilane (TMCS) vapour for one minute. Then, the liquid pre-polymer PDMS was poured on and cured at 80 °C for 1 h. After the cross-linked PDMS structures were peeled off, holes were punched through the elastomer to allow subsequent fluid access to the reservoirs. Afterward, the PDMS pieces were exposed to oxygen plasma and placed on another PDMS slide. Finally, the ensemble was heated again at 80 °C for 1 h, which led to an irreversible PDMS–PDMS bonding. Optical fibres were directly inserted to microfluidic channels through the interface of the two PDMS blocks after bonding. The index of refraction of the PDMS sample we obtained (a 10:1 mixture of RTV 615 A and B components manufactured by GE Bayer Silicones) is 1.41. To get light guided in waveguides and beamsplitters, high-index liquids such as immersion oil (n ¼ 1:52), glycerol (n ¼ 1:47) and ethylene glycol (n ¼ 1:43) are introduced into the microchannels. Both monomode and multimode wave-
3.1. Microfluidic optical components The fabricated waveguides, beamsplitters, lenses and prisms are shown in Fig. 1, where a laser beam or white light has been used to illuminate the liquid channels through optical fibres. Fig. 1(a) displays a photograph of a fabricated Y-shape microfluidic beamsplitter with fibre connections. In this device, the microchannels of 200 lm in width and 50 lm in depth were filled with oil of refractive index of 1.52, which gives a numeric aperture of 0.57 with PDMS claddings. The angle between the two output channels is 20°. The input of the beamsplitter was connected with an optical fibre of core diameter of 8 lm while the two outputs of the beamsplitter were connected with the fibres of 85 lm core diameter. A 3 dB loss was obtained for this device. This relatively large attenuation is due essentially to mismatch of dimensions between waveguides and output fibres. Fig. 1(b) displays a photomicrograph of a X-shape coupler. Laser beam at 532 nm is introduced into the device from one of the two inputs via connected optical fibres and is guided in the microfluidic channels. The guided light is divided equally between the two outputs. The second input is not illuminated in this figure. Fig. 1(c) shows the light propagation in PDMS at the exit of a liquid waveguide with a plane end. The waveguides are 200 lm in width and 30 lm in depth. A laser beam is introduced into the channel and guided to the end of the liquid waveguide. Then it propagates into the PDMS plate, with the expected divergence. In Fig. 1(d), the exit of liquid waveguide is designed with a curvature of 100 lm which acts as
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Q. Kou et al. / Microelectronic Engineering 73–74 (2004) 876–880
Fig. 1. Light propagation in PDMS microfluidic optical components: (a) Y-shape liquid waveguide based beamsplitter with optical fibre connections; (b) X-shape coupler; (c) Liquid waveguide with a plane end, (d) Liquid waveguide with a curved end; (e) PDMS microprisrn and light propagation through the microprism before; (f) and after; (g) oil filling.
a cylindrical lens of a focal distance of 1.4 mm. As can be seen, the beam divergence has been largely reduced due to lens effect. Fig. 1(e) shows an on-chip microprism (air in PDMS) connected with liquid filling channels on the top and at the bottom. The thickness of the prism is 50 lm. On the right of the microprism, a liquid waveguide is patterned to illumine the prism. On the left, a thin air gap is designed to visualise the received light by diffusion. Fig. 1(f) and (g) display the operation of the microprism before (f) and after (g) oil filling with a light incident from its right side. Without the oil (f), light is refracted by the PDMS/ air interface of the prism and visible by the air gap on the left because of the large difference of the refractive index between PDMS and air. With the oil (g), the prism deviates the light only slightly because of much reduced difference of the refractive index between PDMS and oil. 3.2. Fluorescent bead detection In this section, we present a kinetic spectroscopic analyser system for single fluorescent bead
detection. The device consists of a structured microfluidic chip in PDMS containing channels and an integrated optical fibre as shown in Fig. 2(a). A liquid containing florescent beads was injected from the top of the T-shape channel to circulate following the path (1) and the path (2). The channels have the dimensions of approximately 80 lm in width and 30 lm in depth. A multimode optical fibre was inserted into the channel, whose end came in contact with the liquid, in order to receive the maximum signal. The other end of the optical fibre was connected to a spectrometer (USB2000, Ocean Optics Inc.), as shown in Fig. 2(b) for the experimental set-up. This spectrometer covered the whole visible range with a spectral resolution of 1 nm and a temporal resolution of 3 ms. It was equipped with software which made it possible to record the whole visible spectral range and also to record simultaneously fluorescence time evolution events at different wavelengths. Latex beads of 10 lm diameter with green (534 nm) or red (620 nm) fluorescence were mixed in water and excited with the mercury pressure shot-
Q. Kou et al. / Microelectronic Engineering 73–74 (2004) 876–880
879
Fig. 2. Microfluidic chip (a) and experimental set-up (b) for fluorescent particle detection.
