- Email: [email protected]
Hollow AFM cantilever pipette
Microelectronic Engineering 124 (2014) 22–25
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Hollow AFM cantilever pipette Murali Krishna Ghatkesar 1,⇑, Hector Hugo Perez Garza 1, Urs Staufer Micro and Nano Engineering Group, Faculty of Mechanical Maritime, and Materials (3mE) Engineering, Delft University of Technology, Mekelweg 2, Delft 2628 CD, The Netherlands
a r t i c l e
i n f o
Article history: Received 30 October 2013 Received in revised form 17 March 2014 Accepted 9 April 2014 Available online 18 April 2014 Keywords: Hollow atomic force microscope Pipette AFM Femto-liter Dispensing Aspiration
a b s t r a c t Hollow atomic force microscope (AFM) cantilevers were used earlier to dispense liquids of sub 100 nm diameter and inject reagents into one single cell. However, aspiration of liquid was never shown. In this work, we demonstrate pipetting (dispensing and aspiration) of liquid using hollow AFM cantilevers. The microfabricated 155 lm long cantilever had a volume capacity of 2.64 pL, with pipetting flow rate control of 5 fL/s. This functionality enables to move liquid matter at nanoscale, do force-spectroscopy and imaging with the same AFM. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
2. Material and methods
Atomic Force Microscope (AFM) is not only used for imaging and force-spectroscopy but also to manipulate solid-state matter [1,2]. It opened the doors for fluids to reach nanometer scale resolution when AFM was used for the first time to deposit liquids using dip-pen nanolithography technique [3,4]. It gave unprecedented access to deposit small volumes of liquid up to zepto-liters (1021 L) with a spot size of less than 100 nm in diameter [5]. To avoid frequent dipping of the tip, the cantilevers were made hollow to carry liquid directly from the on-chip reservoir to the aperture near the tip of the AFM, from where the liquid is dispensed [6,7]. The aperture was made adjacent to the tip wall without compromising the tip sharpness. This improvement increased the versatility of the device, enabling to deposit liquid even inside the cell nucleus [8,9]. However, up until now all the efforts have been only focused on dispensing liquids. Recently, we have shown that a hollow cantilever can be used to even aspirate (withdraw) liquids using evaporation based pumping [10,11]. However, the self-limiting behavior of this technique forced the aspiration to be stopped after a certain volume threshold is reached. In this work, we show that the hollow AFM cantilever can be used as a standard pipette to achieve both dispensing and aspiration of liquids.
The hollow AFM cantilever was made of SiO2 with Si3N4 tip. It was transparent, and the motion of the fluid was clearly visible with an optical microscope. To fabricate the device, two silicon wafers, one KOH-etched with the fluidic reservoir and the other DRIE-etched with the predefined hollow cantilever channels with Si3N4 tip, were fused together. Subsequently, oxide growth and selective etching were done to get the final device as detailed elsewhere [6,12]. The trapezoidal-shaped fluidic reservoir etched along the h1 1 1i direction of the silicon had a wide opening (square base of 580 lm side) on the upper side of the chip and a narrow opening (square of 15 lm side) on the inner side that was connected to the microfluidic channel of the hollow cantilever. The U-shaped hollow cantilever was 155.7 lm long (Fig. 1) with a SiO2 wall thickness of 1.5 lm. The Si3N4 tip had a radius of 20 nm. The fluidic reservoir and the hollow cantilever had a capacity of 19.8 nL and 2.62 pL respectively. All the dimensions and other device properties are summarized in Table 1. To inject and aspirate various kinds of liquids, the on-chip reservoir was connected to an external syringe pump. To make this fluidic interface possible, an 800 lm-thick polydimethylsiloxane (PDMS) sheet was glued on top of the reservoir. A 9 mm long stainless steel (SS) tube with 200 lm outer diameter and 100 lm inner diameter was used to pierce into the PDMS on top of the fluidic reservoir. The compliance of PDMS gave a good sealing to the tubing that could withstand 2 bars of pressure. The other end of the SS tubing was connected to syringe pump using Tygon tubing. The overview of the fluidic connections is shown in Fig. 2. An aperture of 2 lm in diameter was made near
⇑ Corresponding author. Tel.: +31 15 2782299. 1
E-mail address: [email protected] (M.K. Ghatkesar). These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.mee.2014.04.019 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
M.K. Ghatkesar et al. / Microelectronic Engineering 124 (2014) 22–25
23
Fig. 1. Hollow AFM cantilever chip. The size of the chip is chosen to fit in a commercial AFM system. (a) Front side of the chip with fluid reservoir from where liquid is injected into the cantilever. (b) Back side of the chip where the SiO2 hollow cantilever is located. The microfluidic channel of the hollow cantilever extends till reservoir. (c) U-shaped cantilever, both channels connected to the reservoir. (d) Cross-section of the hollow cantilever. (e) Si3N4 tip. (f) Aperture on the tip wall without compromising the sharpness of the tip.
