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A miniature shape memory alloy pinch valve
Sensors and Actuators 77 Ž1999. 145–148 www.elsevier.nlrlocatersna
A miniature shape memory alloy pinch valve Catherine M. Pemble ) , Bruce C. Towe Bioengineering, Arizona State UniÕersity, Tempe, AZ, USA Received 26 August 1998; accepted 10 February 1999
Abstract A normally closed miniature pinch valve employing a Nitinol shape memory alloy actuator to control flow in silicone microbore tubing has been designed, fabricated, and tested. The valve can withstand long term exposure to fluids because only the tubing interior is contacted by the regulated fluid. The valve has application to microflow chemistry and has the advantage of compact and simple construction. At a differential pressure of 3 psi the valve permits a maximum flow rate of about 0.28 mlrs and has a 398 mW power dissipation. The maximum differential pressure the valve can withstand is greater than 30 psi. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Miniature pinch valve; Memory alloy actuator; Silicone microbore tubing
1. Introduction Compact and reliable fluid valves of simple construction are desirable for microfluidics, microchemistry, sensors, and implantable drug delivery systems. For these applications, several types of valves are available. Miniature solenoid valves are often favored because of relatively smaller sizes, extended cycle life, and chemical inertness. Bimetallic diaphragm valves consisting of a diaphragm actuator with a central boss mated to an etched silicon valve orifice w1x offer control over very low flow rates. Heat actuated shape memory alloy ŽSMA. thin film microdevices w2x are being integrated into applications such as miniature valves, connectors, and switches. SMA film actuators produce large forces and displacements within small spaces at voltages compatible with electronics. Other elastic-membrane microvalves use pneumatic pressure to deform a membrane that closes a microfluidic channel w3x. Many of the above valves are larger than desired and may allow undesirable fluid contact with actuator components, which can lead to corrosion and clogging. Fabrication of some valves involves numerous steps including etching and sputter-depositing, of SMA for example, Žfollowed by heat treating. requiring a clean room environment. Thus, such problems can prevent their use in lifesupport applications.
)
Corresponding author
We have developed a miniature shape memory alloy pinch valve actuator for silicone microbore tubing. The actuator valve uses readily available SMA actuator wire and has a compact valve structure. The nickel–titanium SMA wire is corrosion resistant, biocompatible, and inexpensive, and it can be cycled millions of times. This pinch valve involves no micromachining.
2. Design Our particular application requires that the valve fit within a compact biomedical implant and that there be the possibility of a densely packed array of valves. Actuation times of several seconds are acceptable. The actuator must not be affected by small variations in the temperature of the surroundings and must be suited to control a microliter per minute flow system. 2.1. Shape memory alloy Nitinol is a nickel–titanium shape memory alloy that has gained popularity in medical devices because of its biocompatibility, fatigue resistance, and body-temperature superelasticity w4x. Nitinol exhibits a recoverable, repeatable, and rapid change in crystalline structure with temperature cycling between the martensite and austenite phases. The lower temperature crystalline form can absorb some plastic deformation that is reversed in the higher tempera-
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 1 5 7 - 0
146
C.M. Pemble, B.C. Tower Sensors and Actuators 77 (1999) 145–148
tion of a superelastic cantilever and relieves pinch pressure on the silicone tubing thereby allowing flow. 2.2. Analysis of the ÕalÕe operation
Fig. 1. Illustration of the pinch valve structure, including the electrical contacts on the actuator wire and the locations of epoxy fixtures.
