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Plasma treatments of wool fiber surface for microfluidic applications
Materials Research Bulletin 69 (2015) 65–70
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Plasma treatments of wool fiber surface for microfluidic applications So-Hyoun Jeon a , Ki-Hwan Hwang a , Jin Su Lee a , Jin-Hyo Boo a, * , Sang H. Yun b, ** a b
Department of Chemistry, Sungkyunkwan University, 440-746 Suwon, Republic of Korea Institute of Basic Science, Sungkyunkwan University, 440-746 Suwon, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 9 February 2015
Recent progress in health diagnostics has led to the development of simple and inexpensive systems. Thread-based microfluidic devices allow for portable and inexpensive field-based technologies enabling medical diagnostics, environmental monitoring, and food safety analysis. However, controlling the flow rate of wool thread, which is a very important part of thread-based microfluidic devices, is quite difficult. For this reason, we focused on thread-based microfluidics in the study. We developed a method of changing the wettability of hydrophobic thread, including wool thread. Thus, using natural wool thread as a channel, we demonstrate herein that the manipulation of the liquid flow, such as micro selecting and micro mixing, can be achieved by applying plasma treatment to wool thread. In addition to enabling the flow control of the treated wool channels consisting of all natural substances, this procedure will also be beneficial for biological sensing devices. We found that wools treated with various gases have different flow rates. We used an atmospheric plasma with O2, N2 and Ar gases. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Surfaces Electron microscopy Infrared spectroscopy Microstructure Surface properties
1. Introduction So far, the developing world has been given access to many kinds of medical diagnostic technologies [1,2]. However, most medical diagnostic technologies are too expensive or require conditions which can make them very hard to use. Such approaches cannot satisfy the needs of the majority of people afflicted with many kinds of disease, because the majority of them have access to poorly resourced health care and cannot pay very much for health care. Microfluidics is known as one of the most promising detectors for monitoring diseases. It has the advantages of low cost, light weight and simplicity [3,4]. Even though microfluidics have many advantages, more study is required into the transport, mixing and switching methods [5,6]. In this paper, we suggest a simple method of making microfluidic devices [7–9]. We used thread-based microfluidics, which is one kind of microfluidic device. To define the best transport system, we investigated wool thread modified using a DBD (Dielectric barrier discharge) plasma device [10–12]. We were able to control the wicking process in the thread-based microfluidic device using the modified wool thread. We used various gases to modify the wool thread, which is hydrophobic.
* Corresponding author. Tel.: +82 31 290 7072. ** Corresponding author. E-mail addresses: [email protected] (J.-H. Boo), [email protected] (S.H. Yun). http://dx.doi.org/10.1016/j.materresbull.2015.02.025 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
Wool fiber consists of three layers. The outmost layer of scales is called the cuticle, the middle layer is called the cortex and the inner core is referred to as the medulla Joseph [13]. The surface of the outermost cuticle cells is a fatty acid layer of 18-methyleicosanoic acid covalently bound to the protein layer of the wool cuticle via a thioester linkage [14]. This fatty layer makes the surface of the wool hydrophobic, which prevents it from being directly used as a microfluidic channel [15]. To utilize wool thread, the wetting properties of the surface should be modified to introduce hydrophilicity. We were able to produce hydrophilic wool thread by using a DBD plasma with various gases. The penetration of the activating species in the DBD plasma into the fiber is shallow, so the middle and inner layers were hardly affected [16]. This is why the DBD plasma is used as the method of modifying the surface properties of the wool fiber without changing the middle and inner layers. For these reasons, we focus on the outmost layer to demonstrate why the wettability was changed. We obtained SEM images of the pristine wool fibers and those treated with the plasma. We found that the damage to the surface increases as the plasma treatment time increases [17]. We used oxygen, nitrogen and argon gases and air as the plasma generating gas. We also controlled the plasma treatment time. The wool fibers were treated with the plasma for 15, 30, 45 and 60 min. The FTIR spectra were obtained to investigate which functional group was changed. Finally, we made a simple microfluidics channel with the treated wool fibers and successfully transferred a solution containing water and soluble ink. We confirmed that it
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Fig. 1. (A) SEM image of pristine wool fiber. Inset: contact angle of water on the pristine wool fabric. (B) structure of outmost layer of wool fiber. (C), (D) structure of protein matrix.
