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Design and testing of a polymeric microgripper for cell manipulation
Microelectronic Engineering 84 (2007) 1219–1222 www.elsevier.com/locate/mee
Design and testing of a polymeric microgripper for cell manipulation Belen Solano *, David Wood Microsystems Research Group, School of Engineering, Durham University, Durham DH1 3LE, UK Available online 3 February 2007
Abstract This work presents the design, fabrication and testing of a thermally actuated microgripper for the manipulation of single cells and other biological particles. This microgripper has been fabricated with a particular combination of surface micromachining techniques that permit the release of stress-free polymer (SU8)/metal (Au) structures without the use of sacrificial layers. The inclusion of novel thermal actuators, which completely eliminate the parasitic resistance of the cold arm, improves considerably the efficiency of the system and therefore enables large displacements at low input voltages and operating temperatures. Two types of microgripper have been fabricated and tested producing displacements of up to 262 lm at 1.94 V input voltage and 78 mW power. Micromanipulation experiments have successfully demonstrated the gripping, holding and positioning of a micro-sized object. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Micromanipulation; Bio-MEMS; Microgripper; Electrothermal actuators; SU8 polymer
1. Introduction Experiments that enable the investigation of how individual cells perform their specialised functions, and how they interact between each other, are of crucial importance for the progress of biology and medicine. When studying complex interactions between and inside cells, it is often necessary to hold, sort and transport biological samples in dry or aqueous environments. Current bio-manipulation techniques and tools – including optical tweezers, electro kinetic forces, magnetic tweezers, acoustic traps, hydrodynamic flows and pipettes – are powerful for particular micro-applications. However, these require expensive experimental set-ups and lack the flexibility and ease of use offered by mechanical-end effectors which are in direct contact with the sample but which do not interfere with it either optically or electrically. In recent years, a variety of microgrippers have been developed for the manipulation of micro-sized objects. Different mechanisms of actuation have been used for microgripper applications such as piezoelectric [1], electrostatic *
Corresponding author. Tel.: +44 191 3342525; fax: + 44 191 3342498. E-mail address: [email protected] (B. Solano).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.153
[2–4], SMA [5] or electrothermal [6–10]. Amongst them, electrothermal actuation is the preferred mechanism for biological purposes since it is able to produce large deflections at low activation voltages. In particular, SU8 polymeric electrothermal grippers demonstrate additional advantages such as good biocompatibility and low operating temperatures. Most of these electrothermal grippers [7,9] consist of a polymer structure with a metal heater deposited on top that defines one or two hot arm U-shaped actuators. Even if in some cases very good performance is achieved [9], limited attention has been paid to the thermal and geometrical optimisation of the structure. A recent development [6], proposes a microgripper fabricated with metallic heaters on top of a silicon heat conducting structure embedded in SU8 that permit a better heat diffusion through the thickness of the polymer layer. Despite demonstrating better performance than some other grippers [7], the integration of silicon conducting elements makes the overall structure stiffer and smaller displacements are obtained for the same input power. In this work we present an electro-thermally actuated polymeric microgripper with optimised thermal efficiency and geometry. The total suppression of the heat generation in the cold arm permits not only larger deflections for the
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B. Solano, D. Wood / Microelectronic Engineering 84 (2007) 1219–1222
same input power but also the possibility to integrate active elements in that arm such as force sensors or extra resistors to produces bi-directional in-plane movements. 2. The microgripper 2.1. Device structure The microgripper device consists of two thermal actuators positioned face-to-face plus a pair of extended prongs (Fig. 1). The main body of the microgripper is composed of a multi-layer structure which encapsulates a thin metallic heater in between two layers of dielectric polymeric material. When a current is applied through the conducting layer, the dissipated power heats the surrounding polymeric material and the whole structure deflects in-plane due to differential thermal expansion of its constituent parts. The base material of the microgripper is the polymer, SU8 (Microchem Corp.), which gives shape to the anchors, the actuators and the extension tips. Joule heating is produced via the thin layer of gold deposited and patterned on top of the thick SU8. The choice of materials has been based on the favourable mechanical and chemical properties of the highly cross-linked SU8, along with its biocompatibility, ease of fabrication and low cost. After fabrication the polymeric microgripper is bonded to a PCB, leaving the microgripper overhanging. 2.2. Novel thermal actuator design The principle of operation of the microgripper is the Ushaped thermal actuator. The actuation structure, composed of two adjacent micro-beams, deflects at its tips by the asymmetrical thermal expansion of its constituent parts which have variable cross-sections and lengths. The inplane deflection is controlled by the difference in temperature established in between the hot and cold arms.