ture form. The force needed to deform the material is much less than the force it can exert when heated w4x. Nitinol superelastic and actuator wires are used in the pinch valve. Superelastic behavior occurs when the alloy forms stress-induced martensite above the transition temperature, resulting in a strain to relieve the stress. As soon as the stress is removed, the material reverts to undeformed austenite. Thus, the superelastic wire can act as a spring that provides a constant force over a large strain range w4x. Nitinol shape memory alloy actuator wires are small diameter wires that contract by several percent of their length when heated and easily elongate with the application of a bias force when allowed to cool. Depending on the application, different transition temperatures may be favored—wires with higher transition temperatures require a longer amount of time to heat up, but will cool much faster than wires with lower transition temperatures w5x. The heating and cooling rates depend on the temperature gradient between the transition and ambient temperatures. The actuator consists of a Nitinol contraction wire in tension with a Nitinol superelastic wire as shown in Fig. 1. This normally closed valve has been fabricated using a 0.152 mm diameter contraction wire and a 0.584 mm diameter superelastic wire. In the prototype, one end of the structure is fixed to a flat surface using epoxy. This epoxy, also used to bond the two wires Žat both ends., is high performance, heat resistant Hysol EA9460. Electrical resistance heating of the contraction wire causes upward deflec-
The analysis of the valve operation includes determination of the force required to close the silicone tube, the pressure of the fluid within the tube, the deflection of the superelastic wire, and the tension of the actuator wire. Our application required a compact pinch valve that could withstand a differential pressure up to 3 psi Ž20.7 kPa.. The silicone tubing has an inner diameter of 0.64 mm and an outer diameter of 1.2 mm. The modulus of elasticity for the superelastic wire was calculated from Shape Memory Applications Inc., literature to be approximately 55 GPa. The moment of inertia for the superelastic wire was calculated to be 5.72 = 10y7 cm4 . The force with which a 0.152 mm diameter actuator wire can pull is 3.234 N w5x. The 3.234 N is well below the value of applied strain at which the material changes phase and undergoes large strain; thus, the material behaves linearly. The flow characteristics of this valve depend on the distance of the tube from the base of the cantilever. An equation for the elastic curvature of the superelastic wire in the relaxed closed position was derived w6x to determine where the silicone tubing must be placed. The placement of the tubing must be such that the force required to close the tube is achieved by the superelastic wire. The derivation from the free body diagram analysis gives the following equation: yPx 2 ÕŽ x. s
6 EI
Ž x y 3l .
Ž 1.
where Õ Ž x . is the vertical height of bending at position x, P is the force exerted by the tube, x is the variable position along the horizontal, l is the point at which P is applied Žnot necessarily at the end of the cantilever., E is the modulus of elasticity, and I is the moment of inertia. The dimensional variables are depicted in Fig. 2. Õ Ž x . was set at 0.56 mm, which is the vertical thickness of the silicone tube when it is fully closed. The force required to
Fig. 2. Ža. Location of variables with respect to the bending superelastic wire. Žb. Alternate view of variables with valve in the relaxed closed position.
C.M. Pemble, B.C. Tower Sensors and Actuators 77 (1999) 147–148
147
the actuator wire must be 5.3 mm above the superelastic wire; thus, the actuator wire is 1.13 cm in length. A picture of the actual pinch valve is shown in Fig. 3.
3. Results and discussion
Fig. 3. Picture of assembled pinch valve with truncated silicone tubing.
close the silicone tube and stop flow at 3 psi is 1.603 N, which was measured by applying weights until the tube closed. By setting x equal to l and solving for l, it was determined that the silicone tube should be positioned 6.91 mm from the base of the cantilever. A second equation was derived w6x for the open position of the valve involving the application of a force Žby the contraction wire. at the free end of the cantilever. This equation is as follows: ÕŽ x. s
yT sin u x 2 6 EI
Ž x y 3 L.
Ž 2.