was possible to apply this procedure to microfluidic devices with a very simple process. 2. Experimental
The fibers were obtained from the wool fabric and put on the plate of the DBD plasma device. The plasma is generated by the gas and AC power [19]. The wool fibers were treated by the plasma for 15, 30, 45 and 60 min with each gas.
2.1. Preparation of wool fiber The liquid wetting dynamics on a fabric are influenced by the fabric geometry and fiber surface properties. To demonstrate the effect of the surface properties, we used a single kind of wool fiber. We measured the contact angle to investigate the wettability of the wool fabric. The inset image of Fig. 1(A) shows the snapshots of a water droplet on the pristine wool fabric [18]. The image shows that it is absolutely hydrophobic. As shown in Fig. 1(A), wool fiber consists of many layers. Because the penetration of the activating species in the DBD plasma into the fiber is shallow, we focus on the change of the surface. The outmost layer of the wool fiber is called the cuticle, which is a fatty acid layer of 18-methyleicosanoic acid covalently bound to the protein layer via a thioester linkage, as shown in Fig. 1(B). This fatty layer makes the wool fiber hydrophobic. The protein matrix structure is shown in Fig. 1(C) and (D). It contains disulfide bonds, hydrogen bonds and ion–ion interactions. 2.2. DBD plasma treatment Fig. 2 shows the schematic structure of the DBD plasma device. We used O2, N2 and Ar gases and air to generate the plasma.
Fig. 2. The schematic structure of DBD plasma device.
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2.3. Measure of wicking rate As shown in Fig. 3(E), a simple channel of wool fiber is made to investigate the wicking rate of each fiber. On the slide glass, we attached double sided tape from 3M to attach the fibers to the glass. A Kimtech wiper was cut into a rectangular block with a length and width of 5 mm and 75 mm, respectively, and attached to the right end side of the glass for the purpose of loading the ink solution. The pristine and treated wool fibers were attached to the double sided tape. 40 ml of blue ink solution was loaded on the Kimtech wipers. We took a picture when the wicking of the solution was completed and calculated the wicking rate from the measured wicking time. Four channels for the oxygen, nitrogen, argon and air plasmas, respectively, were fabricated to investigate the effect of the treatment time. On each channel, the pristine wool and wool fibers which were treated for 15, 30, 45 and 60 min were attached. 2.4. Fabrication of simple microfluidic device We made a simple microfluidic device to investigate the micro-mixing properties. Double sided tape from 3M was attached to the slide glass. Before attaching it to the glass, we made two 5 mm holes, which are located on the right side of the device for the loading of the ink solution. The surface of the glass is hydrophilic, so the ink solution was able to be placed on the holes. On the left side of the device, two pieces of Kimtech wipers were attached for
67
the purpose of detecting the mixing solution. The Kimtech wipers was cut onto circles with a diameter of 5 mm. Two fibers were attached to the tape in the shape as shown in Fig. 7. 20 ml of blue and red ink solutions were loaded on each hole at the same time. When the mixing color was observed on the Kimtech wipers, indicating that the micro-mixing was successful, we took a picture, as shown in Fig. 7. This showed that it was possible to apply this procedure to the microfluidic device. 3. Results and discussion 3.1. Wicking rate according to treatment time Fig. 3 shows the wicking of the wool according to the plasma treatment time. Each glass includes wool fibers which were treated with either the air, argon, nitrogen or oxygen plasma. As shown in the first slide glass, the wicking speed increases with increasing treatment time [17]. This phenomenon is also observed for the other glasses. The short red lines indicate how long the ink solution was transported and the wicking rates of the wool fibers are calculated using the measured wicking time, as shown in Fig. 4. 3.2. Damage on surface of the wool fibers (scanning electron microscopy images) To investigate the surface morphology of the wool fibers, scanning electron microscopy (SEM) was used. As shown in Fig. 5,
Fig. 3. (A) Wool fibers treated with oxygen plasma. (B) Wool fibers treated with argon plasma. (C) Wool fibers treated with nitrogen plasma. (D) Wool fibers treated with air plasma. (A), (B), (C), (D) Pristine wool fiber and fibers treated for 15, 30, 45 and 60 min, respectively from upside. (E) The schematic structure of channels. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
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each wool fiber was damaged to an extent which increased with increasing treatment time. The oxygen plasma was so powerful that a treatment time of just 30 min was sufficient, however no more damage was caused to the fibers which were treated for 45 and 60 min. Some of the fatty acid on the surface of the wool fibers was damaged. For these reasons, the wettability of the wool fiber was increased [20,21].