arc lamp of the microscope (Zeiss, Axiovert 200) equipped with a low-pass filter (wavelengths cut up to 500 nm). As can be seen in Fig. 3(a), the recorded green and red peaks were well separated, but a small portion of the green bead fluorescence was merged in the peak of red beads, which explained why we had detected residual intensity of
green beads in the red wavelength channel (see Fig. 3(c)). In Fig. 3(b), a histogram of detection events for the green beads following the path (1) is given. In this case, the wavelength channel is set to 534 nm. Thus, only when a green bead passed in front of fibre, a fluorescence peak was recorded. The different peak intensities corresponded to
Fig. 3. (a) Spectrum of both green and red fluorescent beads, (b) histogram of individual green fluorescent bead detection events, (c) simultaneous counting of individual beads of two different colours and (d) fluorescence decrease of a green bead leaving the detection zone.
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Q. Kou et al. / Microelectronic Engineering 73–74 (2004) 876–880
different bead positions compared to the detection fibre core surface. To count one bead, three recording points are necessary. The time resolution of the spectrometer we used is 3 ms, which gives a upper counting limit of 400,000 beads per hour. In Fig. 3(c), a histogram of simultaneous detection events for the green beads (upper line) and the red beads (lower line) is given. We noted that in this figure, one red bead and two green beads are detected during a period less than 300 ms. Because of the diaphonic phenomena, as shown in Fig. 3(a), the peak intensities of green beads are observable in the red channel, but they are very weak. Fig. 3(d) illustrates a situation where the system detects the green beads passing through the path (2). The large decreasing fluorescence peak indicated that one bead was leaving away from the detection fibre. We suppose that, when a bead leaves the fibre, the fluorescence intensity decreases proportionally with the solid angle between the bead and the detection fibre surface. This decreasing can be calculated, which is inversely proportional to the square of the distance between the bead and the fibre end surface. In the case of Fig. 3(d), the length of the channel is 600 lm and the curve has a width of about 8 s. By analysing the decay curve, the circulation speed of the bead (70 lm s1 ) can then be obtained. Finally, the system we developed allows the on-chip simultaneous spectral and temporal analysis of the individual fluorescent particles, for which no equivalent work was reported before to our knowledge.
4. Conclusions We have demonstrated several optical functionalities integrated in microfluidic chips. Optical components such as waveguides, beamsplitters, lenses and prisms have been obtained by soft lithography and by filling the microfluidic channels with high refractive index solutions. In addition, spectroscopic detection of microbeads of different colours has been shown. It is worthwhile to note that these optical components, themselves in liquid state, can be easily integrated to microfluidic systems which open a new way for optical detection required in microfabricated analyser devices. Integration of such micro-optical components into functional devices for biological and chemical applications is underway.
References [1] J.M.K. Ng, I. Gitlin, A.D. Stroock, G.M. Whitesides, Electrophoresis 23 (2002) 3461. [2] J.M. Ruano, A. Glidle, A. Cleary, A. Walmsley, J.S. Aitchison, J.M. Cooper, Biosens. Bioelectron. 18 (2003) 175. [3] K.B. Morgensen, J. El-Ali, A. Wolff, J.P. Kutter, Appl. Opt. 42 (2003) 4092. [4] J.C. McDonald, D.C. Duffy, J.R. Anderson, D.T. Chiu, H. Wu, O.J.A. Schueller, G.M. Whitesides, Electrophoresis 21 (2000) 27. [5] Y. Chen, A. Pepin, Electrophoresis 22 (2001) 187. [6] V. Studer, A. Pepin, Y. Chen, Appl. Phys. Lett. 80 (2002) 3614.