Fig. 1. SEM photo of a fabricated microgripper.
Fig. 2. Schematic of: (a) standard U-shaped thermal actuator, and (b) novel U-shaped thermal actuator.
In conventional U-shaped actuators (Fig. 2a) current flows through both arms reducing considerably the effective temperature difference (DT) between them. With our proposed architecture for the U-shaped actuator (Fig. 2b), current flows in and out from the hot arm and no parasitic Joule heating is produced in the cold arm which in turn will maximise the deflection for a given input power. After the application of the current through the conducting layer, the hot arm will expand more than the cold arm. As both arms are connected at their free ends, an in-plane deflection around the z axis will take place. 3. Fabrication The microgripper has been fabricated using a novel combination of standard surface micromachining techniques in a three-mask process. This process, shown in Fig. 3, enables the release of encapsulated stress free gold/SU8 structures and membranes with incorporated and accessible electrical contact pads. It does not require the use of polymeric or metal sacrificial layers and it relies on the induced diminished adhesion between gold/SU8 and the silicon substrate. The process can be described as follows. First, a thin layer (30 nm) of OMNICOATTM is spin deposited on a Si wafer. The thickness of this layer increases or reduces the adhesion of the SU8 structures in the release step and can be adapted to other geometries. Then, a layer of SU8-2 (1.5 lm) is spin deposited and patterned using UV-lithography. After that, a thin layer of gold (240 nm) is thermally evaporated on top of the SU8 and patterned. Then a layer of SU8-10 (30 lm) is spin deposited and patterned. During the development of the second layer of SU8, the whole structure will start to peel off from the Si wafer without residual stress or damage to the electrical pads. The quality of the release process will depend on the geometry of the structure to be released, the thickness of the layers and the cooling down times applied after post
B. Solano, D. Wood / Microelectronic Engineering 84 (2007) 1219–1222
1. Spin deposition and baking of OmnicoatTM (30 nm)
Deflection [ m]
2. Spin deposition of SU8 (2 m), patterning and development
3. Thermal evaporation of gold and patterning
4. Spin deposition of SU8 (30 m), patterning and release step at the same time as development
280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 -20 -0.2
Omnicoat
SU8
Au
Si
(84°C) Model 2 Model 1
(54°C)
(52°C)
(30˚C)
(41°C)
(10˚C) (20°C) (6.5°C) 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Input Voltage [V] Fig. 5. Measured deflection versus input voltage for models 1 and 2. The numbers in brackets represent the temperature above ambient for a selection of data points.
Fig. 3. Fabrication process.
