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Microfluidic connectors by ultrasonic welding
Microelectronic Engineering 86 (2009) 1354–1357
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Microfluidic connectors by ultrasonic welding S.H. Ng a,*, Z.F. Wang a, N.F. de Rooij b a b
Singapore Institute of Manufacturing Technology, Microfluidic Devices Manufacturing Programme, 71 Nanyang Drive, Singapore 638075, Singapore Institute of Microtechnology, University of Neuchâtel, Jaquet-Droz 1, 2002 Neuchâtel, Switzerland
a r t i c l e
i n f o
Article history: Received 27 August 2008 Received in revised form 20 January 2009 Accepted 20 January 2009 Available online 29 January 2009 Keywords: Microfluidics Dead-volume Connector Ultrasonic Welding Polymer
a b s t r a c t We attempted ultrasonic welding as a technique for the bonding of connectors to microfluidic devices. Different schemes of the method were explored based on different designs of the connectors to achieve a strong, minimal dead-volume connection. Results showed that without the use of inserts, there would be unwanted flow of melt material into the conduit of the connector causing blockage. With the small scale of welding, the use of inserts and proper fixture designs were necessary to block or direct melt flow during the process. The process times were well within 1 s. Two connectors were also simultaneously welded on a microfluidic device to demonstrate the possibility of further increase in productivity in the technique. The device was able to withstand a minimum of 6 bars (gauge) pressure. Where dead-volume is not a concern, it was possible to create connection without the use of an insert by employing different connector geometry and changing the scheme of the welding. It was found out that the width of the welding zone is limited by the width of the sleeve (application of ultrasonic energy to the connector). Hence, the welding zone would proceed no further towards the conduit and causing blockage. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Microfluidic devices have gained tremendous interest in both academic and industrial research due to key advantages such as fast response times and low analyte consumption. Polymer-based microfluidic devices have seen tremendous interest in recent years due to the low material cost and their associated mass production techniques. While a lot of work has been focused on ‘‘within chip” research such as creating micro mixers, micro valves, on-chip micro motors, and so on; one of the non-trivial challenges is the efficient connection of the microfluidic device to external devices such as syringe pumps, autosamplers, pretreatment and analysis devices. This has been referred to as the macro-to-micro interface, interconnect, connector, or world-to-chip interface in the research community. In many microfluidic device or lab chip, there will be inlet and/or outlet holes where fluid can flow in and out. The first question is: what is a suitable technique to connect fluid supply or tubes (in most cases) to these holes? The second question to be addressed is: how can the dead-volume be minimized in such connections? A dead-volume refers to places in the fluidic channel where fluid flow velocity is zero or close to zero (except at the channel walls) such that there is a stagnation or ‘‘fluid trap.” This is a serious problem in microfluidics and can lead to contamination and dilution issues. It also leads to increased reagent usage. In micro total analysis systems (lTAS), dead-volume could lead to the
* Corresponding author. Tel.: +65 67938382; fax: +65 67916377. E-mail address: [email protected] (S.H. Ng). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.048
generation of spurious signals or excessive base-line signals. CAC. Lin et al. [1] reported ‘‘band broadening” in the detection signal of capillary electrophoresis due to the dilution effect caused by dead-volume in the connection. Most connection methods so far have been based on adhesive, force-fitting, screw fittings or combinations of the above-mentioned techniques [2–8]. Although polymer-based microfluidic devices are gaining popularity and the bonding issues getting more important, there is still limited application of ultrasonic welding in the fabrication of microfluidic devices. Truckenmuller et al. [9] reported using micro ultrasonic welding in the sealing of microchannels and assembly of a micropump. In this paper, we attempted the use of ultrasonic welding for the creation of microfluidic connectors and explore the different schemes to achieve it. The interdiffusion of material between the two interfaces during ultrasonic welding resulted in an interlocking network of polymers chains across the welding zone, giving rise to a homogeneous weld that is sufficiently strong for most microfluidics applications. 2. Ultrasonic welding of plastics Ultrasonic welding is one of the most popular methods for joining thermoplastics [10]. It is used in mass production because it is both fast and economical. Cycles times are typically on the order of a second. The equipment is compact and the process easily automated. During the process, low amplitude (2.5–250 lm) high frequency (20–40 kHz) vibrations are applied to the workpiece [11].
