- Email: [email protected]
Easy-to-realise Polyvinylsiloxane Microfluidic Connectors for PDMS Chips
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 120 (2015) 675 – 678
EUROSENSORS 2015
Easy-to-realise polyvinylsiloxane microfluidic connectors for PDMS chips S. van den Drieschea,c*, J.V. Pimentela,c, D. Puchberger-Enenglb, L. Brandhoffa,c, M.J. Vellekoopa,c a b
Institute for Microsensors,actuators and -systems, University of Bremen, Otto-Hahn-Allee NW1, 28359, Bremen, Germany Institute of Sensor and Actuator Systems, Vienna University of Technology, Gusshausstrasse 27-29, 1040, Vienna, Austria c Microsystems Center Bremen (MCB), Bremen,Germany
Abstract
The connection of tubes to a polydimethylsiloxane (PDMS) device can usually not be realised easily. Tube connectors embedded in the PDMS structure have a large foot print. Thick layers of PDMS in which the tubes or needles can hold hinders the visualisation of sample within the device. In this contribution we present microfluidic connectors for PDMS chips realized by easy-to-use elastomeric impression material polyvinylsiloxane (PVS). PVS cylinders were added to a pre-cured PDMS chip and ready to use within five minutes without the need of exposure to high temperatures or the use of additional glues. The robustness of the PVS connectors has been demonstrated through pull-tests and liquid pressure tests. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2015The The Authors. Published by Elsevier Ltd. ©2015 (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015. Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
Keywords: polyvinylsiloxane (PVS); polydimethylsiloxane (PDMS); microfluidics;biocompatible; bonding port
1. Introduction PDMS (polydimethylsiloxane) is a biocompatible, optical transparent and cost effective silicone elastomer. It is commonly used as a base material for microfluidic devices. It can be structured by soft lithography and bonded to a glass, silicon, PMMA (polymethylmethacrylate), or other base substrate to fabricate microfluidic channels and
* Corresponding author. Tel.: +49-421-218-62652; fax: +49-421-218-98-62652 E-mail address: [email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
doi:10.1016/j.proeng.2015.08.720
676
S. van den Driesche et al. / Procedia Engineering 120 (2015) 675 – 678
microstructures [1–3]. Also a multilayer soft lithography method was designed where multiple layers of PDMS are bonded at 80 °C for 1.5 hours [4]. Something that cannot be easily realised with PDMS is the connection of tubes. This connection can be realised by using special designed holders with integrated tube connectors, or thick layers of PDMS of more than 2 mm in which a tube or needle can hold connecting the microfluidic chip [5]. These special holders are custom-made and expensive. Thick layers of PDMS have the disadvantage that the optical path-length through this material will be also large, hindering the visualisation of the sample in the channel. Saarela et al. presented a method to realise reusable PDMS fluidic connectors with a reduced occupied area per inlet by using a silicon etched casting setup with dummy fibres around which PDMS can be cured [6]. This method also has the advantage that the inlets are well aligned within the microfluidic chip. In this contribution, we present microfluidic chip connectors realised by easy-to-use elastomeric impression material PVS which is fully compatible to cured PDMS. This flexible, biocompatible, addition curing PVS material has been used by dentists as impression material in the past three decades [7,8]. The attractiveness of this material lays in its curing at room temperature, its short working time of up to two minutes, its very low linear dimensional change (0.1%), and its capability to strongly adhere to cured PDMS. The PVS connectors were created and ready to use all within 5 minutes without the need of exposure to high temperatures or the use of additional glues. The robustness of the PVS connectors has been demonstrated through pull-tests and liquid pressure tests. 2. Materials and Methods We have found that the polymer-polymer adhesion between the two silicones PVS and PDMS is very effective to connect tubes to PDMS. PVS cylinders were created by mixing the two base pastes (Mueller-Omicron Betasil® Vario Light) and filling it into a mould (see Figure 1). The cylinders have a diameter of 3 mm, a height of 5 mm, and an inner through hole of 0.6 mm. With this mould nine cylinders can be made simultaneously within 5 minutes.
(a)
(b)
Fig. 1. View of the connector mould for nine PVS connectors. (a) Two component PVS paste loaded into a metal mould sandwiched between needle alignment plates to realize aligned tube holes; (b) Nine moulded PVS connectors. The needles and top alignment plate have been removed.
