- Email: [email protected]
Femtosecond laser assisted fabrication of networked semi-occlusive microfluidic channel on fused silica glass surface
Accepted Manuscript Title: Femtosecond laser assisted fabrication of networked semi-occlusive microfluidic channel on fused silica glass surface Author: Guang Li Guoying Feng Shutong Wang Hua Zhang Zhuping Wang Shouhuan Zhou PII: DOI: Reference:
S0030-4026(17)30453-9 http://dx.doi.org/doi:10.1016/j.ijleo.2017.04.050 IJLEO 59092
To appear in: Received date: Revised date: Accepted date:
10-10-2016 16-3-2017 14-4-2017
Please cite this article as: G. Li, G. Feng, S. Wang, H. Zhang, Z. Wang, S. Zhou, Femtosecond laser assisted fabrication of networked semi-occlusive microfluidic channel on fused silica glass surface, Optik - International Journal for Light and Electron Optics (2017), http://dx.doi.org/10.1016/j.ijleo.2017.04.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Femtosecond laser assisted fabrication of networked semi-occlusive microfluidic channel on fused silica glass surface Guang Li a , Guoying Feng a,* , Shutong Wang a , Hua Zhang a , Zhuping Wang a ,
a
us
North China Research Institute of Electro-Optics, Beijing, China, 100015
cr
Sichuan University, Institute of Laser & Micro/Nano Engineering, College of Electronics and Information
Engineering, No.24 South Section 1, First Ring Road, Chengdu, China, 610064 b
ip t
Shouhuan Zhou a,b
Abstract: Networked semi-occlusive microfluidic was demonstrated via fabricating channel on the surface of the
an
silica substrate using femtosecond laser direct writing (FLDW) followed by chemical etching and covering a thin layer of polydimethylsiloxane (PDMS) film on substrate. The microfluidic channel formation mechanism is
M
identified as femtosecond laser irradiation leading to substrate material modification,and modified areas acquiring an increased solubility to hydrofluoric (HF) acid solution. The morphologies of networked microfluidic were
d
characterized by using optical microscopy and scanning electronic microscopy. Furthermore, in order to test flowing
Ac ce pt e
characteristics of the microfluidic, we conducted the experiment of liquid injection.
Keywords: networked microfluidic; femtosecond laser; direct writing; liquid injection
1
Introduction
During the last two decades, due to microfluidic system’s ability of highly integrating and minimizing system, microfluidic system has attracted considerable attention. Meanwhile, microfluidic system can be used for series of chemical and biological analysis applications [1-4]. Microfluidic channels are the key components of a micro-total analysis system (µ-TAS) [5], thus the research about them is imminent. So far, photolithography is still a main way of microfluidic channels fabrication which is actually a two-dimensional planar fabrication technology [6, 7].
*
Corresponding author. E-mail addresses: [email protected] 1
Page 1 of 10
Therefore,
fabrication
of
three-dimensional
(3D)
microfluidic
structures
by
photolithography-based techniques requires additional stack and bond, leading to increase in complexity and cost. A main method for achieving 3D microfluidic structures in transparent
ip t
substrates is to use femtosecond laser direct writing (FLDW) as demonstrated by many groups [8-13]. The microfluidic structures fabricated by FLDW which is a maskless fabrication technique have been found comprehensive applications, such as single cell manipulation, and
label-free
protein detection
[14];
microfluidic waveguide lasers
cr
analytics,
[15];
us
nano-aquarium for dynamic observation of living cells [16]; optofluidic sensors with various functions including refractive index monitoring [17], etc. Generally, microfluidic channels can be
an
formed in transparent substrates via FLDW followed by chemical etching [11, 18-24]. However, the morphology of the microchannels fabricated via this way is always conical in shape. This is due to the limited contrast ratio of etching selectivity between the laser exposed and unexposed
M
regions. Since the chemical etching always begins from the surface of the substrate and progresses toward the middle area of the channels, the region close to the entrance of the
d
channels will always suffer a longer etching period compared to the middle region. More recently,
Ac ce pt e
we have developed a new technique which firstly fabricates channel on the surface of the substrate using FLDW followed by chemical etching and then covers a thin layer of PDMS film on substrate to form semi-occlusive microchannels. The conical feature brought by etching selectivity can be effectively reduced, because the channels are fabricated on the surface of substrate, thus there is no problem of suffering different etching period. Via this technique, we can fabricate various homogeneous networked semi-occlusive microfluidic on the surface of silica.