exposure baking (PEB). The electrical contact pads of the gold layer are accessible through windows defined in the first layer of SU8. In a subsequent process the microgripper is diced, bonded and wired onto a PCB. 4. Experiments 4.1. Performance characteristics Two possible configurations of the microgripper have been fabricated (Fig. 4): one with the two hot arms facing each other and a second one with the two cold arms facing
Model 1 L h1 Lj
1221
Wh
W Au
Model 2 L h2 Lj
W Au
Wh Lf Wf
each other. In the first case, the normally closed mode (model 1), actuation is needed only for the release operation and no actuation is needed when holding the object. In the second case (model 2), the normally open mode, the actuators need to be operated continuously to ensure the gripping of the sample. The geometry of the actuators is slightly different in each case. In model 1, the hot and cold arms have identical geometries. In model 2, the cold arm is twice the width of the hot arm and the deflection is magnified with a flexure. This flexure is three times thinner than the hot arm and acts as a hinge. A constant current source was used to actuate the microgrippers. Current levels ranging from 5 to 40 mA were applied. The measured displacements represent the opening in between the two arms of the gripper from its initial position. For each data point, the voltage reading was used to estimate the resistance change of the metallization layer and to determine the average temperature of the hot arm. Fig. 5 shows the deflection and average temperature change for models 1 and 2 versus input voltage. The dots are the measured deflection data and the numbers between brackets represents the increase of temperature above ambient temperature (20 °C).The maximum power consumed for models 1 and 2 is calculated to be 77 mW and 64 mA, respectively. For displacements of the order of the size of biological cells (2–150 lm) average temperatures are less than to 80 °C making this device suitable for biomanipulation. 4.2. Micromanipulation experiments
Fig. 4. Schematic drawing of the two different microgripper models. Critical dimensions: Lh1 = 2000 lm; Lh2 = 1500 lm; Lj = 1600 lm; Jg1 = 60 lm; Jg = 140 lm; g = 60 lm; Wh = 140 lm; Wf = 40 lm; Lf = 360 lm; WAu = 40 lm. Out-of-plane thickness = 30 lm.
The ability of the microgripper to manipulate microscale objects has been experimentally verified by grasping, holding and positioning gold coated SU8 micro-cylinders with an approximate diameter of 100 lm. The sequence of photos shown in Fig. 6 demonstrates the successful manipula-
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B. Solano, D. Wood / Microelectronic Engineering 84 (2007) 1219–1222
Fig. 6. Micromanipulation sequence where the microgripper (model 1) is used to pick and place a gold coated SU8 object.
tion of a single micro-cylinder in a grid of 500 lm spaced dots. The experiments were conducted in air in a clean room environment at 20 °C.
by One North-East and the County Durham sub-regional Partnership fund. References
5. Conclusions Electro-thermally actuated microgrippers with novel integrated thermal actuators have been designed, fabricated and characterised. The choice of biocompatible materials together with the low actuation voltages required, and large deflection produced at low temperatures, make this microgripper highly suitable for bio-manipulation experiments in air or in aqueous media. Future work includes the realisation of experiments in biological media and the inclusion of a second resistor on the ‘cold’ arm to provide bi-directional movement. Acknowledgements The authors acknowledge A.J. Gallant for the help with the manipulation experiments. This work was part-funded
[1] M. Carrozza, A. Eisinberg, A. Menciassi, J. Micromech. Microeng. 10 (2000) 271–276. [2] C. Kim, A. Pisano, R. Muller, J. MEMS 1 (1992). [3] B.E. Volland, H. Heerlein, I.W. Rangelow, Microelectron. Eng. 61 (2002) 1015–1023. [4] R. Wierzbicki, K. Houston, H. Heerlein, W. Barth, T. Debski, A. Eisinberg, A. Mendicassi, M.C. Carrozza, P. Dario, Microelectron. Eng. 83 (2006) 1651–1654. [5] I. Roch, P. Bidaud, D. Collard, L. Buchaillot, J. Micromech. Microeng. 13 (2003) 330–336. [6] T.C. Duc, G. Lau, J. Wei, P. Sarro, in: MicroMechanics Europe Workshop, Southampton, UK, September 3–5, 2006, p. 197. [7] N. Nguyen, S. Ho, C. Low, J. Micromech. Microeng. 14 (2004) 969– 974. [8] H. Du, C. Su, M. Lim, W. Jin, Smart Mater. Struct. 8 (1999) 616– 622. [9] N. Chronis, L. Lee, IEEE J. MEMS 14 (2005) 857–863. [10] K. Ivanova, T. Ivanov, A. Badar, B.E. Volland, I.W. Rangelow, D. Andrijasevic, F. Sumecz, S. Fisher, M. Spitzbart, W. Brenner, I. Kostic, Microelectron. Eng. 83 (2006) 1393–1395.