The PVS cylinders that are held by a needle are contacted with an uncured PVS sheet. The needle secures the perfect alignment to the chip. After that they are placed in the PDMS inlets and cured for 2 minutes, the PVS connectors are ready to be used. In Figure 2 (a) a PDMS-glass microfluidic chip with two PVS connectors is depicted. PEEK tubing with an outer diameter of 1/32” (0.8 mm) was clamped into the 0.6 mm connection.
S. van den Driesche et al. / Procedia Engineering 120 (2015) 675 – 678
3. Results and Discussion The functionality of the PVS connectors was tested by performing pull-tests and pressure-tests. Pull-tests were made to determine the bonding strength between PVS and PDMS. Seven connectors bonded to PDMS were placed in the pull-test setup (XYZ Condor 100). The force required to release the bonding ports from a PDMS sheet is given in Table 1. (a)
(b)
Fig. 2. (a) Photograph of PVS connectors with 0.8 mm outer diameter PEEK tubes connected to a PDMS-glass based microfluidic chip. The 0.8 mm diameter PEEK tubes are clamped in the 0.6 mm diameter PVS bonding port connectors. The connectors have a diameter of 3 mm and a height of 5 mm. The PDMS layer consists of imprinted channels with a height of 100 μm and a width of 1 to 3.5 mm. To seal the channel, the PDMS was oxygen-plasma bonded to glass; (b) The bonding strength of 3 mm diameter PVS connectors fixed to PDMS was measured with pull tests (XYZ Condor 100). The minimum amount of force required to break the PVS - PDMS bond was 3.6 N (which corresponds to 525 kPa). The pull-test took 8 seconds. Table 1. Pull-test results of seven PVS connectors. PVS connector
Force
Force/Area
[N]
[kPa]
Breakage point
1
4.8
714
PVS breaks
2
5.3
786
PVS breaks
3
4.9
716
PVS breaks
4
3.9
581
PVS breaks
5
3.9
581
PVS breaks
6
3.6
525
Bond breaks
7
3.8
562
Bond breaks
The plot depicted in Figure 2 (b) shows that the minimum amount of force required to release 3 mm in diameter PVS connectors from a PDMS layer is 3.6 N (PVS connector number 6, Table 1). This lowest measured breaking force of the seven connecters corresponds to a bonding strength of approx. 525 kPa which is stronger than oxygen plasma PDMS-PDMS bonding [9] and is strong enough to hold a tube in place. Additionally, water pressure-tests were made to test the connection between the PVS-PDMS bond. Seven PDMS chips were prepared each with two bonded PVS connectors. PEEK tubings were clamped in each bonding port. The pressure of the water flowing through the chip was increased by 50 kPa every 30 seconds by using an EFD 1000 DVE fluid dispenser until leakage occurred or the maximum pressure of 650 kPa was reached. The minimum withstood pressure of the microfluidic devices was 350 kPa. For microfluidic devices this applied pressure is more than sufficient [10]. Three of the seven chips even withstood a pressure of 650 kPa without leaking (see Table 2).
677
678
S. van den Driesche et al. / Procedia Engineering 120 (2015) 675 – 678 Table 2. Pressure-test results of seven PDMS chips each with two PVS connectors. PDMS chip
Withstood pressure
Leakage point
[kPa] 1
350
PDMS/PVS bond
2
400
PDMS/PVS bond
3
400
PDMS/PVS bond
4
650
No leakage
5
350
PDMS/PVS bond
6
650
No leakage
7
650
No leakage
4. Conclusions PVS is an attractive material that can be easily moulded and bonded to PDMS at room temperature without the need of additional glues. PVS bonding ports were created by fixing moulded PVS cylinders, with an inner hole to connect PEEK tubing, to PDMS chips within 5 minutes. Pull-test and pressure-test results shows the high bonding strength between the two silicones. The presented PDMS-chip connector by moulded PVS is an easy way of connecting PDMS microfluidic devices.
Acknowledgments This project is a part of an EU Marie Curie Initial Training Network (ITN): biomedical engineering for cancer and brain disease diagnosis and therapy development, EngCaBra. Proj. no. PITN-GA-2010-264417. The authors would like to thank Parth Jhaveri for designing the connector mould as part of his master project at IMSAS, University of Bremen.