Table 1 Hollow cantilever pipette characteristics. Parameter
Units
Cantilever dimensions (each leg) Length (L); Width(w); Height(H) Fluidic channel dimensions (each leg) Length(Lch); Width(Wch); Height (Wch) Tip pyramid dimensions Base (Wpy) Height(hpy); Wall thickness (Tpy-wall) Leg separation Aperture diameter on the tip Resonance frequency – air filled (fa) Resonance frequency – water filled (fw) Estimated mass of water inside the cantilever from the frequency shift (Dmw ) Spring constant (air filled) (k) Quality factor (air filled) (Q) Quality factor (water filled) (Qw) Length of the microfluidic channel (Including the cantilever length) Flow rate (minimum) (Qmin) Hydrodynamic resistance (Rhyd)
155 lm; 6.4 lm; 4.9 lm 153.5 lm; 3.7 lm; 2.2 lm 8 lm; 2.2 lm; 0.2 lm 6 lm 2 lm 150.063 kHz 149.928 kHz 3.4 pg
(a)
the apex of the cantilever tip using focused ion beam. A 10 nm thick gold layer was deposited on the tip-side of the cantilever to avoid charging-effect during FIB, which also helps to reflect the laser beam during measurements. The other side of the cantilever was still transparent to visualize the fluid flow. Laser beam-deflection method was used to measure the cantilever deflection and the resonance frequency. 3. Results Flow rate was set on the external syringe pump to control the amount of pumping and aspiration of deionized water inside the transparent hollow channels. Initially, the cantilever was empty. Then, liquid was injected into the cantilever until the fluidic channels got filled (Fig. 3, panel 1) and a droplet was formed near the tip (Fig. 3, panel 2). Once the droplet was hanging from the tip, the liquid was fully aspirated back inside the channels (Fig. 3, panels 3 and 4) by switching from injection to withdraw option on the syringe pump. See Supplementary material for the videos. All the
4.3 N/m 457 468 684 lm 5 fL/s 1.5 1017 Pa-s/m3
(b)
Fig. 2. (a) A schematic of cross-sectional view of the fluidic connection from the on-chip reservoir to the external syringe pump. (b) The photograph of the actual setup. The stainless tube is pierced into the glued PDMS on top of the fluid reservoir. Two layers of PDMS are used to give some holding area to the tube. It formed a good seal and gave flexibility to the tube during external connections. The other end of the stainless steel tube is connected to syringe pump using Tygon tubing.
24
M.K. Ghatkesar et al. / Microelectronic Engineering 124 (2014) 22–25
(a) Injection
(b) Aspiration
Fig. 3. The pictures shown here are snapshots from a movie (see Supplementary material). The cantilever is seen from the top with tip pointing towards bottom side at the free end. (a) Injection of deionized water into the cantilever by external syringe pump. Big solid arrows indicate the direction of fluid flow. In Panel 1 the cantilever is filled with water. By further injecting the liquid, a water droplet is formed near the apex as shown in Panel 2. (b) Aspiration of liquid into the cantilever by switching to withdraw mode on the syringe pump. In panel 3, fluid getting retracted with applied negative pressure (direction is shown by solid arrows). The blue square inset shows the magnified view of the fluid meniscus inside the cantilever. Finally the empty cantilever after entire liquid has been aspirated is shown in panel 4. The droplet formed shown in panel 2 was completely aspirated into the cantilever. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
calculated as sum of the cantilever channels volume and the hollow pyramid volume from V ¼ ½2 ðLch W ch Hch Þþ ½1=3 hpy ðW py 2 T pywall Þ2 (see Table 1) amounting to 2.62 pL. For water (density of 1000 Kg/m3), this corresponds to a mass of 2.62 pg. The difference in the estimated and the calculated mass is attributed to the variation in the channel dimensions and the cantilever length during fabrication. The mass resolution is defined by the ability to measure the minimum frequency shift, which is limited by the quality factor of the resonance peak.