where T is the tension exerted by the actuator wire, u is the angle at which the actuator is bonded to the non-fixed end of the superelastic wire with respect to the horizontal, and L is the length of the cantilever. As previously discussed, T is equal to 3.234 N, and L was decided to be 1 cm for our application. Õ Ž x . was set at 0.88 mm, such that the silicone tube is open to half the inner diameter. x was set equal to l Žfrom Eq. Ž1.. at 6.91 mm. It was determined that u is equal to 27.8 degrees. Simple geometry concludes that at the fixed end of the valve structure
Using DC actuation and water as the regulated fluid, the valves have been cycled repeatedly within the manufacturer’s guidelines Žfor heating. of the wires and no changes in performance have been observed. In addition, the valve allows no leakage while in the closed position even after the repeated cycles. The manufacturer’s actuator wire literature w5x recommends applying a current of 400 mA to the 0.152 mm diameter wire. Within these guidelines, tens of millions of cycles are possible w5x. We measured flow rate over a range of currents starting at 20 mA and extending to 500 mA. A rough proportionality between flow and current occurred between 260 mA and 320 mA. The greatest flow rate Ž0.28 mlrs. occurred at a current of 420 mA with a pressure head of 3 psi. The electrical resistance of the contraction wire was measured to be 2.26 V Žat 2 Vrcm. and the transition temperature was 90 degrees C. The valve opened in less than 1 s and closed in less than 2.5 s. There was no significant heating of the silicone tubing by the superelastic wire resulting from thermal conduction from the contraction wire. The contraction wire power dissipation calculated for the designed valve is 398 mW Žwhen the activating current is 420 mA.. This value compares favorably to miniature solenoid valves that, for example, exhibit minimum values around 625 mW w7x. The valve can operate as onroff or potentially can be used to vary flow by changing the electrical current flow within a certain range. Flow rate was measured as the time
Fig. 4. Flow rate Žat 3 psi. while varying current applied to the valve. The black bars indicate maximum and minimum flow rates at the corresponding currents; a moving average trendline is plotted.
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C.M. Pemble, B.C. Tower Sensors and Actuators 77 (1999) 145–148
Fig. 5. Flow rate while varying pressure of valve flow system. Three different currents were tested.
required to fill a specified volume Žthree trials were performed at each current and averages were calculated.. Flow versus applied current is shown in Fig. 4. There is a threshold current needed to achieve the transition temperature of the actuator wire. It was determined experimentally that the prototype valve withstood a pressure well over 30 psi while closed. However, theoretical calculations were made for a maximum pressure of 3 psi. We speculate that the characteristic of the superelastic wire to act as a nonlinear spring and absorb large amounts of strain energy w4x accounts for the prototype valve’s improved performance in resisting pressure compared to theory. The flow rate as a function of pressure on the valve at various currents is shown in Fig. 5. The flow through the valve, within experimental error, behaves in a linear fashion with respect to pressure.
4. Conclusions The design, fabrication, and testing of a miniature pinch valve utilizing the properties of shape memory alloy have been successfully performed in a simple and uncomplicated manner. The use of Nitinol superelastic and actuator wires favorably enhances the operation, reliability, and longevity of the pinch valve. The small pinch valve pro-
vides comparable pressure handling capacity and corrosive fluid handling compatibility to larger valves.
References w1x H. Jerman, Microvalve technology, Measurements and Control 166 Ž1994. 137–145. w2x G. Spera, Implantable pumps improve drug delivery, strengthen weak hearts, Medical Device and Diagnostic Industry 19 Ž1997. 59. w3x A.M. Young, T.M. Bloomstein, S.T. Palmacci, M.A. Hollis, Elasticmembrane microvalves for microfluidic network integration, SolidState Research Report, Lincoln Laboratory, Massachusetts Institute of Technology, 4 Ž1997. 23–29. w4x Shape Memory Applications Inc., Introduction to Shape Memory and Superelasticity, Santa Clara, CA Ž1997.. w5x Dynalloy, Technical Characteristics of FLEXINOLe Actuator Wires, Irvine, CA Ž1997.. w6x R.C. Hibbeler, Statics and Mechanics of Materials, Macmillan Publishing, New York Ž1993. 701–706. w7x The Lee Company, Electro-fluidic systems, Component Catalog with Engineering Reference Material, 6th edn. Ž1994. 21–23. Catherine Pemble received her BSE degree in biomedical engineering from Mercer University, Macon, GA, in 1997. She is currently a graduate student at Arizona State University working to earn her MS in bioengineering. Bruce Towe received the PhD degree from Pennsylvania State University in 1978 and is currently Professor of Engineering in the Bioengineering program at Arizona State University. He is currently working on implantable drug delivery systems.