In this paper, we demonstrate that, for the first time to the best of our knowledge, semi-occlusive microfluidic channel can be achieved by fabricating channel on the surface of the substrate and then covering a layer of about 500 µm thick PDMS film on it. Eventually we fabricated a homogeneous networked semi-occlusive microfluidic on the surface of silica, and then carried out a test experiment of liquid injection. This networked microfluidic systerm can be
2
Page 2 of 10
used as micro-storage of gas or liquid, microfluidic laser cavity and cracks of bed rock model for the research of oil flowing, etc.
Experiment
ip t
2
In the experiment, we used a fused silica as the substrate material which was cut into 10 mm×10 mm×1 mm coupons with all surfaces polished. Experimental setup schematic of the
cr
FLDW process to fabricate microfluidic channels is shown in Fig. 1. The experiment was carried
us
out with a commercial Ti:sapphire regenerative amplifier laser system (Legend Elite, Coherent), which generates laser pulses with centre wavelength at 800 nm, pulse duration of 60 fs and
an
maximum single pulse energy of 0.3 mJ at a repetition rate of 1 kHz.The average power of the laser beam used to direct writing was controlled by a combination of polarizer and wave plate. A 20× objective with a NA of 0.4 was employed to focus the laser beam. The sample could be
M
optionally translated by a personal-computer-controlled XYZ stage (M-111.1DG, Physik Instrumente) with a resolution of 6.9 nm. The average power of femtosecond laser was chosen to
d
be 160 mW, and the translating speed of the stage was 0.01 mm/s (laser fluence was 1.27×105
Ac ce pt e
J/cm2). A charge couple device (CCD) connected to a personal computer was used for monitoring the whole FLDW process in real time.
Fig. 1. (color online)Experimental setup schematic of the FLDW process.
In our experiment, as shown in Fig. 2, the whole process consists of four steps (schematics are shown in Fig. 2(a-d), respectively): (1) FLDW on the surface of the silica to form networked modified region; (2) dipping the laser irradiated substrate in 10% aqueous solution of HF at 25 3
Page 3 of 10
°C in ultrasonic bath about 30 min for removing the modified material to form homogeneous microfluidic channel; (3) covering a layer of 500 µm thick PDMS film on the substrate; (4) injecting Rh6G into microfluidic channel in order to test flowing characteristics of the
ip t
microfluidic. All experiments were conducted in air environment. Prior to irradiation, the fused silica was cleaned in acetone and deionized water in an ultrasonic bath for 15 min each. After the laser irradiation, the sample was cleaned in alcohol and deionized water in an ultrasonic bath for
cr
15 min to remove the plume dust deposited in the ablation area. After etching, we dipped the
us
substrate in enough calcium chloride aqueous solution to achieve the combination of residual fluoride ions and calcium ions and form calcium fluoride precipitation; then we used 10%
an
aqueous solution of sodium hydroxide to neutralize residual hydrogen ions for 3 min, and then douched the substrate with water about 10 min. The acid-base property of substrate was tested by phteststrips. Neutralization and douching were conducted until the result of pH text was
M
alkalescence. The surface morphology was observed using optical microscopy (OM, Keyence
Ac ce pt e
d
VHX 650) and scanning electronic microscope (SEM, Hitachi SU8220).