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Time Fig. 4. Calculated wicking rate according to gas species and treatment time.
The FTIR spectra were measured to investigate the change of the functional groups on the surface of the wool fibers. As shown in Fig. 6, the peak at 1700 cm 1 is attributed to the thioester group [22]. To compare the FTIR spectra of the wool fibers as a function of the treatment time, the spectra of the wool fibers which were treated with the same gas but for different treatment times are shown together. Fig. 6(A) shows that the peak for the thioester group decreases with increasing treatment time when oxygen gas
Fig. 5. SEM images of wool fibers which treated with air plasma for 15 (A), 30 (B), 45 (C), 60 min (D), treated with argon plasma for 15 (E), 30 (F), 45(G), 60 min (H), treated with nitrogen plasma for 15 (I), 30 (J), 45 (K), 60 min (L), and treated with oxygen plasma for 5 (I), 15 (J), 30 (K).
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was used. This phenomenon is also observed in Fig. 6(B) and (C) which shows the spectra for the wool fibers treated with the argon and nitrogen plasmas, respectively. Under the influence of the DBD plasma treatment, the cysteine groups present on the surface of the wool fiber were changed into cysteric acid groups. This is the main reason for the increase in the wettability. The improvement of the wettability is due to the removal of the fatty acid layer which is on the surface of the outmost cuticle. 3.4. Fabrication of simple microfluidic device We designed a simple microfluidic device, as shown in Fig. 7. Wool fibers which were treated with the oxygen, argon and nitrogen plasmas for 30, 45, and 60 min were used for making the device, because the fibers treated with the air plasma or treated for only 15 min were not rapid enough to apply to the microfluidic device. Two kinds of ink solutions were loaded on the holes on the right end side of the device at the same time. Red and blue ink solutions were dropped on the upper and lower holes, respectively. After a few minutes, we found that the Kimtech wipers were dyed a purple color, which is a mixture of red and blue. The micro-mixing of the solutions is successful on this device. It indicates that the
wool fibers could be applied to a microfluidic device with a very simple and low-cost process. 4. Conclusions The effects of a DBD plasma with various gases, viz. oxygen, argon, nitrogen and air, on wool fiber were investigated. The treatment time was also controlled and the wicking rate was found to increase with increasing treatment time. SEM images were obtained to investigate the surface morphology. The damage to the wool surface increased with increasing treatment time. The FTIR spectra were measured to investigate the change of the functional groups on the surface. The peak for the thioester group decreased with increasing treatment time. The cysteine groups, which link the fatty acid and protein layers, were changed into cysteric acid groups. In other words, the improvement of the wettability is due to the removal of the fatty acid layer which is on the surface of the outmost cuticle. The wicking rates of the wool fibers increase in the order: oxygen, argon, nitrogen and air plasma. As a result, the oxygen plasma is the most effective to change the wettability of the wool fiber.