4. Discussion The results indicate that the hollow microfluidic cantilever could inject and aspirate liquid like a standard pipette. The setup (Fig. 2) used to promote the pipetting capability of the device has many elements that offer hydrodynamic resistance to the fluid flow. An electrical circuit equivalent of the fluidic elements is shown in Fig. 5. The syringe pump is like a current source for a set flow rate. Then the various fluidic resistors due to syringe, tygon tube, stainless steel tube, on-chip reservoir, rectangular microfluidic cantilever channel and the aperture on the cantilever tip are connected in series. The syringe barrel and tygon tube are elastic in nature, and expand when pressure is applied, therefore they also act as capacitors. Their capacitance value determines the delay in response between the injection and aspiration steps. The dominant resistance in this network usually comes from the aperture on the tip; but in our case, as the aperture is 2 lm in diameter, which is almost the size of the channel height, we neglect its resistance. The next dominant hydrodynamic resistance contributor in the network is the microfluidic channel of the cantilever. The two legs of the microfluidic channel of the cantilever form parallel resistors that are connected to the reservoir. Even though the cantilever length is 155 lm, the channel extends to the reservoir with a total length of 684 lm. The hydrodynamic resistance (Rhyd ) of a rectangular channel with w > h is given by
Rhyd ¼ Fig. 4. The fundamental resonance frequency of the vibrating cantilever is shown. A negative frequency shift of 135 Hz was obtained when the cantilever was filled with water. The frequency shifted back to the original value when it was again empty. The quality factor was almost constant indicating that filling the cantilever with water does not increase damping.
experiments were performed in ambient conditions. The fundamental resonance when the cantilever was filled with air (fa) and filled with water (fw) was measured to be 150.063 kHz and 149.928 kHz respectively (Fig. 4). The quality factor was relatively unchanged. A frequency shift of 135 Hz corresponds to an estimated distributed mass (Dmw ) uptake of 3.4 pg. The hollow cantilever can hold up to 2.62 pL volume of liquid as calculated from the channel dimensions of the device including the hollow tip
Dp 12gL ¼ 3 Q h ðw 0:63hÞ
Where, Dp is pressure difference between inlet and outlet, Q is fluid flow rate, g is fluid’s dynamic viscosity, L is length of the microfluidic channel, h is channel height and w is channel width. Using the dimension values summarized in the Table 1, the Rhyd obtained for single cantilever leg is 3.3 1017 Pa-s/m3. The effective resistance is the parallel combination of two legs. Therefore, the hydrodynamic resistance of the network is 1.65 1017 Pa-s/m3. For pipetting through a 2 lm aperture on the cantilever tip a minimum flow rate of 117.2 fL/s was obtained (see video1 in the Supplementary material). The challenge is to have reproducible control on pipetting. The capacitive elements depending on their elastic modulus, length, and hysteresis will influence the switching speed between dispensing and aspiration. In a separate experiment with cantilever
Fig. 5. Electrical equivalent of the hydrodynamic resistance at various stages in the setup.
M.K. Ghatkesar et al. / Microelectronic Engineering 124 (2014) 22–25
arrays connected in series (see video2 in the Supplementary material), a minimum flow rate of 5 fL/s could be controlled. Our next efforts are to get better control on the flow rate, pipetting multiple liquids and use smaller apertures for even smaller volume control. One of the issues with small nozzles is clogging, which will also be addressed in our future experiments. Pipetting tests inside the liquid environment will also be performed. 5. Conclusion We have demonstrated pipetting (both injection and aspiration) functionality in a microfluidic hollow AFM cantilever. The transparent SiO2 cantilever channels were 153.5 lm long, 3.7 lm wide and 2.2 lm thick. For a 2 lm aperture a hydrodynamic resistance of 1.5 1017 Pa-s/m3 was obtained. A minimum flow control of 5 fL/s was obtained by using an external syringe pump. Due to the small size and unique capability, it can be used to inject and withdraw liquids from containers as small as a single living cell in the liquid environment. Acknowledgements This work is supported by NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners. We acknowledge the members of the Micro and Nano Engineering group for their valuable suggestions and the fruitful discussions. We also thank the members of the technical staff of the DIMES Technology Center at TU Delft for the support provided during the fabrication of the devices.