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A typical ultrasonic welder consists of a power supply, actuator and a converter-booster-stack. The power supply converts the 50/ 60 Hz mains current to 20–40 kHz electrical energy. The actuator generates the stroke and welding pressure. The ultrasonic vibrations are produced in the converter-booster-horn stack. The converter contains piezoelectric or magnetostrictive elements that convert electrical energy to mechanical energy. The converter is normally attached to a booster that amplifies the vibration. The booster in turn is connected to the horn (or sonotrode) that contacts the workpiece during ultrasonic welding. The workpiece is held firmly in an anvil or fixture below the horn. In the ultrasonic welding of plastics, localised melting happens at the contact interface. The melt is squeezed out of the welding zone due to the pressure applied and solidifies when the energy is cut off. Fig. 2. Setup for Scheme 1 using an insert. The horn contacts the connector directly.
3. Experiment The ultrasonic bonding of polymethyl methacrylate (PMMA) connectors to PMMA substrates were carried out. The connectors were 5–10 mm long rod with external diameters of 3 mm. A 1 mm diameter conduit went through the central axis of the connectors. They were fabricated by machining PMMA rods on a lathe. Different connector designs were explored as seen in Fig. 1. An ultrasonic welder (Branson 921 AES) with a maximum power of 2 kW and maximum clamping force of 2.8 kN was employed. The converter had a frequency of 20 kHz. A 1:2.5 booster was used. The stepped rectangular horn was made of aluminium. Fixtures were fabricated for the aligning and securing the connector to the substrate. The test substrate was a 1 mm thick PMMA chip with a 1 mm diameter hole drilled through. Figs. 2 and 3 show the schematic of the setups for the two schemes. The substrate was secured tightly in the fixture and tolerances were tight to prevent large areas of flash. In Scheme 1, an insert was used to prevent melt from entering the conduit. The horn contacts the connector directly. In Scheme 2, the horn contacts a cylindrical sleeve instead of the connector. Similarly, the fixture helped direct the flow of the melt. An insert could also be used, if necessary.
50 lm. The minimum amplitude for bonding to occur was 41.5 lm. Fig. 4 shows a cross sectional, scanning electron micrograph of a connector ultrasonically welded to a substrate by Scheme 1 but without using the insert. There was a blockage in the conduit by the molten material as the interface melts. Most of the molten material was directed towards the conduit as the outside of the connector was surrounded by the fixture. A small flash could still be seen at the outer edge of the connector. The connector and substrate were completely welded together as seen by the homogeneity of the welding zone. Since the flow of molten material was also restricted by the fixture below the substrate, most the flow went up along the conduit of the connector. As a re-
4. Results and discussion To look at the feasibility of ultrasonically welding the connector to the substrate without the use of inserts, the CD1 connector was used together with Scheme 1 but with the insert removed. The amplitude of the ultrasonic vibrations was varied from 25 to
Fig. 3. Setup for Scheme 2 without using an insert. The horn contacts a sleeve instead of the connector.
Fig. 1. Connector designs (CD) 1, 2 and 3.
Fig. 4. Scanning electron micrograph of the cross section of a blocked connector.
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Fig. 5. Connector shortening and blockage without using insert.
sult of the flow of material away from the welding zone, the overall length of the connector was also reduced as seen in Fig. 5. The definitions of the dimensional parameters used in the figure are as follows: a1 original length of the connector s thickness of the substrate a2 length of the connector after welding b length of conduit unblocked We looked at the percentage reduction in the length of the connector, a as well as the percentage of blockage of the conduit, b after the welding process:
a (%) = 100 (a1
a2)/a1 b (%) = 100 (a2 + s b)/(a2 + s) Fig. 6 shows plots of reduction in connector length and degree of blockage of the conduit versus weld time and trigger force. Increasing the weld time or trigger force resulted in increases in both a and b. Weld time is the amount of time that amplitude and pressure are applied to the part. Generally, increasing the weld time increases the weld strength. Weld time that is too long may result in excessive melting and flashing. In this case, the increases in weld time results in more melt material; hence further shortening the connector and causing a bigger blockage. At 300 ms, the
Fig. 6. Effects of (1) weld time and (2) trigger force on the shortening of connector and blockage of conduit.