References [1] D.C. Duffy, J.C. McDonald, O.J. Schueller, G.M. Whitesides (1998): Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane). Anal. Chem. 70, 4974–84. [2] T. Fujii (2002): PDMS-based microfluidic devices for biomedical applications. Microelectron. Eng. 61-62, 907–914. [3] S. van den Driesche, V. Rao, D. Puchberger-Enengl, W. Witarski, M.J. Vellekoop (2012): Continuous cell from cell separation by traveling wave dielectrophoresis. Sensors Actuators B Chem. 170, 207–214. [4] M.A. Unger, H.P. Chou, T. Thorsen, .A. Scherer, S.R. Quake (2000): Monolithic microfabricated valves and pumps by multilayer soft lithography, Science 288, 113–116. [5] P.M. van Midwoud, G.M.M. Groothuis, M.T. Merema, E. Verpoorte (2010): Microfluidic biochip for the perifusion of precision-cut rat liver slices for metabolism and toxicology studies, Biotechnol. Bioeng. 105, 184–94. [6] V. Saarela, S. Franssila, S. Tuomikoski, S. Marttila, P. Östman, T. Sikanen, T. Kotiaho, R. Kostiainen (2006): Re-usable multi-inlet PDMS fluidic connector, Sensors Actuators B Chem.114, 552–557. [7] W.W. Chee, T.E. Donovan (1992): Polyvinyl siloxane impression materials: A review of properties and techniques, J. Prosthet. Dent. 68, 728– 732. [8] D. Nowakowska, Z. Raszewski, J. Saczko, J. Kulbacka, W. Wieckiewicz (2014): Polymerization time compatibility index of polyvinyl siloxane impression materials with conventional and experimental gingival margin displacement agents, J. Prosthet. Dent. 112, 168–75. [9] M.A. Eddings, M.A. Johnson, B.K. Gale (2008): Determining the optimal PDMS-PDMS bonding technique for microfluidic devices, J. Micromech. Microeng.18, 067001. [10] S.K. Sia, G.M. Whitesides (2003): Microfluidic devices fabricated in poly(dimethylsiloxane) for biological studies, Electrophoresis 24, 356376.
ScienceDirect Procedia Engineering 120 (2015) 675 – 678
EUROSENSORS 2015
Easy-to-realise polyvinylsiloxane microfluidic connectors for PDMS chips S. van den Drieschea,c*, J.V. Pimentela,c, D. Puchberger-Enenglb, L. Brandhoffa,c, M.J. Vellekoopa,c a b
Institute for Microsensors,actuators and -systems, University of Bremen, Otto-Hahn-Allee NW1, 28359, Bremen, Germany Institute of Sensor and Actuator Systems, Vienna University of Technology, Gusshausstrasse 27-29, 1040, Vienna, Austria c Microsystems Center Bremen (MCB), Bremen,Germany
Abstract
The connection of tubes to a polydimethylsiloxane (PDMS) device can usually not be realised easily. Tube connectors embedded in the PDMS structure have a large foot print. Thick layers of PDMS in which the tubes or needles can hold hinders the visualisation of sample within the device. In this contribution we present microfluidic connectors for PDMS chips realized by easy-to-use elastomeric impression material polyvinylsiloxane (PVS). PVS cylinders were added to a pre-cured PDMS chip and ready to use within five minutes without the need of exposure to high temperatures or the use of additional glues. The robustness of the PVS connectors has been demonstrated through pull-tests and liquid pressure tests. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2015The The Authors. Published by Elsevier Ltd. ©2015 (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015. Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
Keywords: polyvinylsiloxane (PVS); polydimethylsiloxane (PDMS); microfluidics;biocompatible; bonding port
1. Introduction PDMS (polydimethylsiloxane) is a biocompatible, optical transparent and cost effective silicone elastomer. It is commonly used as a base material for microfluidic devices. It can be structured by soft lithography and bonded to a glass, silicon, PMMA (polymethylmethacrylate), or other base substrate to fabricate microfluidic channels and
* Corresponding author. Tel.: +49-421-218-62652; fax: +49-421-218-98-62652 E-mail address: [email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
doi:10.1016/j.proeng.2015.08.720
676
S. van den Driesche et al. / Procedia Engineering 120 (2015) 675 – 678