On-chip optical components and microfluidic systems Q. Kou a
a,*
, I. Yesilyurt a, V. Studer a, M. Belotti a, E. Cambril a, Y. Chen
a
Laboratoire de Photonique et de Nanostructures, CNRS Route de Nozay, 91460 Marcoussis, France Available online 15 April 2004
Abstract We present the concept and the fabrication of on-chip optical components based on new microfluidic functionalities. Soft lithography has been used for the fabrication of microfluidic devices made of poly-dimethylsiloxane (PDMS). By filling the microchannels with a liquid of high refractive index, waveguides can be obtained. Other types of microoptical components such as beamsplitters, couplers, lenses and prisms are also fabricated. By inserting optical fibres into the microchannels, optical measurements are performed to show the expected effects. In addition, the dynamic spectroscopic detection of micro-beads of different colours has been demonstrated, and this should be applicable in various chemical and biological applications. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Microfluidic optical component; Liquid waveguide; Soft lithography; Fluorescent particle detection
1. Introduction Microfluidic devices have been developed for advanced applications in biotechnology, analytical and clinical chemistry. Basically, a microfluidic system is a miniaturization of standard analytical laboratory equipment which is able to perform variable transport and sensing operations on liquids. With microfluidic systems, it is possible to work with volumes of the picolitres range and thus much smaller sample quantities as well as shorter analysis time are needed. Microfluidic components such as microchannels, valves, mixers, switches * Corresponding author. Permanent address: Laboratoire de Photophysique Moleculaire, CNRS 91405, Orsay cedex, France. E-mail address: [email protected] (Q. Kou).
and pumps are being actively developed for different integrated microfluidic devices. In contrast, much less efforts have been devoted to the fabrication of optical components and their integration into microfluidic devices [1]. Moreover, most fabricated components consist of semiconductors or solid state devices [2,3]. To create new functionalities of microfluidic systems, it is highly desirable to fabricate microfluidic on-chip components for optical sensing and analysis in combining multitudes of other building blocks such as mechanical and electronic structures. In this work, we report on the design and the fabrication of microfluidic optical structures such as waveguides, beamsplitters, lenses and prisms by using soft lithography. For waveguides and beamsplitters, the cladding is solid and the core is liquid. The refractive index of the core is higher
0167-9317/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2004.03.068
Q. Kou et al. / Microelectronic Engineering 73–74 (2004) 876–880
877
than that of the cladding, so that total internal reflection occurs. For the fabricated lenses and prisms, the expected performances of light focusing and deflection are obtained. Because of the flexibility of the method, optical fibres can be coupled directly into the chip, allowing testing of the different optical functionalities. In particular, spectroscopic analysis can be performed in an efficient way for fluorescent particle detection.
guides and beamsplitters have been designed, but only multimode optical components were studied in this work since we were primarily interested in channel dimensions which are compatible with our cell-biology applications [6]. To make microfluidic devices for spectroscopic analysis, light has not necessarily to be guided. Low index liquids such as ethanol (n ¼ 1:36) and water (n ¼ 1:33) are then used to fill microchannels.