Fig. 7. Scanning electron micrograph of the cross section of a welded connector using Scheme 1.
connector is not bonded to the substrate. It was reported that the weld time and amplitude of vibration were the principal factors affecting the joint property of ultrasonically welded thermoplastics [12]. The trigger force is the force required to start the ultrasonic portion of the weld cycle. Hence, a higher compressive stress existed in the welding zone due to higher trigger force. This resulted in the increase in blockage and connection shortening. In the range of conditions tested, there was flow of melt material into the conduit (with varying degrees). Lower degree of blockage generally means weaker bonding, and vice versa. Without using an insert, the flow of melt into the conduit could not be stopped. More experiments were conducted using insert as shown in Scheme 1. Fig. 7 shows the cross section of a well bonded connector (CD2) using a scheme with insert. The welding zone was homogeneous and the conduit unblocked, creating a minimal deadvolume path that fluids could pass through. The insert was able to prevent the flow of melt material into the conduit. The taper at the top of the connector was created after the ultrasonic welding. Fig. 8 shows the cross section of a connector of different design (CD3) where no insert was required. The taper in the design of the connector acted like an energy director initiating the melting of material. Although no insert was used, the conduit was not blocked because there was still some distance between the welding zone and the conduit. However, a dead-volume was generated because the taper did not collapse completely. The dead-volume region is the area shown in the figure where there could be a tremendous slow down of fluid velocity or stagnation of fluid due to the geometry, as compared to the fluid velocity traveling through the rest of the conduit (whereas in Fig. 7, there are no such regions that could cause fluid to be trapped). This scheme could be used if dead-vol-
Fig. 8. Scanning electron micrograph of the cross section of a welded connector using Scheme 2.
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of using insert in the conduit to prevent blockage due to melt material when creating a connection with minimal dead-volume. Proper fixture design is also required to impede or direct melt material. In most lab chips, there will be multiple ports for fluid to flow in and out. The bonding can be easily configured to achieve multiple bonding of connectors simultaneously. The horn will then consist of multiple contact points with each connector. The bonding process can be automated for mass production. Acknowledgement This research is funded by the Agency for Science, Technology and Research (A*STAR), Singapore. Fig. 9. Simultaneously bonded multiple connectors on a device.
ume is not a concern. It was found out that the width of the welding zone is limited by the width of the sleeve (application of ultrasonic energy to the connector). Hence, the welding zone would proceed no further towards the conduit and causing blockage. To further demonstrate the technique, a solvent bonded monolithic microfluidic device made of PMMA was created (see Fig. 9). Then, two connectors were bonded simultaneously in a single welding step. A flow test was conducted and did not show any blockage in the microchannel. There was no noticeable deformation in the microchannel as a result of the ultrasonic welding when observed under a microscope. The chip was also pressure tested with the bubble emission technique as described by Ng et al. [13]. It was able to withstand a minimum of 6 bars (gauge) pressure for at least 10 min. 5. Conclusions Ultrasonic welding as a technique for the creation of microfluidics connector has been explored. The study shows the importance
References [1] C.-C. Lin, G.-B. Lee, S.-H. Chen, Electrophoresis 23 (2002) 3550. [2] B.L. Gray, S.D. Collins, R.L. Smith, Sensors and Actuators A 112 (2004) 18. [3] C.-H. Chiou, G.-B. Lee, Journal of Micromechanics and Microengineering 14 (2004) 1484. [4] A.M. Christensen, D.A. Chang-Yen, B.K. Gale, Journal of Micromechanics and Microengineering 15 (2005) 928. [5] A. Puntambekar, C.H. Ahn, Journal of Micromechanics and Microengineering 12 (2002) 35. [6] B.L. Gray, D. Jaeggi, N.J. Mourlas, B.P. van Drieenhuizen, K.R. Williams, N.I. Maluf, G.T.A. Kovacs, Sensors and Actuators A: Physical 77 (1999) 57. [7] J.-H. Tsai, L. Lin, Journal of Micromechanics and Microengineering 11 (2001) 577. [8] R.F. Renzi, US patent 6926313B1, 2005. [9] R. Truckenmuller, Y. Cheng, R. Ahrens, H. Bahrs, G. Fischer, J. Lehmann, Microsystem Technologies 12 (2006) 1027. [10] V.K. Stokes, Polymer Engineering and Science 29 (1989) 1310. [11] A. Benatar, R.V. Eswaran, S.K. Nayar, Polymer Engineering and Science 29 (1989) 1689. [12] S.-J. Liu, W.-F. Lin, B.-C. Chang, G.-M. Wu, Advances in Polymer Technology 18 (1999) 125. [13] S.H. Ng, R.T. Tjeung, Z.F. Wang, A.C.W. Lu, I. Rodriguez, N.F. de Rooij, Microsystem Technologies 14 (2008) 753.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Microfluidic connectors by ultrasonic welding S.H. Ng a,*, Z.F. Wang a, N.F. de Rooij b a b