Design and testing of a polymeric microgripper for cell manipulation Belen Solano *, David Wood Microsystems Research Group, School of Engineering, Durham University, Durham DH1 3LE, UK Available online 3 February 2007
Abstract This work presents the design, fabrication and testing of a thermally actuated microgripper for the manipulation of single cells and other biological particles. This microgripper has been fabricated with a particular combination of surface micromachining techniques that permit the release of stress-free polymer (SU8)/metal (Au) structures without the use of sacrificial layers. The inclusion of novel thermal actuators, which completely eliminate the parasitic resistance of the cold arm, improves considerably the efficiency of the system and therefore enables large displacements at low input voltages and operating temperatures. Two types of microgripper have been fabricated and tested producing displacements of up to 262 lm at 1.94 V input voltage and 78 mW power. Micromanipulation experiments have successfully demonstrated the gripping, holding and positioning of a micro-sized object. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Micromanipulation; Bio-MEMS; Microgripper; Electrothermal actuators; SU8 polymer
1. Introduction Experiments that enable the investigation of how individual cells perform their specialised functions, and how they interact between each other, are of crucial importance for the progress of biology and medicine. When studying complex interactions between and inside cells, it is often necessary to hold, sort and transport biological samples in dry or aqueous environments. Current bio-manipulation techniques and tools – including optical tweezers, electro kinetic forces, magnetic tweezers, acoustic traps, hydrodynamic flows and pipettes – are powerful for particular micro-applications. However, these require expensive experimental set-ups and lack the flexibility and ease of use offered by mechanical-end effectors which are in direct contact with the sample but which do not interfere with it either optically or electrically. In recent years, a variety of microgrippers have been developed for the manipulation of micro-sized objects. Different mechanisms of actuation have been used for microgripper applications such as piezoelectric [1], electrostatic *
Corresponding author. Tel.: +44 191 3342525; fax: + 44 191 3342498. E-mail address: [email protected] (B. Solano).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.153
[2–4], SMA [5] or electrothermal [6–10]. Amongst them, electrothermal actuation is the preferred mechanism for biological purposes since it is able to produce large deflections at low activation voltages. In particular, SU8 polymeric electrothermal grippers demonstrate additional advantages such as good biocompatibility and low operating temperatures. Most of these electrothermal grippers [7,9] consist of a polymer structure with a metal heater deposited on top that defines one or two hot arm U-shaped actuators. Even if in some cases very good performance is achieved [9], limited attention has been paid to the thermal and geometrical optimisation of the structure. A recent development [6], proposes a microgripper fabricated with metallic heaters on top of a silicon heat conducting structure embedded in SU8 that permit a better heat diffusion through the thickness of the polymer layer. Despite demonstrating better performance than some other grippers [7], the integration of silicon conducting elements makes the overall structure stiffer and smaller displacements are obtained for the same input power. In this work we present an electro-thermally actuated polymeric microgripper with optimised thermal efficiency and geometry. The total suppression of the heat generation in the cold arm permits not only larger deflections for the
1220
B. Solano, D. Wood / Microelectronic Engineering 84 (2007) 1219–1222
same input power but also the possibility to integrate active elements in that arm such as force sensors or extra resistors to produces bi-directional in-plane movements. 2. The microgripper 2.1. Device structure The microgripper device consists of two thermal actuators positioned face-to-face plus a pair of extended prongs (Fig. 1). The main body of the microgripper is composed of a multi-layer structure which encapsulates a thin metallic heater in between two layers of dielectric polymeric material. When a current is applied through the conducting layer, the dissipated power heats the surrounding polymeric material and the whole structure deflects in-plane due to differential thermal expansion of its constituent parts. The base material of the microgripper is the polymer, SU8 (Microchem Corp.), which gives shape to the anchors, the actuators and the extension tips. Joule heating is produced via the thin layer of gold deposited and patterned on top of the thick SU8. The choice of materials has been based on the favourable mechanical and chemical properties of the highly cross-linked SU8, along with its biocompatibility, ease of fabrication and low cost. After fabrication the polymeric microgripper is bonded to a PCB, leaving the microgripper overhanging. 2.2. Novel thermal actuator design The principle of operation of the microgripper is the Ushaped thermal actuator. The actuation structure, composed of two adjacent micro-beams, deflects at its tips by the asymmetrical thermal expansion of its constituent parts which have variable cross-sections and lengths. The inplane deflection is controlled by the difference in temperature established in between the hot and cold arms.