25
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mee.2014.04.019. References [1] T. Junno, K. Deppert, L. Montelius, L. Samuelson, Appl. Phys. Lett. 66 (1995) 3627–3629. [2] S. Thalhammer, R.W. Stark, S. Muller, J. Wienberg, W.M. Heckl, J. Struct. Biol. 119 (1997) 232–237. [3] R.D. Piner, J. Zhu, F. Xu, S.H. Hong, C.A. Mirkin, Science 283 (1999) 661–663. [4] D.S. Ginger, H. Zhang, C.A. Mirkin, Angew. Chem. Int. Ed. 43 (2004) 30–45. [5] K. Salaita, S.W. Lee, X.F. Wang, L. Huang, T.M. Dellinger, C. Liu, C.A. Mirkin, Small 1 (2005) 940–945. [6] T. Hug, T. Biss, N. De Rooij, U. Staufer, Solid-State Sensors, Actuators and Microsystems, 2005. Digest of Technical Papers. TRANSDUCERS’05. The 13th International Conference on, IEEE (2005), pp. 1191–1194. [7] K.H. Kim, N. Moldovan, H.D. Espinosa, Small 1 (2005) 632–635. [8] A. Meister, M. Gabi, P. Behr, P. Studer, J. Voros, P. Niedermann, J. Bitterli, J. Polesel-Maris, M. Liley, H. Heinzelmann, T. Zambelli, Nano Lett. 9 (2009) 2501– 2507. [9] P. Stiefel, F.I. Schmidt, P. Dorig, P. Behr, T. Zambelli, J.A. Vorholt, J. Mercer, Nano Lett. 12 (2012) 4219–4227. [10] H.H.P. Garza, M.K. Ghatkesar, U. Staufer, Institution of Engineering and Technology, (2013). [11] F. Heuck, T. Hug, T. Akiyama, P.L.T.M. Frederix, A. Engel, A. Meister, H. Heinzelmann, N.F. de Rooij, U. Staufer, Microelectron. Eng. 85 (2008) 1302– 1305. [12] F.C.A. Heuck, PhD Thesis, Mechanical, Maritime, and Materials Engineering, Delft University of Technology, The Netherlands, 2010.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Hollow AFM cantilever pipette Murali Krishna Ghatkesar 1,⇑, Hector Hugo Perez Garza 1, Urs Staufer Micro and Nano Engineering Group, Faculty of Mechanical Maritime, and Materials (3mE) Engineering, Delft University of Technology, Mekelweg 2, Delft 2628 CD, The Netherlands
a r t i c l e
i n f o
Article history: Received 30 October 2013 Received in revised form 17 March 2014 Accepted 9 April 2014 Available online 18 April 2014 Keywords: Hollow atomic force microscope Pipette AFM Femto-liter Dispensing Aspiration
a b s t r a c t Hollow atomic force microscope (AFM) cantilevers were used earlier to dispense liquids of sub 100 nm diameter and inject reagents into one single cell. However, aspiration of liquid was never shown. In this work, we demonstrate pipetting (dispensing and aspiration) of liquid using hollow AFM cantilevers. The microfabricated 155 lm long cantilever had a volume capacity of 2.64 pL, with pipetting flow rate control of 5 fL/s. This functionality enables to move liquid matter at nanoscale, do force-spectroscopy and imaging with the same AFM. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
2. Material and methods
Atomic Force Microscope (AFM) is not only used for imaging and force-spectroscopy but also to manipulate solid-state matter [1,2]. It opened the doors for fluids to reach nanometer scale resolution when AFM was used for the first time to deposit liquids using dip-pen nanolithography technique [3,4]. It gave unprecedented access to deposit small volumes of liquid up to zepto-liters (1021 L) with a spot size of less than 100 nm in diameter [5]. To avoid frequent dipping of the tip, the cantilevers were made hollow to carry liquid directly from the on-chip reservoir to the aperture near the tip of the AFM, from where the liquid is dispensed [6,7]. The aperture was made adjacent to the tip wall without compromising the tip sharpness. This improvement increased the versatility of the device, enabling to deposit liquid even inside the cell nucleus [8,9]. However, up until now all the efforts have been only focused on dispensing liquids. Recently, we have shown that a hollow cantilever can be used to even aspirate (withdraw) liquids using evaporation based pumping [10,11]. However, the self-limiting behavior of this technique forced the aspiration to be stopped after a certain volume threshold is reached. In this work, we show that the hollow AFM cantilever can be used as a standard pipette to achieve both dispensing and aspiration of liquids.