Singapore Institute of Manufacturing Technology, Microfluidic Devices Manufacturing Programme, 71 Nanyang Drive, Singapore 638075, Singapore Institute of Microtechnology, University of Neuchâtel, Jaquet-Droz 1, 2002 Neuchâtel, Switzerland
a r t i c l e
i n f o
Article history: Received 27 August 2008 Received in revised form 20 January 2009 Accepted 20 January 2009 Available online 29 January 2009 Keywords: Microfluidics Dead-volume Connector Ultrasonic Welding Polymer
a b s t r a c t We attempted ultrasonic welding as a technique for the bonding of connectors to microfluidic devices. Different schemes of the method were explored based on different designs of the connectors to achieve a strong, minimal dead-volume connection. Results showed that without the use of inserts, there would be unwanted flow of melt material into the conduit of the connector causing blockage. With the small scale of welding, the use of inserts and proper fixture designs were necessary to block or direct melt flow during the process. The process times were well within 1 s. Two connectors were also simultaneously welded on a microfluidic device to demonstrate the possibility of further increase in productivity in the technique. The device was able to withstand a minimum of 6 bars (gauge) pressure. Where dead-volume is not a concern, it was possible to create connection without the use of an insert by employing different connector geometry and changing the scheme of the welding. It was found out that the width of the welding zone is limited by the width of the sleeve (application of ultrasonic energy to the connector). Hence, the welding zone would proceed no further towards the conduit and causing blockage. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Microfluidic devices have gained tremendous interest in both academic and industrial research due to key advantages such as fast response times and low analyte consumption. Polymer-based microfluidic devices have seen tremendous interest in recent years due to the low material cost and their associated mass production techniques. While a lot of work has been focused on ‘‘within chip” research such as creating micro mixers, micro valves, on-chip micro motors, and so on; one of the non-trivial challenges is the efficient connection of the microfluidic device to external devices such as syringe pumps, autosamplers, pretreatment and analysis devices. This has been referred to as the macro-to-micro interface, interconnect, connector, or world-to-chip interface in the research community. In many microfluidic device or lab chip, there will be inlet and/or outlet holes where fluid can flow in and out. The first question is: what is a suitable technique to connect fluid supply or tubes (in most cases) to these holes? The second question to be addressed is: how can the dead-volume be minimized in such connections? A dead-volume refers to places in the fluidic channel where fluid flow velocity is zero or close to zero (except at the channel walls) such that there is a stagnation or ‘‘fluid trap.” This is a serious problem in microfluidics and can lead to contamination and dilution issues. It also leads to increased reagent usage. In micro total analysis systems (lTAS), dead-volume could lead to the
* Corresponding author. Tel.: +65 67938382; fax: +65 67916377. E-mail address: [email protected] (S.H. Ng). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.048
generation of spurious signals or excessive base-line signals. CAC. Lin et al. [1] reported ‘‘band broadening” in the detection signal of capillary electrophoresis due to the dilution effect caused by dead-volume in the connection. Most connection methods so far have been based on adhesive, force-fitting, screw fittings or combinations of the above-mentioned techniques [2–8]. Although polymer-based microfluidic devices are gaining popularity and the bonding issues getting more important, there is still limited application of ultrasonic welding in the fabrication of microfluidic devices. Truckenmuller et al. [9] reported using micro ultrasonic welding in the sealing of microchannels and assembly of a micropump. In this paper, we attempted the use of ultrasonic welding for the creation of microfluidic connectors and explore the different schemes to achieve it. The interdiffusion of material between the two interfaces during ultrasonic welding resulted in an interlocking network of polymers chains across the welding zone, giving rise to a homogeneous weld that is sufficiently strong for most microfluidics applications. 2. Ultrasonic welding of plastics Ultrasonic welding is one of the most popular methods for joining thermoplastics [10]. It is used in mass production because it is both fast and economical. Cycles times are typically on the order of a second. The equipment is compact and the process easily automated. During the process, low amplitude (2.5–250 lm) high frequency (20–40 kHz) vibrations are applied to the workpiece [11].