Fig. 2. (color online)Flow chart of the whole fabrication process: (a) FLDW on the surface of the silica; (b) chemical selective etching with HF; (c) covering a layer of 500 µm thick PDMS film on the substrate; (d) injecting Rh6G into microfluidic channel．
4
Page 4 of 10
3
Results and discussions Top view optical micrograph of microfluidic channels fabricated on the surface of fused
silica substrate by FLDW is shown in Fig. 3(a). Fig. 3(b) is the enlargement of Fig. 3(a). It can
ip t
be easily found that there were a handful of recasts around ablation area, besides, embossment and scallops existed on the straight channels which seriously impact the roughness of microfluidic channels. After etching process, top view optical micrograph of microfluidic
cr
channels is shown in Fig. 3(c). Fig. 3(d) is the enlargement of Fig. 3(c). After etching, recasts
us
almost disappeared, besides, the microfluidic channels become more homogeneous. Both the magnification of the Fig. 3(a) and Fig. 3(c) are 500, the magnification of Fig. 3(b) and Fig. 3(d)
an
are 2000. By comparing the optical micrographs of microfluidic channels after direct writing with after chemical etching, it can be easily found that the roughness and morphology of microfluidic channels both effectively improve after etching. Meanwhile, after etching, width of
Ac ce pt e
d
M
microfluidic channels increases some extent.
Fig. 3. (color online)Optical micrographs of microfluidic channels: (a) after direct writing; (b) partial enlargement of Figure(a) ;(c)after etching; (d)partial enlargement of Figure(c).
5
Page 5 of 10
The width of networked microfluidic channel measured at different straight channels is shown in Fig. 4, each data point is the result of the average of five values measured at different positions of the same straight channel. Fig. 4(a) is the width of networked microfluidic channel
ip t
before etching. Obviously, the width of microfluidic channel is about 7.25 µm. Fig. 4(b) is the width of networked microfluidic channel after etching and the width of microfluidic channel is about 8.25 µm. Apparently, etching process increase the width of microfluidic channel about 1
cr
µm. In addition, according to date points and corresponding error distribution, we may breezily
us
draw the conclusion that global and regional of microfluidic channel both become more
Ac ce pt e
d
M
an
homogeneous after etching.
Fig. 4. The width of networked microfluidic channel at different straight channel: (a) before etching and (b) after etching.
The scanning electron microscope (SEM) images of the surface morphology of the sample after etching are shown in Fig. 5. The overall outline of the networked microfluidic channel is shown in Fig. 5(a); Fig. 5(b) is the partial enlargement of microfluidic channel, focus is located on the surface of the substrate; when focus is located at bottom of microfluidic channel, morphology of microfluidic channel is shown in Fig. 5(c). It is clear that the microfluidic channel after etching process still shows relatively high surface roughness, which can be attributed that SEM has a greater depth of field compared with optical microscope and there is always a certain inclination in innerwalls of the channel particularly near the surface of the
6
Page 6 of 10
substrate (silver zonal region in Fig. 5(c)). The reason of silver zonal region existing is that focus position is fixed in FLDW process, thus the spot becomes larger with the increase of machining depth, furthermore, the femtosecond laser is Gaussian used in experiment, as the spot becomes
ip t
larger, areas that can achieve damage threshod of substrate will become smaller, so the width of channel will decrease with depth increase of channel, eventually the fabricated microfluidic
an
us
cr
channel presents slope feature in the direction of microfluidic channel depth.
M
Fig. 5. (color online)SEM image of networked microfluidic channels after etching:(a)overall outline; (b)partial enlargement of microfluidic channel when focus is located on the surface of the substrate; (c)when focus is located
Ac ce pt e
d
at bottom of microfluidic channel.
In order to test flowing characteristics of the semi-occlusive microfluidic, we conducted the experiment of liquid injection. After covering the etched microfluidic channels with a thin layer of PDMS film, we used pipette dropping a drop Rh6G to one end of microfluidic channel, then we filmed the whole process of liquid flowing with optical microscope under 200 times magnification(video see Appendix).We captured some critical moment in the process, as we can see in the Fig. 6. Fig. 6(a) shows the networked microfluidic channel fabricated by FLDW on silica substrate after selective chemical etching. The networked microfluidic channel covered by a layer of 500 µm thick PDMS film is shown in Fig. 6(b). The networked microfluidic channel after covered by PDMS film and partially filled with Rh6G is shown in Fig. 6(c). The networked microfluidic channel after covered by PDMS film and completely filled with Rh6G is shown in Fig. 6(d).