A miniature shape memory alloy pinch valve Catherine M. Pemble ) , Bruce C. Towe Bioengineering, Arizona State UniÕersity, Tempe, AZ, USA Received 26 August 1998; accepted 10 February 1999
Abstract A normally closed miniature pinch valve employing a Nitinol shape memory alloy actuator to control flow in silicone microbore tubing has been designed, fabricated, and tested. The valve can withstand long term exposure to fluids because only the tubing interior is contacted by the regulated fluid. The valve has application to microflow chemistry and has the advantage of compact and simple construction. At a differential pressure of 3 psi the valve permits a maximum flow rate of about 0.28 mlrs and has a 398 mW power dissipation. The maximum differential pressure the valve can withstand is greater than 30 psi. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Miniature pinch valve; Memory alloy actuator; Silicone microbore tubing
1. Introduction Compact and reliable fluid valves of simple construction are desirable for microfluidics, microchemistry, sensors, and implantable drug delivery systems. For these applications, several types of valves are available. Miniature solenoid valves are often favored because of relatively smaller sizes, extended cycle life, and chemical inertness. Bimetallic diaphragm valves consisting of a diaphragm actuator with a central boss mated to an etched silicon valve orifice w1x offer control over very low flow rates. Heat actuated shape memory alloy ŽSMA. thin film microdevices w2x are being integrated into applications such as miniature valves, connectors, and switches. SMA film actuators produce large forces and displacements within small spaces at voltages compatible with electronics. Other elastic-membrane microvalves use pneumatic pressure to deform a membrane that closes a microfluidic channel w3x. Many of the above valves are larger than desired and may allow undesirable fluid contact with actuator components, which can lead to corrosion and clogging. Fabrication of some valves involves numerous steps including etching and sputter-depositing, of SMA for example, Žfollowed by heat treating. requiring a clean room environment. Thus, such problems can prevent their use in lifesupport applications.
)
Corresponding author
We have developed a miniature shape memory alloy pinch valve actuator for silicone microbore tubing. The actuator valve uses readily available SMA actuator wire and has a compact valve structure. The nickel–titanium SMA wire is corrosion resistant, biocompatible, and inexpensive, and it can be cycled millions of times. This pinch valve involves no micromachining.
2. Design Our particular application requires that the valve fit within a compact biomedical implant and that there be the possibility of a densely packed array of valves. Actuation times of several seconds are acceptable. The actuator must not be affected by small variations in the temperature of the surroundings and must be suited to control a microliter per minute flow system. 2.1. Shape memory alloy Nitinol is a nickel–titanium shape memory alloy that has gained popularity in medical devices because of its biocompatibility, fatigue resistance, and body-temperature superelasticity w4x. Nitinol exhibits a recoverable, repeatable, and rapid change in crystalline structure with temperature cycling between the martensite and austenite phases. The lower temperature crystalline form can absorb some plastic deformation that is reversed in the higher tempera-
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 9 . 0 0 1 5 7 - 0
146
C.M. Pemble, B.C. Tower Sensors and Actuators 77 (1999) 145–148
tion of a superelastic cantilever and relieves pinch pressure on the silicone tubing thereby allowing flow. 2.2. Analysis of the ÕalÕe operation
Fig. 1. Illustration of the pinch valve structure, including the electrical contacts on the actuator wire and the locations of epoxy fixtures.