S.H. Ng et al. / Microelectronic Engineering 86 (2009) 1354–1357
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A typical ultrasonic welder consists of a power supply, actuator and a converter-booster-stack. The power supply converts the 50/ 60 Hz mains current to 20–40 kHz electrical energy. The actuator generates the stroke and welding pressure. The ultrasonic vibrations are produced in the converter-booster-horn stack. The converter contains piezoelectric or magnetostrictive elements that convert electrical energy to mechanical energy. The converter is normally attached to a booster that amplifies the vibration. The booster in turn is connected to the horn (or sonotrode) that contacts the workpiece during ultrasonic welding. The workpiece is held firmly in an anvil or fixture below the horn. In the ultrasonic welding of plastics, localised melting happens at the contact interface. The melt is squeezed out of the welding zone due to the pressure applied and solidifies when the energy is cut off. Fig. 2. Setup for Scheme 1 using an insert. The horn contacts the connector directly.
3. Experiment The ultrasonic bonding of polymethyl methacrylate (PMMA) connectors to PMMA substrates were carried out. The connectors were 5–10 mm long rod with external diameters of 3 mm. A 1 mm diameter conduit went through the central axis of the connectors. They were fabricated by machining PMMA rods on a lathe. Different connector designs were explored as seen in Fig. 1. An ultrasonic welder (Branson 921 AES) with a maximum power of 2 kW and maximum clamping force of 2.8 kN was employed. The converter had a frequency of 20 kHz. A 1:2.5 booster was used. The stepped rectangular horn was made of aluminium. Fixtures were fabricated for the aligning and securing the connector to the substrate. The test substrate was a 1 mm thick PMMA chip with a 1 mm diameter hole drilled through. Figs. 2 and 3 show the schematic of the setups for the two schemes. The substrate was secured tightly in the fixture and tolerances were tight to prevent large areas of flash. In Scheme 1, an insert was used to prevent melt from entering the conduit. The horn contacts the connector directly. In Scheme 2, the horn contacts a cylindrical sleeve instead of the connector. Similarly, the fixture helped direct the flow of the melt. An insert could also be used, if necessary.
50 lm. The minimum amplitude for bonding to occur was 41.5 lm. Fig. 4 shows a cross sectional, scanning electron micrograph of a connector ultrasonically welded to a substrate by Scheme 1 but without using the insert. There was a blockage in the conduit by the molten material as the interface melts. Most of the molten material was directed towards the conduit as the outside of the connector was surrounded by the fixture. A small flash could still be seen at the outer edge of the connector. The connector and substrate were completely welded together as seen by the homogeneity of the welding zone. Since the flow of molten material was also restricted by the fixture below the substrate, most the flow went up along the conduit of the connector. As a re-
4. Results and discussion To look at the feasibility of ultrasonically welding the connector to the substrate without the use of inserts, the CD1 connector was used together with Scheme 1 but with the insert removed. The amplitude of the ultrasonic vibrations was varied from 25 to
Fig. 3. Setup for Scheme 2 without using an insert. The horn contacts a sleeve instead of the connector.
Fig. 1. Connector designs (CD) 1, 2 and 3.
Fig. 4. Scanning electron micrograph of the cross section of a blocked connector.
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Fig. 5. Connector shortening and blockage without using insert.
sult of the flow of material away from the welding zone, the overall length of the connector was also reduced as seen in Fig. 5. The definitions of the dimensional parameters used in the figure are as follows: a1 original length of the connector s thickness of the substrate a2 length of the connector after welding b length of conduit unblocked We looked at the percentage reduction in the length of the connector, a as well as the percentage of blockage of the conduit, b after the welding process:
a (%) = 100 (a1
a2)/a1 b (%) = 100 (a2 + s b)/(a2 + s) Fig. 6 shows plots of reduction in connector length and degree of blockage of the conduit versus weld time and trigger force. Increasing the weld time or trigger force resulted in increases in both a and b. Weld time is the amount of time that amplitude and pressure are applied to the part. Generally, increasing the weld time increases the weld strength. Weld time that is too long may result in excessive melting and flashing. In this case, the increases in weld time results in more melt material; hence further shortening the connector and causing a bigger blockage. At 300 ms, the
Fig. 6. Effects of (1) weld time and (2) trigger force on the shortening of connector and blockage of conduit.