7
Page 7 of 10
ip t cr us an
M
Fig. 6. (color online)Optical micrographs of networked microfluidic channels: (a) fabricated by FLDW on silica substrate after selective chemical etching; (b) covered by a layer of 500 µm thick PDMS film; (c) partially filled
Ac ce pt e
d
with Rh6G; (d) completely filled with Rh6G.
The mechanism of microfluidic channel fabrication in silica by femtosecond laser writing is that, the intense femtosecond laser beam tightly focused into a confined small area induces multiphoton absorption within substrate material leading to material alteration due to an extremely high photon density. Because of physical and chemical modification associated with the multiphoton absorption, the irradiated areas acquire an increased solubility to aqueous hydrofluoric acid [25]. As we know, the microfluidic channels were filled with air before injecting Rh6G. As Rh6G was injected in microfluidic channels under the capillary force [26], the bottom space of microfluidic channels were occupied by Rh6G( we can see a stream of fluid flowing through the other side of microfluidic channels at 17th second of video) and the air was squashed to the top of microfluidic channels. More Liquid in microfluidic channels results in greater pressure and more gas dissolved in liquid. So with the increase of Rh6G in microfluidic
8
Page 8 of 10
channels, air gradually dissolves in Rh6G and flows out from the end of microfluidic channels. Eventually, microfluidic channels were completely filled with Rh6G and presented amaranth.
Conclusion
ip t
4
To summarize, we demonstrate the fabrication of networked semi-occlusive microfluidic via femtosecond laser direct writing on the silica followed by covering a thin layer of PDMS film
cr
on substrate, and test flowing characteristics of the microfluidic via conducting the experiment
us
of liquid injection, providing an alternative solution for fabrication of semi-occlusive homogeneous microfluidic systems.
an
Acknowledgments
This work was supported by Major Program of National Natural Science Foundation of China
M
(11574221).
Ac ce pt e
d
Video 1 The process of injecting Rh6G into microfluidic References
[1] X. Fan, I.M. White, Optofluidic microsystems for chemical and biological analysis, Nat Photonics. 5 (2011) 591-597.
[2] D. Qin, Y. Xia, et al., Microsystem technology in chemistry and life science, Top Curr Chem. 194 (1999) 1-20. [3] D.B. Weibel, G.M. Whitesides, Applications of microfluidics in chemical biology, Curr Opin Chem Biol. 10 (2006) 584-591.
[4] H. Abramczyk, B. Brozek-Pluska, et al., Photostability of biological systems—Femtosecond dynamics of zinc tetrasulfonated phthalocyanine at cancerous and noncancerous human Breast tissues, Journal of Photochemistry & Photobiology A Chemistry. 332 (2017) 10-24.
[5] A. Arora, G. Simone, et al., Latest developments in micro total analysis systems, Anal Chem. 82 (2010) 4830-4847.
[6] L. Hai-Hua, C. Jian, et al., Research of photolithography technology based on surface plasmon, Chin Phys B. 19 (2010) 114203. [7] M.S. Giridhar, K. Seong, et al., Femtosecond pulsed laser micromachining of glass substrates with application to microfluidic devices, Appl Opt. 43 (2004) 4584-4589. [8] R.R. Gattass, E. Mazur, Femtosecond laser micromachining in transparent materials, Nat Photonics. 2 (2008) 219-225. 9
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[9] B.-B. Xu, Y.-L. Zhang, et al., Fabrication and multifunction integration of microfluidic chips by femtosecond laser direct writing, Lab Chip. 13 (2013) 1677-1690. [10] P. Paiè, F. Bragheri, et al., Straightforward 3D hydrodynamic focusing in femtosecond laser fabricated microfluidic channels, Lab Chip. 14 (2014) 1826-1833. [11] Y.-L. Zhang, Q.-D. Chen, et al., Designable 3D nanofabrication by femtosecond laser direct writing, Nano Today. 5 (2010) 435-448.
ip t
[12] K. Sugioka, Y. Hanada, et al., 3D integration of microcomponents in a single glass chip by femtosecond laser direct writing for biochemical analysis, Appl Surf Sci. 253 (2007) 6595-6598.
[13] C. Li, X. Shi, et al., Fabrication of three-dimensional microfluidic channels in glass by femtosecond pulses,
cr
Optics Communications. 282 (2009) 657-660.