The hollow AFM cantilever was made of SiO2 with Si3N4 tip. It was transparent, and the motion of the fluid was clearly visible with an optical microscope. To fabricate the device, two silicon wafers, one KOH-etched with the fluidic reservoir and the other DRIE-etched with the predefined hollow cantilever channels with Si3N4 tip, were fused together. Subsequently, oxide growth and selective etching were done to get the final device as detailed elsewhere [6,12]. The trapezoidal-shaped fluidic reservoir etched along the h1 1 1i direction of the silicon had a wide opening (square base of 580 lm side) on the upper side of the chip and a narrow opening (square of 15 lm side) on the inner side that was connected to the microfluidic channel of the hollow cantilever. The U-shaped hollow cantilever was 155.7 lm long (Fig. 1) with a SiO2 wall thickness of 1.5 lm. The Si3N4 tip had a radius of 20 nm. The fluidic reservoir and the hollow cantilever had a capacity of 19.8 nL and 2.62 pL respectively. All the dimensions and other device properties are summarized in Table 1. To inject and aspirate various kinds of liquids, the on-chip reservoir was connected to an external syringe pump. To make this fluidic interface possible, an 800 lm-thick polydimethylsiloxane (PDMS) sheet was glued on top of the reservoir. A 9 mm long stainless steel (SS) tube with 200 lm outer diameter and 100 lm inner diameter was used to pierce into the PDMS on top of the fluidic reservoir. The compliance of PDMS gave a good sealing to the tubing that could withstand 2 bars of pressure. The other end of the SS tubing was connected to syringe pump using Tygon tubing. The overview of the fluidic connections is shown in Fig. 2. An aperture of 2 lm in diameter was made near
⇑ Corresponding author. Tel.: +31 15 2782299. 1
E-mail address: [email protected] (M.K. Ghatkesar). These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.mee.2014.04.019 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
M.K. Ghatkesar et al. / Microelectronic Engineering 124 (2014) 22–25
23
Fig. 1. Hollow AFM cantilever chip. The size of the chip is chosen to fit in a commercial AFM system. (a) Front side of the chip with fluid reservoir from where liquid is injected into the cantilever. (b) Back side of the chip where the SiO2 hollow cantilever is located. The microfluidic channel of the hollow cantilever extends till reservoir. (c) U-shaped cantilever, both channels connected to the reservoir. (d) Cross-section of the hollow cantilever. (e) Si3N4 tip. (f) Aperture on the tip wall without compromising the sharpness of the tip.
Table 1 Hollow cantilever pipette characteristics. Parameter
Units
Cantilever dimensions (each leg) Length (L); Width(w); Height(H) Fluidic channel dimensions (each leg) Length(Lch); Width(Wch); Height (Wch) Tip pyramid dimensions Base (Wpy) Height(hpy); Wall thickness (Tpy-wall) Leg separation Aperture diameter on the tip Resonance frequency – air filled (fa) Resonance frequency – water filled (fw) Estimated mass of water inside the cantilever from the frequency shift (Dmw ) Spring constant (air filled) (k) Quality factor (air filled) (Q) Quality factor (water filled) (Qw) Length of the microfluidic channel (Including the cantilever length) Flow rate (minimum) (Qmin) Hydrodynamic resistance (Rhyd)
155 lm; 6.4 lm; 4.9 lm 153.5 lm; 3.7 lm; 2.2 lm 8 lm; 2.2 lm; 0.2 lm 6 lm 2 lm 150.063 kHz 149.928 kHz 3.4 pg
(a)
the apex of the cantilever tip using focused ion beam. A 10 nm thick gold layer was deposited on the tip-side of the cantilever to avoid charging-effect during FIB, which also helps to reflect the laser beam during measurements. The other side of the cantilever was still transparent to visualize the fluid flow. Laser beam-deflection method was used to measure the cantilever deflection and the resonance frequency. 3. Results Flow rate was set on the external syringe pump to control the amount of pumping and aspiration of deionized water inside the transparent hollow channels. Initially, the cantilever was empty. Then, liquid was injected into the cantilever until the fluidic channels got filled (Fig. 3, panel 1) and a droplet was formed near the tip (Fig. 3, panel 2). Once the droplet was hanging from the tip, the liquid was fully aspirated back inside the channels (Fig. 3, panels 3 and 4) by switching from injection to withdraw option on the syringe pump. See Supplementary material for the videos. All the
4.3 N/m 457 468 684 lm 5 fL/s 1.5 1017 Pa-s/m3
(b)
Fig. 2. (a) A schematic of cross-sectional view of the fluidic connection from the on-chip reservoir to the external syringe pump. (b) The photograph of the actual setup. The stainless tube is pierced into the glued PDMS on top of the fluid reservoir. Two layers of PDMS are used to give some holding area to the tube. It formed a good seal and gave flexibility to the tube during external connections. The other end of the stainless steel tube is connected to syringe pump using Tygon tubing.