Fig. 7. Scanning electron micrograph of the cross section of a welded connector using Scheme 1.
connector is not bonded to the substrate. It was reported that the weld time and amplitude of vibration were the principal factors affecting the joint property of ultrasonically welded thermoplastics [12]. The trigger force is the force required to start the ultrasonic portion of the weld cycle. Hence, a higher compressive stress existed in the welding zone due to higher trigger force. This resulted in the increase in blockage and connection shortening. In the range of conditions tested, there was flow of melt material into the conduit (with varying degrees). Lower degree of blockage generally means weaker bonding, and vice versa. Without using an insert, the flow of melt into the conduit could not be stopped. More experiments were conducted using insert as shown in Scheme 1. Fig. 7 shows the cross section of a well bonded connector (CD2) using a scheme with insert. The welding zone was homogeneous and the conduit unblocked, creating a minimal deadvolume path that fluids could pass through. The insert was able to prevent the flow of melt material into the conduit. The taper at the top of the connector was created after the ultrasonic welding. Fig. 8 shows the cross section of a connector of different design (CD3) where no insert was required. The taper in the design of the connector acted like an energy director initiating the melting of material. Although no insert was used, the conduit was not blocked because there was still some distance between the welding zone and the conduit. However, a dead-volume was generated because the taper did not collapse completely. The dead-volume region is the area shown in the figure where there could be a tremendous slow down of fluid velocity or stagnation of fluid due to the geometry, as compared to the fluid velocity traveling through the rest of the conduit (whereas in Fig. 7, there are no such regions that could cause fluid to be trapped). This scheme could be used if dead-vol-
Fig. 8. Scanning electron micrograph of the cross section of a welded connector using Scheme 2.
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of using insert in the conduit to prevent blockage due to melt material when creating a connection with minimal dead-volume. Proper fixture design is also required to impede or direct melt material. In most lab chips, there will be multiple ports for fluid to flow in and out. The bonding can be easily configured to achieve multiple bonding of connectors simultaneously. The horn will then consist of multiple contact points with each connector. The bonding process can be automated for mass production. Acknowledgement This research is funded by the Agency for Science, Technology and Research (A*STAR), Singapore. Fig. 9. Simultaneously bonded multiple connectors on a device.
ume is not a concern. It was found out that the width of the welding zone is limited by the width of the sleeve (application of ultrasonic energy to the connector). Hence, the welding zone would proceed no further towards the conduit and causing blockage. To further demonstrate the technique, a solvent bonded monolithic microfluidic device made of PMMA was created (see Fig. 9). Then, two connectors were bonded simultaneously in a single welding step. A flow test was conducted and did not show any blockage in the microchannel. There was no noticeable deformation in the microchannel as a result of the ultrasonic welding when observed under a microscope. The chip was also pressure tested with the bubble emission technique as described by Ng et al. [13]. It was able to withstand a minimum of 6 bars (gauge) pressure for at least 10 min. 5. Conclusions Ultrasonic welding as a technique for the creation of microfluidics connector has been explored. The study shows the importance
References [1] C.-C. Lin, G.-B. Lee, S.-H. Chen, Electrophoresis 23 (2002) 3550. [2] B.L. Gray, S.D. Collins, R.L. Smith, Sensors and Actuators A 112 (2004) 18. [3] C.-H. Chiou, G.-B. Lee, Journal of Micromechanics and Microengineering 14 (2004) 1484. [4] A.M. Christensen, D.A. Chang-Yen, B.K. Gale, Journal of Micromechanics and Microengineering 15 (2005) 928. [5] A. Puntambekar, C.H. Ahn, Journal of Micromechanics and Microengineering 12 (2002) 35. [6] B.L. Gray, D. Jaeggi, N.J. Mourlas, B.P. van Drieenhuizen, K.R. Williams, N.I. Maluf, G.T.A. Kovacs, Sensors and Actuators A: Physical 77 (1999) 57. [7] J.-H. Tsai, L. Lin, Journal of Micromechanics and Microengineering 11 (2001) 577. [8] R.F. Renzi, US patent 6926313B1, 2005. [9] R. Truckenmuller, Y. Cheng, R. Ahrens, H. Bahrs, G. Fischer, J. Lehmann, Microsystem Technologies 12 (2006) 1027. [10] V.K. Stokes, Polymer Engineering and Science 29 (1989) 1310. [11] A. Benatar, R.V. Eswaran, S.K. Nayar, Polymer Engineering and Science 29 (1989) 1689. [12] S.-J. Liu, W.-F. Lin, B.-C. Chang, G.-M. Wu, Advances in Polymer Technology 18 (1999) 125. [13] S.H. Ng, R.T. Tjeung, Z.F. Wang, A.C.W. Lu, I. Rodriguez, N.F. de Rooij, Microsystem Technologies 14 (2008) 753.