[14] W. Hellmich, C. Pelargus, et al., Single cell manipulation, analytics, and label‐free protein detection in
us
microfluidic devices for systems nanobiology, Electrophoresis. 26 (2005) 3689-3696.
[15] D.V. Vezenov, B.T. Mayers, et al., A low-threshold, high-efficiency microfluidic waveguide laser, J Am Chem Soc. 127 (2005) 8952-8953.
an
[16] Y. Hanada, K. Sugioka, et al., Nano-aquarium for dynamic observation of living cells fabricated by femtosecond laser direct writing of photostructurable glass, Biomed Microdevices. 10 (2008) 403-410. [17] A. Crespi, Y. Gu, et al., Three-dimensional Mach-Zehnder interferometer in a microfluidic chip for spatially-resolved label-free detection, Lab Chip. 10 (2010) 1167-1173.
M
[18] V. Maselli, R. Osellame, et al., Fabrication of long microchannels with circular cross section using astigmatically shaped femtosecond laser pulses and chemical etching, Appl Phys Lett. 88 (2006) 191107. [19] Y. Bellouard, A. Said, et al., Fabrication of high-aspect ratio, micro-fluidic channels and tunnels using
d
femtosecond laser pulses and chemical etching, Opt Express. 12 (2004) 2120-2129. [20] D.N. Vitek, D.E. Adams, et al., Temporally focused femtosecond laser pulses for low numerical aperture
Ac ce pt e
micromachining through optically transparent materials, Opt Express. 18 (2010) 18086-18094. [21] C. Hnatovsky, R. Taylor, et al., Fabrication of microchannels in glass using focused femtosecond laser radiation and selective chemical etching, Appl Phys A. 84 (2006) 47-61. [22] D. Wu, S.Z. Wu, et al., Hybrid femtosecond laser microfabrication to achieve true 3D glass/polymer composite biochips with multiscale features and high performance: the concept of ship‐in‐a‐bottle biochip, Laser & Photonics Reviews. 8 (2014) 458-467.
[23] X. Meng, Q. Yang, et al., Fabrication of 3D solenoid microcoils in silica glass by femtosecond laser wet etch and microsolidics, Proceedings of SPIE - The International Society for Optical Engineering. 9449 (2015). [24] C. Liu, Y. Liao, et al., Compact 3D microfluidic channel structures embedded in glass fabricated by femtosecond laser direct writing, Journal of Laser Micro Nanoengineering. 8 (2013) 170. [25] S. Kiyama, S. Matsuo, et al., Examination of etching agent and etching mechanism on femotosecond laser microfabrication of channels inside vitreous silica substrates†, The Journal of Physical Chemistry C. 113 (2009) 11560-11566. [26] M. Gleiche, L.F. Chi, et al., Nanoscopic channel lattices with controlled anisotropic wetting, Nature. 403 (2000) 173-175.
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S0030-4026(17)30453-9 http://dx.doi.org/doi:10.1016/j.ijleo.2017.04.050 IJLEO 59092
To appear in: Received date: Revised date: Accepted date:
10-10-2016 16-3-2017 14-4-2017
Please cite this article as: G. Li, G. Feng, S. Wang, H. Zhang, Z. Wang, S. Zhou, Femtosecond laser assisted fabrication of networked semi-occlusive microfluidic channel on fused silica glass surface, Optik - International Journal for Light and Electron Optics (2017), http://dx.doi.org/10.1016/j.ijleo.2017.04.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Femtosecond laser assisted fabrication of networked semi-occlusive microfluidic channel on fused silica glass surface Guang Li a , Guoying Feng a,* , Shutong Wang a , Hua Zhang a , Zhuping Wang a ,
a
us
North China Research Institute of Electro-Optics, Beijing, China, 100015
cr
Sichuan University, Institute of Laser & Micro/Nano Engineering, College of Electronics and Information
Engineering, No.24 South Section 1, First Ring Road, Chengdu, China, 610064 b
ip t
Shouhuan Zhou a,b
Abstract: Networked semi-occlusive microfluidic was demonstrated via fabricating channel on the surface of the
an
silica substrate using femtosecond laser direct writing (FLDW) followed by chemical etching and covering a thin layer of polydimethylsiloxane (PDMS) film on substrate. The microfluidic channel formation mechanism is
M
identified as femtosecond laser irradiation leading to substrate material modification,and modified areas acquiring an increased solubility to hydrofluoric (HF) acid solution. The morphologies of networked microfluidic were
d
characterized by using optical microscopy and scanning electronic microscopy. Furthermore, in order to test flowing
Ac ce pt e
characteristics of the microfluidic, we conducted the experiment of liquid injection.