microstructures [1–3]. Also a multilayer soft lithography method was designed where multiple layers of PDMS are bonded at 80 °C for 1.5 hours [4]. Something that cannot be easily realised with PDMS is the connection of tubes. This connection can be realised by using special designed holders with integrated tube connectors, or thick layers of PDMS of more than 2 mm in which a tube or needle can hold connecting the microfluidic chip [5]. These special holders are custom-made and expensive. Thick layers of PDMS have the disadvantage that the optical path-length through this material will be also large, hindering the visualisation of the sample in the channel. Saarela et al. presented a method to realise reusable PDMS fluidic connectors with a reduced occupied area per inlet by using a silicon etched casting setup with dummy fibres around which PDMS can be cured [6]. This method also has the advantage that the inlets are well aligned within the microfluidic chip. In this contribution, we present microfluidic chip connectors realised by easy-to-use elastomeric impression material PVS which is fully compatible to cured PDMS. This flexible, biocompatible, addition curing PVS material has been used by dentists as impression material in the past three decades [7,8]. The attractiveness of this material lays in its curing at room temperature, its short working time of up to two minutes, its very low linear dimensional change (0.1%), and its capability to strongly adhere to cured PDMS. The PVS connectors were created and ready to use all within 5 minutes without the need of exposure to high temperatures or the use of additional glues. The robustness of the PVS connectors has been demonstrated through pull-tests and liquid pressure tests. 2. Materials and Methods We have found that the polymer-polymer adhesion between the two silicones PVS and PDMS is very effective to connect tubes to PDMS. PVS cylinders were created by mixing the two base pastes (Mueller-Omicron Betasil® Vario Light) and filling it into a mould (see Figure 1). The cylinders have a diameter of 3 mm, a height of 5 mm, and an inner through hole of 0.6 mm. With this mould nine cylinders can be made simultaneously within 5 minutes.
(a)
(b)
Fig. 1. View of the connector mould for nine PVS connectors. (a) Two component PVS paste loaded into a metal mould sandwiched between needle alignment plates to realize aligned tube holes; (b) Nine moulded PVS connectors. The needles and top alignment plate have been removed.
The PVS cylinders that are held by a needle are contacted with an uncured PVS sheet. The needle secures the perfect alignment to the chip. After that they are placed in the PDMS inlets and cured for 2 minutes, the PVS connectors are ready to be used. In Figure 2 (a) a PDMS-glass microfluidic chip with two PVS connectors is depicted. PEEK tubing with an outer diameter of 1/32” (0.8 mm) was clamped into the 0.6 mm connection.
S. van den Driesche et al. / Procedia Engineering 120 (2015) 675 – 678
3. Results and Discussion The functionality of the PVS connectors was tested by performing pull-tests and pressure-tests. Pull-tests were made to determine the bonding strength between PVS and PDMS. Seven connectors bonded to PDMS were placed in the pull-test setup (XYZ Condor 100). The force required to release the bonding ports from a PDMS sheet is given in Table 1. (a)
(b)
Fig. 2. (a) Photograph of PVS connectors with 0.8 mm outer diameter PEEK tubes connected to a PDMS-glass based microfluidic chip. The 0.8 mm diameter PEEK tubes are clamped in the 0.6 mm diameter PVS bonding port connectors. The connectors have a diameter of 3 mm and a height of 5 mm. The PDMS layer consists of imprinted channels with a height of 100 μm and a width of 1 to 3.5 mm. To seal the channel, the PDMS was oxygen-plasma bonded to glass; (b) The bonding strength of 3 mm diameter PVS connectors fixed to PDMS was measured with pull tests (XYZ Condor 100). The minimum amount of force required to break the PVS - PDMS bond was 3.6 N (which corresponds to 525 kPa). The pull-test took 8 seconds. Table 1. Pull-test results of seven PVS connectors. PVS connector
Force
Force/Area
[N]
[kPa]
Breakage point
1
4.8
714
PVS breaks
2
5.3
786
PVS breaks
3
4.9
716
PVS breaks
4
3.9
581
PVS breaks
5
3.9
581
PVS breaks
6
3.6
525
Bond breaks
7
3.8
562
Bond breaks
The plot depicted in Figure 2 (b) shows that the minimum amount of force required to release 3 mm in diameter PVS connectors from a PDMS layer is 3.6 N (PVS connector number 6, Table 1). This lowest measured breaking force of the seven connecters corresponds to a bonding strength of approx. 525 kPa which is stronger than oxygen plasma PDMS-PDMS bonding [9] and is strong enough to hold a tube in place. Additionally, water pressure-tests were made to test the connection between the PVS-PDMS bond. Seven PDMS chips were prepared each with two bonded PVS connectors. PEEK tubings were clamped in each bonding port. The pressure of the water flowing through the chip was increased by 50 kPa every 30 seconds by using an EFD 1000 DVE fluid dispenser until leakage occurred or the maximum pressure of 650 kPa was reached. The minimum withstood pressure of the microfluidic devices was 350 kPa. For microfluidic devices this applied pressure is more than sufficient [10]. Three of the seven chips even withstood a pressure of 650 kPa without leaking (see Table 2).