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Fig. 7. Simple microfluidic devices with wool fiber which are treated with oxygen plasma for 5 min (A), 15 min (B), 30 min (C), and with argon plasma for 30 min (D), 45 min (E), 60 min (F), with nitrogen plasma for 30 min (G), 45 min (H), 60 min (I). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
A simple microfluidic device was fabricated with the treated wool fibers. It transferred the solutions successfully and performed micro-mixing. This indicates that the DBD plasma treatment of the wool fibers can be applied to microfluidic devices. It is very promising because the process is simple and inexpensive. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2010-0027963) and Agency for Defense Development through Chemical and Biological Defense Research Center (CBD-12). References [1] [2] [3] [4]
T. San Park, W. Li, K.E. McCracken, J.-Y. Yoon, Lab Chip 13 (2013) 4832–4840. A.K. Yetisen, M.S. Akram, C.R. Lowe, Lab Chip 13 (2013) 2210–2251. X. Li, D.R. Ballerini, W. Shen, Biomicrofluidics 6 (2012) 011301. A.W. Martinez, S.T. Phillips, G.M. Whitesides, Proc. Natl. Acad. Sci. U. S. A. 105 (2008) 19606–19611.
[5] A.R. Rezk, A. Qi, J.R. Friend, W.H. Li, L.Y. Yeo, Lab Chip 12 (2012) 773–779. [6] J. Zhang, X. Wei, X. Xue, Z. Jiang, J. Nanosci. Nanotechnol. 14 (2014) 7419–7426. [7] M. Reches, K.A. Mirica, R. Dasgupta, M.D. Dickey, M.J. Butte, G.M. Whitesides, ACS Appl. Mater. Interfaces 2 (2010) 1722–1728. [8] X. Li, J. Tian, W. Shen, ACS Appl. Mater. Interfaces 2 (2009) 1–6. [9] P. Bhandari, T. Narahari, D. Dendukuri, Lab Chip 11 (2011) 2493–2499. [10] R. Molina, P. Jovan9 ci c, F. Comelles, E. Bertran, P. Erra, J. Adhes. Sci. Technol. 16 (2002) 1469–1485. [11] K. Chi-wai, C. Kwong, M.Y. Chun-wah, AUTEX Res. J. 4 (2004) 37–44. [12] X. Wang, G. Cao, W. Xu, J. Appl. Polym. Sci. 112 (2009) 1959–1966. [13] D. Chen, L. Tan, H. Liu, J. Hu, Y. Li, F. Tang, Langmuir 26 (2009) 4675–4679. [14] A.P. Negri, H.J. Cornell, D.E. Rivett, Text. Res. J. 63 (1993) 109–115. [15] L. Coderch, O. Lopez, A. De La Maza, A. Manich, J. Parra, J. Cegarra, Text. Res. J. 67 (1997) 131–136. [16] C.W. Kan, K. Chan, C.W.M. Yuen, M.H. Miao, J. Mater. Process. Technol. 83 (1998) 180–184. [17] C. Kan, Modern Research and Educational Topics in Microscopy, vol. 2, 2007, pp. 683–689. [18] B. Bateup, J. Cook, H. Feldtman, B. Fleischfresser, Text. Res. J. 46 (1976) 720–723. [19] M. Kushner, J. Appl. Phys. 53 (1982) 2939–2946. [20] F. Ferrero, M. Periolatto, Ultrason. Sonochem. 19 (2012) 601–606. [21] E. El-Khatib, W. Raslan, A. El-Halwagy, S. Galab, Res. J. Text. Apparel 17 (2013) 124–132.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Plasma treatments of wool fiber surface for microfluidic applications So-Hyoun Jeon a , Ki-Hwan Hwang a , Jin Su Lee a , Jin-Hyo Boo a, * , Sang H. Yun b, ** a b
Department of Chemistry, Sungkyunkwan University, 440-746 Suwon, Republic of Korea Institute of Basic Science, Sungkyunkwan University, 440-746 Suwon, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 9 February 2015
Recent progress in health diagnostics has led to the development of simple and inexpensive systems. Thread-based microfluidic devices allow for portable and inexpensive field-based technologies enabling medical diagnostics, environmental monitoring, and food safety analysis. However, controlling the flow rate of wool thread, which is a very important part of thread-based microfluidic devices, is quite difficult. For this reason, we focused on thread-based microfluidics in the study. We developed a method of changing the wettability of hydrophobic thread, including wool thread. Thus, using natural wool thread as a channel, we demonstrate herein that the manipulation of the liquid flow, such as micro selecting and micro mixing, can be achieved by applying plasma treatment to wool thread. In addition to enabling the flow control of the treated wool channels consisting of all natural substances, this procedure will also be beneficial for biological sensing devices. We found that wools treated with various gases have different flow rates. We used an atmospheric plasma with O2, N2 and Ar gases. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Surfaces Electron microscopy Infrared spectroscopy Microstructure Surface properties