Fig. 1. SEM photo of a fabricated microgripper.
Fig. 2. Schematic of: (a) standard U-shaped thermal actuator, and (b) novel U-shaped thermal actuator.
In conventional U-shaped actuators (Fig. 2a) current flows through both arms reducing considerably the effective temperature difference (DT) between them. With our proposed architecture for the U-shaped actuator (Fig. 2b), current flows in and out from the hot arm and no parasitic Joule heating is produced in the cold arm which in turn will maximise the deflection for a given input power. After the application of the current through the conducting layer, the hot arm will expand more than the cold arm. As both arms are connected at their free ends, an in-plane deflection around the z axis will take place. 3. Fabrication The microgripper has been fabricated using a novel combination of standard surface micromachining techniques in a three-mask process. This process, shown in Fig. 3, enables the release of encapsulated stress free gold/SU8 structures and membranes with incorporated and accessible electrical contact pads. It does not require the use of polymeric or metal sacrificial layers and it relies on the induced diminished adhesion between gold/SU8 and the silicon substrate. The process can be described as follows. First, a thin layer (30 nm) of OMNICOATTM is spin deposited on a Si wafer. The thickness of this layer increases or reduces the adhesion of the SU8 structures in the release step and can be adapted to other geometries. Then, a layer of SU8-2 (1.5 lm) is spin deposited and patterned using UV-lithography. After that, a thin layer of gold (240 nm) is thermally evaporated on top of the SU8 and patterned. Then a layer of SU8-10 (30 lm) is spin deposited and patterned. During the development of the second layer of SU8, the whole structure will start to peel off from the Si wafer without residual stress or damage to the electrical pads. The quality of the release process will depend on the geometry of the structure to be released, the thickness of the layers and the cooling down times applied after post
B. Solano, D. Wood / Microelectronic Engineering 84 (2007) 1219–1222
1. Spin deposition and baking of OmnicoatTM (30 nm)
Deflection [ m]
2. Spin deposition of SU8 (2 m), patterning and development
3. Thermal evaporation of gold and patterning
4. Spin deposition of SU8 (30 m), patterning and release step at the same time as development
280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 -20 -0.2
Omnicoat
SU8
Au
Si
(84°C) Model 2 Model 1
(54°C)
(52°C)
(30˚C)
(41°C)
(10˚C) (20°C) (6.5°C) 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Input Voltage [V] Fig. 5. Measured deflection versus input voltage for models 1 and 2. The numbers in brackets represent the temperature above ambient for a selection of data points.
Fig. 3. Fabrication process.