2. Fabrication of microfluidic components
3. Results and discussions
Elastomeric poly-dimethylsiloxane (PDMS) was used as basic material to fabricate microstructures of different optical components by using soft lithography [4,5]. For fast prototyping of optical components, we have used a high-resolution (3600 pixel per square millimetre) printer to pattern a transparent plastic film as a mask for contact printing optical lithography. The designed optical circuits were then replicated into a thin layer of AZ100 photoresist deposited on a silicon wafer by using a UV light exposure. Typically, the spin-coated resist thickness, which defines the channel depth, was chosen to be between 10 and 50 lm. Before elastomer (PDMS) casting, the resist patterns were exposed to trimethylchlorosilane (TMCS) vapour for one minute. Then, the liquid pre-polymer PDMS was poured on and cured at 80 °C for 1 h. After the cross-linked PDMS structures were peeled off, holes were punched through the elastomer to allow subsequent fluid access to the reservoirs. Afterward, the PDMS pieces were exposed to oxygen plasma and placed on another PDMS slide. Finally, the ensemble was heated again at 80 °C for 1 h, which led to an irreversible PDMS–PDMS bonding. Optical fibres were directly inserted to microfluidic channels through the interface of the two PDMS blocks after bonding. The index of refraction of the PDMS sample we obtained (a 10:1 mixture of RTV 615 A and B components manufactured by GE Bayer Silicones) is 1.41. To get light guided in waveguides and beamsplitters, high-index liquids such as immersion oil (n ¼ 1:52), glycerol (n ¼ 1:47) and ethylene glycol (n ¼ 1:43) are introduced into the microchannels. Both monomode and multimode wave-
3.1. Microfluidic optical components The fabricated waveguides, beamsplitters, lenses and prisms are shown in Fig. 1, where a laser beam or white light has been used to illuminate the liquid channels through optical fibres. Fig. 1(a) displays a photograph of a fabricated Y-shape microfluidic beamsplitter with fibre connections. In this device, the microchannels of 200 lm in width and 50 lm in depth were filled with oil of refractive index of 1.52, which gives a numeric aperture of 0.57 with PDMS claddings. The angle between the two output channels is 20°. The input of the beamsplitter was connected with an optical fibre of core diameter of 8 lm while the two outputs of the beamsplitter were connected with the fibres of 85 lm core diameter. A 3 dB loss was obtained for this device. This relatively large attenuation is due essentially to mismatch of dimensions between waveguides and output fibres. Fig. 1(b) displays a photomicrograph of a X-shape coupler. Laser beam at 532 nm is introduced into the device from one of the two inputs via connected optical fibres and is guided in the microfluidic channels. The guided light is divided equally between the two outputs. The second input is not illuminated in this figure. Fig. 1(c) shows the light propagation in PDMS at the exit of a liquid waveguide with a plane end. The waveguides are 200 lm in width and 30 lm in depth. A laser beam is introduced into the channel and guided to the end of the liquid waveguide. Then it propagates into the PDMS plate, with the expected divergence. In Fig. 1(d), the exit of liquid waveguide is designed with a curvature of 100 lm which acts as
878
Q. Kou et al. / Microelectronic Engineering 73–74 (2004) 876–880
Fig. 1. Light propagation in PDMS microfluidic optical components: (a) Y-shape liquid waveguide based beamsplitter with optical fibre connections; (b) X-shape coupler; (c) Liquid waveguide with a plane end, (d) Liquid waveguide with a curved end; (e) PDMS microprisrn and light propagation through the microprism before; (f) and after; (g) oil filling.
a cylindrical lens of a focal distance of 1.4 mm. As can be seen, the beam divergence has been largely reduced due to lens effect. Fig. 1(e) shows an on-chip microprism (air in PDMS) connected with liquid filling channels on the top and at the bottom. The thickness of the prism is 50 lm. On the right of the microprism, a liquid waveguide is patterned to illumine the prism. On the left, a thin air gap is designed to visualise the received light by diffusion. Fig. 1(f) and (g) display the operation of the microprism before (f) and after (g) oil filling with a light incident from its right side. Without the oil (f), light is refracted by the PDMS/ air interface of the prism and visible by the air gap on the left because of the large difference of the refractive index between PDMS and air. With the oil (g), the prism deviates the light only slightly because of much reduced difference of the refractive index between PDMS and oil. 3.2. Fluorescent bead detection In this section, we present a kinetic spectroscopic analyser system for single fluorescent bead
detection. The device consists of a structured microfluidic chip in PDMS containing channels and an integrated optical fibre as shown in Fig. 2(a). A liquid containing florescent beads was injected from the top of the T-shape channel to circulate following the path (1) and the path (2). The channels have the dimensions of approximately 80 lm in width and 30 lm in depth. A multimode optical fibre was inserted into the channel, whose end came in contact with the liquid, in order to receive the maximum signal. The other end of the optical fibre was connected to a spectrometer (USB2000, Ocean Optics Inc.), as shown in Fig. 2(b) for the experimental set-up. This spectrometer covered the whole visible range with a spectral resolution of 1 nm and a temporal resolution of 3 ms. It was equipped with software which made it possible to record the whole visible spectral range and also to record simultaneously fluorescence time evolution events at different wavelengths. Latex beads of 10 lm diameter with green (534 nm) or red (620 nm) fluorescence were mixed in water and excited with the mercury pressure shot-
Q. Kou et al. / Microelectronic Engineering 73–74 (2004) 876–880
879
Fig. 2. Microfluidic chip (a) and experimental set-up (b) for fluorescent particle detection.