1. Introduction So far, the developing world has been given access to many kinds of medical diagnostic technologies [1,2]. However, most medical diagnostic technologies are too expensive or require conditions which can make them very hard to use. Such approaches cannot satisfy the needs of the majority of people afflicted with many kinds of disease, because the majority of them have access to poorly resourced health care and cannot pay very much for health care. Microfluidics is known as one of the most promising detectors for monitoring diseases. It has the advantages of low cost, light weight and simplicity [3,4]. Even though microfluidics have many advantages, more study is required into the transport, mixing and switching methods [5,6]. In this paper, we suggest a simple method of making microfluidic devices [7–9]. We used thread-based microfluidics, which is one kind of microfluidic device. To define the best transport system, we investigated wool thread modified using a DBD (Dielectric barrier discharge) plasma device [10–12]. We were able to control the wicking process in the thread-based microfluidic device using the modified wool thread. We used various gases to modify the wool thread, which is hydrophobic.
* Corresponding author. Tel.: +82 31 290 7072. ** Corresponding author. E-mail addresses: [email protected] (J.-H. Boo), [email protected] (S.H. Yun). http://dx.doi.org/10.1016/j.materresbull.2015.02.025 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
Wool fiber consists of three layers. The outmost layer of scales is called the cuticle, the middle layer is called the cortex and the inner core is referred to as the medulla Joseph [13]. The surface of the outermost cuticle cells is a fatty acid layer of 18-methyleicosanoic acid covalently bound to the protein layer of the wool cuticle via a thioester linkage [14]. This fatty layer makes the surface of the wool hydrophobic, which prevents it from being directly used as a microfluidic channel [15]. To utilize wool thread, the wetting properties of the surface should be modified to introduce hydrophilicity. We were able to produce hydrophilic wool thread by using a DBD plasma with various gases. The penetration of the activating species in the DBD plasma into the fiber is shallow, so the middle and inner layers were hardly affected [16]. This is why the DBD plasma is used as the method of modifying the surface properties of the wool fiber without changing the middle and inner layers. For these reasons, we focus on the outmost layer to demonstrate why the wettability was changed. We obtained SEM images of the pristine wool fibers and those treated with the plasma. We found that the damage to the surface increases as the plasma treatment time increases [17]. We used oxygen, nitrogen and argon gases and air as the plasma generating gas. We also controlled the plasma treatment time. The wool fibers were treated with the plasma for 15, 30, 45 and 60 min. The FTIR spectra were obtained to investigate which functional group was changed. Finally, we made a simple microfluidics channel with the treated wool fibers and successfully transferred a solution containing water and soluble ink. We confirmed that it
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Fig. 1. (A) SEM image of pristine wool fiber. Inset: contact angle of water on the pristine wool fabric. (B) structure of outmost layer of wool fiber. (C), (D) structure of protein matrix.
was possible to apply this procedure to microfluidic devices with a very simple process. 2. Experimental
The fibers were obtained from the wool fabric and put on the plate of the DBD plasma device. The plasma is generated by the gas and AC power [19]. The wool fibers were treated by the plasma for 15, 30, 45 and 60 min with each gas.