Keywords: networked microfluidic; femtosecond laser; direct writing; liquid injection
1
Introduction
During the last two decades, due to microfluidic system’s ability of highly integrating and minimizing system, microfluidic system has attracted considerable attention. Meanwhile, microfluidic system can be used for series of chemical and biological analysis applications [1-4]. Microfluidic channels are the key components of a micro-total analysis system (µ-TAS) [5], thus the research about them is imminent. So far, photolithography is still a main way of microfluidic channels fabrication which is actually a two-dimensional planar fabrication technology [6, 7].
*
Corresponding author. E-mail addresses: [email protected] 1
Page 1 of 10
Therefore,
fabrication
of
three-dimensional
(3D)
microfluidic
structures
by
photolithography-based techniques requires additional stack and bond, leading to increase in complexity and cost. A main method for achieving 3D microfluidic structures in transparent
ip t
substrates is to use femtosecond laser direct writing (FLDW) as demonstrated by many groups [8-13]. The microfluidic structures fabricated by FLDW which is a maskless fabrication technique have been found comprehensive applications, such as single cell manipulation, and
label-free
protein detection
[14];
microfluidic waveguide lasers
cr
analytics,
[15];
us
nano-aquarium for dynamic observation of living cells [16]; optofluidic sensors with various functions including refractive index monitoring [17], etc. Generally, microfluidic channels can be
an
formed in transparent substrates via FLDW followed by chemical etching [11, 18-24]. However, the morphology of the microchannels fabricated via this way is always conical in shape. This is due to the limited contrast ratio of etching selectivity between the laser exposed and unexposed
M
regions. Since the chemical etching always begins from the surface of the substrate and progresses toward the middle area of the channels, the region close to the entrance of the
d
channels will always suffer a longer etching period compared to the middle region. More recently,
Ac ce pt e
we have developed a new technique which firstly fabricates channel on the surface of the substrate using FLDW followed by chemical etching and then covers a thin layer of PDMS film on substrate to form semi-occlusive microchannels. The conical feature brought by etching selectivity can be effectively reduced, because the channels are fabricated on the surface of substrate, thus there is no problem of suffering different etching period. Via this technique, we can fabricate various homogeneous networked semi-occlusive microfluidic on the surface of silica.
In this paper, we demonstrate that, for the first time to the best of our knowledge, semi-occlusive microfluidic channel can be achieved by fabricating channel on the surface of the substrate and then covering a layer of about 500 µm thick PDMS film on it. Eventually we fabricated a homogeneous networked semi-occlusive microfluidic on the surface of silica, and then carried out a test experiment of liquid injection. This networked microfluidic systerm can be
2
Page 2 of 10
used as micro-storage of gas or liquid, microfluidic laser cavity and cracks of bed rock model for the research of oil flowing, etc.
Experiment
ip t
2
In the experiment, we used a fused silica as the substrate material which was cut into 10 mm×10 mm×1 mm coupons with all surfaces polished. Experimental setup schematic of the
cr
FLDW process to fabricate microfluidic channels is shown in Fig. 1. The experiment was carried
us
out with a commercial Ti:sapphire regenerative amplifier laser system (Legend Elite, Coherent), which generates laser pulses with centre wavelength at 800 nm, pulse duration of 60 fs and
an
maximum single pulse energy of 0.3 mJ at a repetition rate of 1 kHz.The average power of the laser beam used to direct writing was controlled by a combination of polarizer and wave plate. A 20× objective with a NA of 0.4 was employed to focus the laser beam. The sample could be
M
optionally translated by a personal-computer-controlled XYZ stage (M-111.1DG, Physik Instrumente) with a resolution of 6.9 nm. The average power of femtosecond laser was chosen to
d
be 160 mW, and the translating speed of the stage was 0.01 mm/s (laser fluence was 1.27×105
Ac ce pt e
J/cm2). A charge couple device (CCD) connected to a personal computer was used for monitoring the whole FLDW process in real time.