24
M.K. Ghatkesar et al. / Microelectronic Engineering 124 (2014) 22–25
(a) Injection
(b) Aspiration
Fig. 3. The pictures shown here are snapshots from a movie (see Supplementary material). The cantilever is seen from the top with tip pointing towards bottom side at the free end. (a) Injection of deionized water into the cantilever by external syringe pump. Big solid arrows indicate the direction of fluid flow. In Panel 1 the cantilever is filled with water. By further injecting the liquid, a water droplet is formed near the apex as shown in Panel 2. (b) Aspiration of liquid into the cantilever by switching to withdraw mode on the syringe pump. In panel 3, fluid getting retracted with applied negative pressure (direction is shown by solid arrows). The blue square inset shows the magnified view of the fluid meniscus inside the cantilever. Finally the empty cantilever after entire liquid has been aspirated is shown in panel 4. The droplet formed shown in panel 2 was completely aspirated into the cantilever. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
calculated as sum of the cantilever channels volume and the hollow pyramid volume from V ¼ ½2 ðLch W ch Hch Þþ ½1=3 hpy ðW py 2 T pywall Þ2 (see Table 1) amounting to 2.62 pL. For water (density of 1000 Kg/m3), this corresponds to a mass of 2.62 pg. The difference in the estimated and the calculated mass is attributed to the variation in the channel dimensions and the cantilever length during fabrication. The mass resolution is defined by the ability to measure the minimum frequency shift, which is limited by the quality factor of the resonance peak.
4. Discussion The results indicate that the hollow microfluidic cantilever could inject and aspirate liquid like a standard pipette. The setup (Fig. 2) used to promote the pipetting capability of the device has many elements that offer hydrodynamic resistance to the fluid flow. An electrical circuit equivalent of the fluidic elements is shown in Fig. 5. The syringe pump is like a current source for a set flow rate. Then the various fluidic resistors due to syringe, tygon tube, stainless steel tube, on-chip reservoir, rectangular microfluidic cantilever channel and the aperture on the cantilever tip are connected in series. The syringe barrel and tygon tube are elastic in nature, and expand when pressure is applied, therefore they also act as capacitors. Their capacitance value determines the delay in response between the injection and aspiration steps. The dominant resistance in this network usually comes from the aperture on the tip; but in our case, as the aperture is 2 lm in diameter, which is almost the size of the channel height, we neglect its resistance. The next dominant hydrodynamic resistance contributor in the network is the microfluidic channel of the cantilever. The two legs of the microfluidic channel of the cantilever form parallel resistors that are connected to the reservoir. Even though the cantilever length is 155 lm, the channel extends to the reservoir with a total length of 684 lm. The hydrodynamic resistance (Rhyd ) of a rectangular channel with w > h is given by
Rhyd ¼ Fig. 4. The fundamental resonance frequency of the vibrating cantilever is shown. A negative frequency shift of 135 Hz was obtained when the cantilever was filled with water. The frequency shifted back to the original value when it was again empty. The quality factor was almost constant indicating that filling the cantilever with water does not increase damping.