exposure baking (PEB). The electrical contact pads of the gold layer are accessible through windows defined in the first layer of SU8. In a subsequent process the microgripper is diced, bonded and wired onto a PCB. 4. Experiments 4.1. Performance characteristics Two possible configurations of the microgripper have been fabricated (Fig. 4): one with the two hot arms facing each other and a second one with the two cold arms facing
Model 1 L h1 Lj
1221
Wh
W Au
Model 2 L h2 Lj
W Au
Wh Lf Wf
each other. In the first case, the normally closed mode (model 1), actuation is needed only for the release operation and no actuation is needed when holding the object. In the second case (model 2), the normally open mode, the actuators need to be operated continuously to ensure the gripping of the sample. The geometry of the actuators is slightly different in each case. In model 1, the hot and cold arms have identical geometries. In model 2, the cold arm is twice the width of the hot arm and the deflection is magnified with a flexure. This flexure is three times thinner than the hot arm and acts as a hinge. A constant current source was used to actuate the microgrippers. Current levels ranging from 5 to 40 mA were applied. The measured displacements represent the opening in between the two arms of the gripper from its initial position. For each data point, the voltage reading was used to estimate the resistance change of the metallization layer and to determine the average temperature of the hot arm. Fig. 5 shows the deflection and average temperature change for models 1 and 2 versus input voltage. The dots are the measured deflection data and the numbers between brackets represents the increase of temperature above ambient temperature (20 °C).The maximum power consumed for models 1 and 2 is calculated to be 77 mW and 64 mA, respectively. For displacements of the order of the size of biological cells (2–150 lm) average temperatures are less than to 80 °C making this device suitable for biomanipulation. 4.2. Micromanipulation experiments
Fig. 4. Schematic drawing of the two different microgripper models. Critical dimensions: Lh1 = 2000 lm; Lh2 = 1500 lm; Lj = 1600 lm; Jg1 = 60 lm; Jg = 140 lm; g = 60 lm; Wh = 140 lm; Wf = 40 lm; Lf = 360 lm; WAu = 40 lm. Out-of-plane thickness = 30 lm.
The ability of the microgripper to manipulate microscale objects has been experimentally verified by grasping, holding and positioning gold coated SU8 micro-cylinders with an approximate diameter of 100 lm. The sequence of photos shown in Fig. 6 demonstrates the successful manipula-
1222
B. Solano, D. Wood / Microelectronic Engineering 84 (2007) 1219–1222
Fig. 6. Micromanipulation sequence where the microgripper (model 1) is used to pick and place a gold coated SU8 object.
tion of a single micro-cylinder in a grid of 500 lm spaced dots. The experiments were conducted in air in a clean room environment at 20 °C.
by One North-East and the County Durham sub-regional Partnership fund. References
5. Conclusions Electro-thermally actuated microgrippers with novel integrated thermal actuators have been designed, fabricated and characterised. The choice of biocompatible materials together with the low actuation voltages required, and large deflection produced at low temperatures, make this microgripper highly suitable for bio-manipulation experiments in air or in aqueous media. Future work includes the realisation of experiments in biological media and the inclusion of a second resistor on the ‘cold’ arm to provide bi-directional movement. Acknowledgements The authors acknowledge A.J. Gallant for the help with the manipulation experiments. This work was part-funded
[1] M. Carrozza, A. Eisinberg, A. Menciassi, J. Micromech. Microeng. 10 (2000) 271–276. [2] C. Kim, A. Pisano, R. Muller, J. MEMS 1 (1992). [3] B.E. Volland, H. Heerlein, I.W. Rangelow, Microelectron. Eng. 61 (2002) 1015–1023. [4] R. Wierzbicki, K. Houston, H. Heerlein, W. Barth, T. Debski, A. Eisinberg, A. Mendicassi, M.C. Carrozza, P. Dario, Microelectron. Eng. 83 (2006) 1651–1654. [5] I. Roch, P. Bidaud, D. Collard, L. Buchaillot, J. Micromech. Microeng. 13 (2003) 330–336. [6] T.C. Duc, G. Lau, J. Wei, P. Sarro, in: MicroMechanics Europe Workshop, Southampton, UK, September 3–5, 2006, p. 197. [7] N. Nguyen, S. Ho, C. Low, J. Micromech. Microeng. 14 (2004) 969– 974. [8] H. Du, C. Su, M. Lim, W. Jin, Smart Mater. Struct. 8 (1999) 616– 622. [9] N. Chronis, L. Lee, IEEE J. MEMS 14 (2005) 857–863. [10] K. Ivanova, T. Ivanov, A. Badar, B.E. Volland, I.W. Rangelow, D. Andrijasevic, F. Sumecz, S. Fisher, M. Spitzbart, W. Brenner, I. Kostic, Microelectron. Eng. 83 (2006) 1393–1395.