Fig. 1. (color online)Experimental setup schematic of the FLDW process.
In our experiment, as shown in Fig. 2, the whole process consists of four steps (schematics are shown in Fig. 2(a-d), respectively): (1) FLDW on the surface of the silica to form networked modified region; (2) dipping the laser irradiated substrate in 10% aqueous solution of HF at 25 3
Page 3 of 10
°C in ultrasonic bath about 30 min for removing the modified material to form homogeneous microfluidic channel; (3) covering a layer of 500 µm thick PDMS film on the substrate; (4) injecting Rh6G into microfluidic channel in order to test flowing characteristics of the
ip t
microfluidic. All experiments were conducted in air environment. Prior to irradiation, the fused silica was cleaned in acetone and deionized water in an ultrasonic bath for 15 min each. After the laser irradiation, the sample was cleaned in alcohol and deionized water in an ultrasonic bath for
cr
15 min to remove the plume dust deposited in the ablation area. After etching, we dipped the
us
substrate in enough calcium chloride aqueous solution to achieve the combination of residual fluoride ions and calcium ions and form calcium fluoride precipitation; then we used 10%
an
aqueous solution of sodium hydroxide to neutralize residual hydrogen ions for 3 min, and then douched the substrate with water about 10 min. The acid-base property of substrate was tested by phteststrips. Neutralization and douching were conducted until the result of pH text was
M
alkalescence. The surface morphology was observed using optical microscopy (OM, Keyence
Ac ce pt e
d
VHX 650) and scanning electronic microscope (SEM, Hitachi SU8220).
Fig. 2. (color online)Flow chart of the whole fabrication process: (a) FLDW on the surface of the silica; (b) chemical selective etching with HF; (c) covering a layer of 500 µm thick PDMS film on the substrate; (d) injecting Rh6G into microfluidic channel．
4
Page 4 of 10
3
Results and discussions Top view optical micrograph of microfluidic channels fabricated on the surface of fused
silica substrate by FLDW is shown in Fig. 3(a). Fig. 3(b) is the enlargement of Fig. 3(a). It can
ip t
be easily found that there were a handful of recasts around ablation area, besides, embossment and scallops existed on the straight channels which seriously impact the roughness of microfluidic channels. After etching process, top view optical micrograph of microfluidic
cr
channels is shown in Fig. 3(c). Fig. 3(d) is the enlargement of Fig. 3(c). After etching, recasts
us
almost disappeared, besides, the microfluidic channels become more homogeneous. Both the magnification of the Fig. 3(a) and Fig. 3(c) are 500, the magnification of Fig. 3(b) and Fig. 3(d)
an
are 2000. By comparing the optical micrographs of microfluidic channels after direct writing with after chemical etching, it can be easily found that the roughness and morphology of microfluidic channels both effectively improve after etching. Meanwhile, after etching, width of
Ac ce pt e
d
M
microfluidic channels increases some extent.
Fig. 3. (color online)Optical micrographs of microfluidic channels: (a) after direct writing; (b) partial enlargement of Figure(a) ;(c)after etching; (d)partial enlargement of Figure(c).
5
Page 5 of 10
The width of networked microfluidic channel measured at different straight channels is shown in Fig. 4, each data point is the result of the average of five values measured at different positions of the same straight channel. Fig. 4(a) is the width of networked microfluidic channel
ip t
before etching. Obviously, the width of microfluidic channel is about 7.25 µm. Fig. 4(b) is the width of networked microfluidic channel after etching and the width of microfluidic channel is about 8.25 µm. Apparently, etching process increase the width of microfluidic channel about 1
cr
µm. In addition, according to date points and corresponding error distribution, we may breezily
us
draw the conclusion that global and regional of microfluidic channel both become more
Ac ce pt e
d
M
an
homogeneous after etching.