arc lamp of the microscope (Zeiss, Axiovert 200) equipped with a low-pass filter (wavelengths cut up to 500 nm). As can be seen in Fig. 3(a), the recorded green and red peaks were well separated, but a small portion of the green bead fluorescence was merged in the peak of red beads, which explained why we had detected residual intensity of
green beads in the red wavelength channel (see Fig. 3(c)). In Fig. 3(b), a histogram of detection events for the green beads following the path (1) is given. In this case, the wavelength channel is set to 534 nm. Thus, only when a green bead passed in front of fibre, a fluorescence peak was recorded. The different peak intensities corresponded to
Fig. 3. (a) Spectrum of both green and red fluorescent beads, (b) histogram of individual green fluorescent bead detection events, (c) simultaneous counting of individual beads of two different colours and (d) fluorescence decrease of a green bead leaving the detection zone.
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Q. Kou et al. / Microelectronic Engineering 73–74 (2004) 876–880
different bead positions compared to the detection fibre core surface. To count one bead, three recording points are necessary. The time resolution of the spectrometer we used is 3 ms, which gives a upper counting limit of 400,000 beads per hour. In Fig. 3(c), a histogram of simultaneous detection events for the green beads (upper line) and the red beads (lower line) is given. We noted that in this figure, one red bead and two green beads are detected during a period less than 300 ms. Because of the diaphonic phenomena, as shown in Fig. 3(a), the peak intensities of green beads are observable in the red channel, but they are very weak. Fig. 3(d) illustrates a situation where the system detects the green beads passing through the path (2). The large decreasing fluorescence peak indicated that one bead was leaving away from the detection fibre. We suppose that, when a bead leaves the fibre, the fluorescence intensity decreases proportionally with the solid angle between the bead and the detection fibre surface. This decreasing can be calculated, which is inversely proportional to the square of the distance between the bead and the fibre end surface. In the case of Fig. 3(d), the length of the channel is 600 lm and the curve has a width of about 8 s. By analysing the decay curve, the circulation speed of the bead (70 lm s1 ) can then be obtained. Finally, the system we developed allows the on-chip simultaneous spectral and temporal analysis of the individual fluorescent particles, for which no equivalent work was reported before to our knowledge.
4. Conclusions We have demonstrated several optical functionalities integrated in microfluidic chips. Optical components such as waveguides, beamsplitters, lenses and prisms have been obtained by soft lithography and by filling the microfluidic channels with high refractive index solutions. In addition, spectroscopic detection of microbeads of different colours has been shown. It is worthwhile to note that these optical components, themselves in liquid state, can be easily integrated to microfluidic systems which open a new way for optical detection required in microfabricated analyser devices. Integration of such micro-optical components into functional devices for biological and chemical applications is underway.
References [1] J.M.K. Ng, I. Gitlin, A.D. Stroock, G.M. Whitesides, Electrophoresis 23 (2002) 3461. [2] J.M. Ruano, A. Glidle, A. Cleary, A. Walmsley, J.S. Aitchison, J.M. Cooper, Biosens. Bioelectron. 18 (2003) 175. [3] K.B. Morgensen, J. El-Ali, A. Wolff, J.P. Kutter, Appl. Opt. 42 (2003) 4092. [4] J.C. McDonald, D.C. Duffy, J.R. Anderson, D.T. Chiu, H. Wu, O.J.A. Schueller, G.M. Whitesides, Electrophoresis 21 (2000) 27. [5] Y. Chen, A. Pepin, Electrophoresis 22 (2001) 187. [6] V. Studer, A. Pepin, Y. Chen, Appl. Phys. Lett. 80 (2002) 3614.