ture form. The force needed to deform the material is much less than the force it can exert when heated w4x. Nitinol superelastic and actuator wires are used in the pinch valve. Superelastic behavior occurs when the alloy forms stress-induced martensite above the transition temperature, resulting in a strain to relieve the stress. As soon as the stress is removed, the material reverts to undeformed austenite. Thus, the superelastic wire can act as a spring that provides a constant force over a large strain range w4x. Nitinol shape memory alloy actuator wires are small diameter wires that contract by several percent of their length when heated and easily elongate with the application of a bias force when allowed to cool. Depending on the application, different transition temperatures may be favored—wires with higher transition temperatures require a longer amount of time to heat up, but will cool much faster than wires with lower transition temperatures w5x. The heating and cooling rates depend on the temperature gradient between the transition and ambient temperatures. The actuator consists of a Nitinol contraction wire in tension with a Nitinol superelastic wire as shown in Fig. 1. This normally closed valve has been fabricated using a 0.152 mm diameter contraction wire and a 0.584 mm diameter superelastic wire. In the prototype, one end of the structure is fixed to a flat surface using epoxy. This epoxy, also used to bond the two wires Žat both ends., is high performance, heat resistant Hysol EA9460. Electrical resistance heating of the contraction wire causes upward deflec-
The analysis of the valve operation includes determination of the force required to close the silicone tube, the pressure of the fluid within the tube, the deflection of the superelastic wire, and the tension of the actuator wire. Our application required a compact pinch valve that could withstand a differential pressure up to 3 psi Ž20.7 kPa.. The silicone tubing has an inner diameter of 0.64 mm and an outer diameter of 1.2 mm. The modulus of elasticity for the superelastic wire was calculated from Shape Memory Applications Inc., literature to be approximately 55 GPa. The moment of inertia for the superelastic wire was calculated to be 5.72 = 10y7 cm4 . The force with which a 0.152 mm diameter actuator wire can pull is 3.234 N w5x. The 3.234 N is well below the value of applied strain at which the material changes phase and undergoes large strain; thus, the material behaves linearly. The flow characteristics of this valve depend on the distance of the tube from the base of the cantilever. An equation for the elastic curvature of the superelastic wire in the relaxed closed position was derived w6x to determine where the silicone tubing must be placed. The placement of the tubing must be such that the force required to close the tube is achieved by the superelastic wire. The derivation from the free body diagram analysis gives the following equation: yPx 2 ÕŽ x. s
6 EI
Ž x y 3l .
Ž 1.
where Õ Ž x . is the vertical height of bending at position x, P is the force exerted by the tube, x is the variable position along the horizontal, l is the point at which P is applied Žnot necessarily at the end of the cantilever., E is the modulus of elasticity, and I is the moment of inertia. The dimensional variables are depicted in Fig. 2. Õ Ž x . was set at 0.56 mm, which is the vertical thickness of the silicone tube when it is fully closed. The force required to
Fig. 2. Ža. Location of variables with respect to the bending superelastic wire. Žb. Alternate view of variables with valve in the relaxed closed position.
C.M. Pemble, B.C. Tower Sensors and Actuators 77 (1999) 147–148
147
the actuator wire must be 5.3 mm above the superelastic wire; thus, the actuator wire is 1.13 cm in length. A picture of the actual pinch valve is shown in Fig. 3.
3. Results and discussion
Fig. 3. Picture of assembled pinch valve with truncated silicone tubing.
close the silicone tube and stop flow at 3 psi is 1.603 N, which was measured by applying weights until the tube closed. By setting x equal to l and solving for l, it was determined that the silicone tube should be positioned 6.91 mm from the base of the cantilever. A second equation was derived w6x for the open position of the valve involving the application of a force Žby the contraction wire. at the free end of the cantilever. This equation is as follows: ÕŽ x. s
yT sin u x 2 6 EI
Ž x y 3 L.
Ž 2.