677
678
S. van den Driesche et al. / Procedia Engineering 120 (2015) 675 – 678 Table 2. Pressure-test results of seven PDMS chips each with two PVS connectors. PDMS chip
Withstood pressure
Leakage point
[kPa] 1
350
PDMS/PVS bond
2
400
PDMS/PVS bond
3
400
PDMS/PVS bond
4
650
No leakage
5
350
PDMS/PVS bond
6
650
No leakage
7
650
No leakage
4. Conclusions PVS is an attractive material that can be easily moulded and bonded to PDMS at room temperature without the need of additional glues. PVS bonding ports were created by fixing moulded PVS cylinders, with an inner hole to connect PEEK tubing, to PDMS chips within 5 minutes. Pull-test and pressure-test results shows the high bonding strength between the two silicones. The presented PDMS-chip connector by moulded PVS is an easy way of connecting PDMS microfluidic devices.
Acknowledgments This project is a part of an EU Marie Curie Initial Training Network (ITN): biomedical engineering for cancer and brain disease diagnosis and therapy development, EngCaBra. Proj. no. PITN-GA-2010-264417. The authors would like to thank Parth Jhaveri for designing the connector mould as part of his master project at IMSAS, University of Bremen.
References [1] D.C. Duffy, J.C. McDonald, O.J. Schueller, G.M. Whitesides (1998): Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane). Anal. Chem. 70, 4974–84. [2] T. Fujii (2002): PDMS-based microfluidic devices for biomedical applications. Microelectron. Eng. 61-62, 907–914. [3] S. van den Driesche, V. Rao, D. Puchberger-Enengl, W. Witarski, M.J. Vellekoop (2012): Continuous cell from cell separation by traveling wave dielectrophoresis. Sensors Actuators B Chem. 170, 207–214. [4] M.A. Unger, H.P. Chou, T. Thorsen, .A. Scherer, S.R. Quake (2000): Monolithic microfabricated valves and pumps by multilayer soft lithography, Science 288, 113–116. [5] P.M. van Midwoud, G.M.M. Groothuis, M.T. Merema, E. Verpoorte (2010): Microfluidic biochip for the perifusion of precision-cut rat liver slices for metabolism and toxicology studies, Biotechnol. Bioeng. 105, 184–94. [6] V. Saarela, S. Franssila, S. Tuomikoski, S. Marttila, P. Östman, T. Sikanen, T. Kotiaho, R. Kostiainen (2006): Re-usable multi-inlet PDMS fluidic connector, Sensors Actuators B Chem.114, 552–557. [7] W.W. Chee, T.E. Donovan (1992): Polyvinyl siloxane impression materials: A review of properties and techniques, J. Prosthet. Dent. 68, 728– 732. [8] D. Nowakowska, Z. Raszewski, J. Saczko, J. Kulbacka, W. Wieckiewicz (2014): Polymerization time compatibility index of polyvinyl siloxane impression materials with conventional and experimental gingival margin displacement agents, J. Prosthet. Dent. 112, 168–75. [9] M.A. Eddings, M.A. Johnson, B.K. Gale (2008): Determining the optimal PDMS-PDMS bonding technique for microfluidic devices, J. Micromech. Microeng.18, 067001. [10] S.K. Sia, G.M. Whitesides (2003): Microfluidic devices fabricated in poly(dimethylsiloxane) for biological studies, Electrophoresis 24, 356376.