2.1. Preparation of wool fiber The liquid wetting dynamics on a fabric are influenced by the fabric geometry and fiber surface properties. To demonstrate the effect of the surface properties, we used a single kind of wool fiber. We measured the contact angle to investigate the wettability of the wool fabric. The inset image of Fig. 1(A) shows the snapshots of a water droplet on the pristine wool fabric [18]. The image shows that it is absolutely hydrophobic. As shown in Fig. 1(A), wool fiber consists of many layers. Because the penetration of the activating species in the DBD plasma into the fiber is shallow, we focus on the change of the surface. The outmost layer of the wool fiber is called the cuticle, which is a fatty acid layer of 18-methyleicosanoic acid covalently bound to the protein layer via a thioester linkage, as shown in Fig. 1(B). This fatty layer makes the wool fiber hydrophobic. The protein matrix structure is shown in Fig. 1(C) and (D). It contains disulfide bonds, hydrogen bonds and ion–ion interactions. 2.2. DBD plasma treatment Fig. 2 shows the schematic structure of the DBD plasma device. We used O2, N2 and Ar gases and air to generate the plasma.
Fig. 2. The schematic structure of DBD plasma device.
S.-H. Jeon et al. / Materials Research Bulletin 69 (2015) 65–70
2.3. Measure of wicking rate As shown in Fig. 3(E), a simple channel of wool fiber is made to investigate the wicking rate of each fiber. On the slide glass, we attached double sided tape from 3M to attach the fibers to the glass. A Kimtech wiper was cut into a rectangular block with a length and width of 5 mm and 75 mm, respectively, and attached to the right end side of the glass for the purpose of loading the ink solution. The pristine and treated wool fibers were attached to the double sided tape. 40 ml of blue ink solution was loaded on the Kimtech wipers. We took a picture when the wicking of the solution was completed and calculated the wicking rate from the measured wicking time. Four channels for the oxygen, nitrogen, argon and air plasmas, respectively, were fabricated to investigate the effect of the treatment time. On each channel, the pristine wool and wool fibers which were treated for 15, 30, 45 and 60 min were attached. 2.4. Fabrication of simple microfluidic device We made a simple microfluidic device to investigate the micro-mixing properties. Double sided tape from 3M was attached to the slide glass. Before attaching it to the glass, we made two 5 mm holes, which are located on the right side of the device for the loading of the ink solution. The surface of the glass is hydrophilic, so the ink solution was able to be placed on the holes. On the left side of the device, two pieces of Kimtech wipers were attached for
67
the purpose of detecting the mixing solution. The Kimtech wipers was cut onto circles with a diameter of 5 mm. Two fibers were attached to the tape in the shape as shown in Fig. 7. 20 ml of blue and red ink solutions were loaded on each hole at the same time. When the mixing color was observed on the Kimtech wipers, indicating that the micro-mixing was successful, we took a picture, as shown in Fig. 7. This showed that it was possible to apply this procedure to the microfluidic device. 3. Results and discussion 3.1. Wicking rate according to treatment time Fig. 3 shows the wicking of the wool according to the plasma treatment time. Each glass includes wool fibers which were treated with either the air, argon, nitrogen or oxygen plasma. As shown in the first slide glass, the wicking speed increases with increasing treatment time [17]. This phenomenon is also observed for the other glasses. The short red lines indicate how long the ink solution was transported and the wicking rates of the wool fibers are calculated using the measured wicking time, as shown in Fig. 4. 3.2. Damage on surface of the wool fibers (scanning electron microscopy images) To investigate the surface morphology of the wool fibers, scanning electron microscopy (SEM) was used. As shown in Fig. 5,
Fig. 3. (A) Wool fibers treated with oxygen plasma. (B) Wool fibers treated with argon plasma. (C) Wool fibers treated with nitrogen plasma. (D) Wool fibers treated with air plasma. (A), (B), (C), (D) Pristine wool fiber and fibers treated for 15, 30, 45 and 60 min, respectively from upside. (E) The schematic structure of channels. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
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each wool fiber was damaged to an extent which increased with increasing treatment time. The oxygen plasma was so powerful that a treatment time of just 30 min was sufficient, however no more damage was caused to the fibers which were treated for 45 and 60 min. Some of the fatty acid on the surface of the wool fibers was damaged. For these reasons, the wettability of the wool fiber was increased [20,21].