experiments were performed in ambient conditions. The fundamental resonance when the cantilever was filled with air (fa) and filled with water (fw) was measured to be 150.063 kHz and 149.928 kHz respectively (Fig. 4). The quality factor was relatively unchanged. A frequency shift of 135 Hz corresponds to an estimated distributed mass (Dmw ) uptake of 3.4 pg. The hollow cantilever can hold up to 2.62 pL volume of liquid as calculated from the channel dimensions of the device including the hollow tip
Dp 12gL ¼ 3 Q h ðw 0:63hÞ
Where, Dp is pressure difference between inlet and outlet, Q is fluid flow rate, g is fluid’s dynamic viscosity, L is length of the microfluidic channel, h is channel height and w is channel width. Using the dimension values summarized in the Table 1, the Rhyd obtained for single cantilever leg is 3.3 1017 Pa-s/m3. The effective resistance is the parallel combination of two legs. Therefore, the hydrodynamic resistance of the network is 1.65 1017 Pa-s/m3. For pipetting through a 2 lm aperture on the cantilever tip a minimum flow rate of 117.2 fL/s was obtained (see video1 in the Supplementary material). The challenge is to have reproducible control on pipetting. The capacitive elements depending on their elastic modulus, length, and hysteresis will influence the switching speed between dispensing and aspiration. In a separate experiment with cantilever
Fig. 5. Electrical equivalent of the hydrodynamic resistance at various stages in the setup.
M.K. Ghatkesar et al. / Microelectronic Engineering 124 (2014) 22–25
arrays connected in series (see video2 in the Supplementary material), a minimum flow rate of 5 fL/s could be controlled. Our next efforts are to get better control on the flow rate, pipetting multiple liquids and use smaller apertures for even smaller volume control. One of the issues with small nozzles is clogging, which will also be addressed in our future experiments. Pipetting tests inside the liquid environment will also be performed. 5. Conclusion We have demonstrated pipetting (both injection and aspiration) functionality in a microfluidic hollow AFM cantilever. The transparent SiO2 cantilever channels were 153.5 lm long, 3.7 lm wide and 2.2 lm thick. For a 2 lm aperture a hydrodynamic resistance of 1.5 1017 Pa-s/m3 was obtained. A minimum flow control of 5 fL/s was obtained by using an external syringe pump. Due to the small size and unique capability, it can be used to inject and withdraw liquids from containers as small as a single living cell in the liquid environment. Acknowledgements This work is supported by NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners. We acknowledge the members of the Micro and Nano Engineering group for their valuable suggestions and the fruitful discussions. We also thank the members of the technical staff of the DIMES Technology Center at TU Delft for the support provided during the fabrication of the devices.
25
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mee.2014.04.019. References [1] T. Junno, K. Deppert, L. Montelius, L. Samuelson, Appl. Phys. Lett. 66 (1995) 3627–3629. [2] S. Thalhammer, R.W. Stark, S. Muller, J. Wienberg, W.M. Heckl, J. Struct. Biol. 119 (1997) 232–237. [3] R.D. Piner, J. Zhu, F. Xu, S.H. Hong, C.A. Mirkin, Science 283 (1999) 661–663. [4] D.S. Ginger, H. Zhang, C.A. Mirkin, Angew. Chem. Int. Ed. 43 (2004) 30–45. [5] K. Salaita, S.W. Lee, X.F. Wang, L. Huang, T.M. Dellinger, C. Liu, C.A. Mirkin, Small 1 (2005) 940–945. [6] T. Hug, T. Biss, N. De Rooij, U. Staufer, Solid-State Sensors, Actuators and Microsystems, 2005. Digest of Technical Papers. TRANSDUCERS’05. The 13th International Conference on, IEEE (2005), pp. 1191–1194. [7] K.H. Kim, N. Moldovan, H.D. Espinosa, Small 1 (2005) 632–635. [8] A. Meister, M. Gabi, P. Behr, P. Studer, J. Voros, P. Niedermann, J. Bitterli, J. Polesel-Maris, M. Liley, H. Heinzelmann, T. Zambelli, Nano Lett. 9 (2009) 2501– 2507. [9] P. Stiefel, F.I. Schmidt, P. Dorig, P. Behr, T. Zambelli, J.A. Vorholt, J. Mercer, Nano Lett. 12 (2012) 4219–4227. [10] H.H.P. Garza, M.K. Ghatkesar, U. Staufer, Institution of Engineering and Technology, (2013). [11] F. Heuck, T. Hug, T. Akiyama, P.L.T.M. Frederix, A. Engel, A. Meister, H. Heinzelmann, N.F. de Rooij, U. Staufer, Microelectron. Eng. 85 (2008) 1302– 1305. [12] F.C.A. Heuck, PhD Thesis, Mechanical, Maritime, and Materials Engineering, Delft University of Technology, The Netherlands, 2010.