where T is the tension exerted by the actuator wire, u is the angle at which the actuator is bonded to the non-fixed end of the superelastic wire with respect to the horizontal, and L is the length of the cantilever. As previously discussed, T is equal to 3.234 N, and L was decided to be 1 cm for our application. Õ Ž x . was set at 0.88 mm, such that the silicone tube is open to half the inner diameter. x was set equal to l Žfrom Eq. Ž1.. at 6.91 mm. It was determined that u is equal to 27.8 degrees. Simple geometry concludes that at the fixed end of the valve structure
Using DC actuation and water as the regulated fluid, the valves have been cycled repeatedly within the manufacturer’s guidelines Žfor heating. of the wires and no changes in performance have been observed. In addition, the valve allows no leakage while in the closed position even after the repeated cycles. The manufacturer’s actuator wire literature w5x recommends applying a current of 400 mA to the 0.152 mm diameter wire. Within these guidelines, tens of millions of cycles are possible w5x. We measured flow rate over a range of currents starting at 20 mA and extending to 500 mA. A rough proportionality between flow and current occurred between 260 mA and 320 mA. The greatest flow rate Ž0.28 mlrs. occurred at a current of 420 mA with a pressure head of 3 psi. The electrical resistance of the contraction wire was measured to be 2.26 V Žat 2 Vrcm. and the transition temperature was 90 degrees C. The valve opened in less than 1 s and closed in less than 2.5 s. There was no significant heating of the silicone tubing by the superelastic wire resulting from thermal conduction from the contraction wire. The contraction wire power dissipation calculated for the designed valve is 398 mW Žwhen the activating current is 420 mA.. This value compares favorably to miniature solenoid valves that, for example, exhibit minimum values around 625 mW w7x. The valve can operate as onroff or potentially can be used to vary flow by changing the electrical current flow within a certain range. Flow rate was measured as the time
Fig. 4. Flow rate Žat 3 psi. while varying current applied to the valve. The black bars indicate maximum and minimum flow rates at the corresponding currents; a moving average trendline is plotted.
148
C.M. Pemble, B.C. Tower Sensors and Actuators 77 (1999) 145–148
Fig. 5. Flow rate while varying pressure of valve flow system. Three different currents were tested.
required to fill a specified volume Žthree trials were performed at each current and averages were calculated.. Flow versus applied current is shown in Fig. 4. There is a threshold current needed to achieve the transition temperature of the actuator wire. It was determined experimentally that the prototype valve withstood a pressure well over 30 psi while closed. However, theoretical calculations were made for a maximum pressure of 3 psi. We speculate that the characteristic of the superelastic wire to act as a nonlinear spring and absorb large amounts of strain energy w4x accounts for the prototype valve’s improved performance in resisting pressure compared to theory. The flow rate as a function of pressure on the valve at various currents is shown in Fig. 5. The flow through the valve, within experimental error, behaves in a linear fashion with respect to pressure.
4. Conclusions The design, fabrication, and testing of a miniature pinch valve utilizing the properties of shape memory alloy have been successfully performed in a simple and uncomplicated manner. The use of Nitinol superelastic and actuator wires favorably enhances the operation, reliability, and longevity of the pinch valve. The small pinch valve pro-
vides comparable pressure handling capacity and corrosive fluid handling compatibility to larger valves.
References w1x H. Jerman, Microvalve technology, Measurements and Control 166 Ž1994. 137–145. w2x G. Spera, Implantable pumps improve drug delivery, strengthen weak hearts, Medical Device and Diagnostic Industry 19 Ž1997. 59. w3x A.M. Young, T.M. Bloomstein, S.T. Palmacci, M.A. Hollis, Elasticmembrane microvalves for microfluidic network integration, SolidState Research Report, Lincoln Laboratory, Massachusetts Institute of Technology, 4 Ž1997. 23–29. w4x Shape Memory Applications Inc., Introduction to Shape Memory and Superelasticity, Santa Clara, CA Ž1997.. w5x Dynalloy, Technical Characteristics of FLEXINOLe Actuator Wires, Irvine, CA Ž1997.. w6x R.C. Hibbeler, Statics and Mechanics of Materials, Macmillan Publishing, New York Ž1993. 701–706. w7x The Lee Company, Electro-fluidic systems, Component Catalog with Engineering Reference Material, 6th edn. Ž1994. 21–23. Catherine Pemble received her BSE degree in biomedical engineering from Mercer University, Macon, GA, in 1997. She is currently a graduate student at Arizona State University working to earn her MS in bioengineering. Bruce Towe received the PhD degree from Pennsylvania State University in 1978 and is currently Professor of Engineering in the Bioengineering program at Arizona State University. He is currently working on implantable drug delivery systems.