Fig. 4. The width of networked microfluidic channel at different straight channel: (a) before etching and (b) after etching.
The scanning electron microscope (SEM) images of the surface morphology of the sample after etching are shown in Fig. 5. The overall outline of the networked microfluidic channel is shown in Fig. 5(a); Fig. 5(b) is the partial enlargement of microfluidic channel, focus is located on the surface of the substrate; when focus is located at bottom of microfluidic channel, morphology of microfluidic channel is shown in Fig. 5(c). It is clear that the microfluidic channel after etching process still shows relatively high surface roughness, which can be attributed that SEM has a greater depth of field compared with optical microscope and there is always a certain inclination in innerwalls of the channel particularly near the surface of the
6
Page 6 of 10
substrate (silver zonal region in Fig. 5(c)). The reason of silver zonal region existing is that focus position is fixed in FLDW process, thus the spot becomes larger with the increase of machining depth, furthermore, the femtosecond laser is Gaussian used in experiment, as the spot becomes
ip t
larger, areas that can achieve damage threshod of substrate will become smaller, so the width of channel will decrease with depth increase of channel, eventually the fabricated microfluidic
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channel presents slope feature in the direction of microfluidic channel depth.
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Fig. 5. (color online)SEM image of networked microfluidic channels after etching:(a)overall outline; (b)partial enlargement of microfluidic channel when focus is located on the surface of the substrate; (c)when focus is located
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at bottom of microfluidic channel.
In order to test flowing characteristics of the semi-occlusive microfluidic, we conducted the experiment of liquid injection. After covering the etched microfluidic channels with a thin layer of PDMS film, we used pipette dropping a drop Rh6G to one end of microfluidic channel, then we filmed the whole process of liquid flowing with optical microscope under 200 times magnification(video see Appendix).We captured some critical moment in the process, as we can see in the Fig. 6. Fig. 6(a) shows the networked microfluidic channel fabricated by FLDW on silica substrate after selective chemical etching. The networked microfluidic channel covered by a layer of 500 µm thick PDMS film is shown in Fig. 6(b). The networked microfluidic channel after covered by PDMS film and partially filled with Rh6G is shown in Fig. 6(c). The networked microfluidic channel after covered by PDMS film and completely filled with Rh6G is shown in Fig. 6(d).
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Fig. 6. (color online)Optical micrographs of networked microfluidic channels: (a) fabricated by FLDW on silica substrate after selective chemical etching; (b) covered by a layer of 500 µm thick PDMS film; (c) partially filled
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with Rh6G; (d) completely filled with Rh6G.
The mechanism of microfluidic channel fabrication in silica by femtosecond laser writing is that, the intense femtosecond laser beam tightly focused into a confined small area induces multiphoton absorption within substrate material leading to material alteration due to an extremely high photon density. Because of physical and chemical modification associated with the multiphoton absorption, the irradiated areas acquire an increased solubility to aqueous hydrofluoric acid [25]. As we know, the microfluidic channels were filled with air before injecting Rh6G. As Rh6G was injected in microfluidic channels under the capillary force [26], the bottom space of microfluidic channels were occupied by Rh6G( we can see a stream of fluid flowing through the other side of microfluidic channels at 17th second of video) and the air was squashed to the top of microfluidic channels. More Liquid in microfluidic channels results in greater pressure and more gas dissolved in liquid. So with the increase of Rh6G in microfluidic
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channels, air gradually dissolves in Rh6G and flows out from the end of microfluidic channels. Eventually, microfluidic channels were completely filled with Rh6G and presented amaranth.
Conclusion
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4
To summarize, we demonstrate the fabrication of networked semi-occlusive microfluidic via femtosecond laser direct writing on the silica followed by covering a thin layer of PDMS film
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on substrate, and test flowing characteristics of the microfluidic via conducting the experiment
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of liquid injection, providing an alternative solution for fabrication of semi-occlusive homogeneous microfluidic systems.
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Acknowledgments
This work was supported by Major Program of National Natural Science Foundation of China
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(11574221).
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Video 1 The process of injecting Rh6G into microfluidic References
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