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The FTIR spectra were measured to investigate the change of the functional groups on the surface of the wool fibers. As shown in Fig. 6, the peak at 1700 cm 1 is attributed to the thioester group [22]. To compare the FTIR spectra of the wool fibers as a function of the treatment time, the spectra of the wool fibers which were treated with the same gas but for different treatment times are shown together. Fig. 6(A) shows that the peak for the thioester group decreases with increasing treatment time when oxygen gas
Fig. 5. SEM images of wool fibers which treated with air plasma for 15 (A), 30 (B), 45 (C), 60 min (D), treated with argon plasma for 15 (E), 30 (F), 45(G), 60 min (H), treated with nitrogen plasma for 15 (I), 30 (J), 45 (K), 60 min (L), and treated with oxygen plasma for 5 (I), 15 (J), 30 (K).
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wavenumber(cm ) Fig. 6. FTIR spectra of wool fibers which are pristine and treated with oxygen plasma (A), and treated with argon plasma (B), and treated with nitrogen plasma (C). Inset images are spectra of the range of 1400–1750 cm 1.
was used. This phenomenon is also observed in Fig. 6(B) and (C) which shows the spectra for the wool fibers treated with the argon and nitrogen plasmas, respectively. Under the influence of the DBD plasma treatment, the cysteine groups present on the surface of the wool fiber were changed into cysteric acid groups. This is the main reason for the increase in the wettability. The improvement of the wettability is due to the removal of the fatty acid layer which is on the surface of the outmost cuticle. 3.4. Fabrication of simple microfluidic device We designed a simple microfluidic device, as shown in Fig. 7. Wool fibers which were treated with the oxygen, argon and nitrogen plasmas for 30, 45, and 60 min were used for making the device, because the fibers treated with the air plasma or treated for only 15 min were not rapid enough to apply to the microfluidic device. Two kinds of ink solutions were loaded on the holes on the right end side of the device at the same time. Red and blue ink solutions were dropped on the upper and lower holes, respectively. After a few minutes, we found that the Kimtech wipers were dyed a purple color, which is a mixture of red and blue. The micro-mixing of the solutions is successful on this device. It indicates that the
wool fibers could be applied to a microfluidic device with a very simple and low-cost process. 4. Conclusions The effects of a DBD plasma with various gases, viz. oxygen, argon, nitrogen and air, on wool fiber were investigated. The treatment time was also controlled and the wicking rate was found to increase with increasing treatment time. SEM images were obtained to investigate the surface morphology. The damage to the wool surface increased with increasing treatment time. The FTIR spectra were measured to investigate the change of the functional groups on the surface. The peak for the thioester group decreased with increasing treatment time. The cysteine groups, which link the fatty acid and protein layers, were changed into cysteric acid groups. In other words, the improvement of the wettability is due to the removal of the fatty acid layer which is on the surface of the outmost cuticle. The wicking rates of the wool fibers increase in the order: oxygen, argon, nitrogen and air plasma. As a result, the oxygen plasma is the most effective to change the wettability of the wool fiber.
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Fig. 7. Simple microfluidic devices with wool fiber which are treated with oxygen plasma for 5 min (A), 15 min (B), 30 min (C), and with argon plasma for 30 min (D), 45 min (E), 60 min (F), with nitrogen plasma for 30 min (G), 45 min (H), 60 min (I). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
A simple microfluidic device was fabricated with the treated wool fibers. It transferred the solutions successfully and performed micro-mixing. This indicates that the DBD plasma treatment of the wool fibers can be applied to microfluidic devices. It is very promising because the process is simple and inexpensive. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2010-0027963) and Agency for Defense Development through Chemical and Biological Defense Research Center (CBD-12). References [1] [2] [3] [4]
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