Fabrication and Characterization of Novel Stretchable Force Sensor Using Melt Electrospinning
ICOMM 2014 No. 71
Junghyuk Ko1, Yonghyun Cho2, Sukhwinder Bhullar3, Martin Byung-Guk Jun4 1
University of Victoria, Mechanical Engineering Department, Canada:
[email protected] University of Victoria, Mechanical Engineering Department, Canada:
[email protected] 3 University of Victoria, Mechanical Engineering Department, Canada:
[email protected] 4 University of Victoria, Mechanical Engineering Department, Canada:
[email protected] 2
Abstract Melt electrospinning can be used to develop stretchable force sensor that can be designed and modulated based on the desired application. This work focuses on developing microstructured polymer based stretchable force sensor. In particular, we have fabricated poly (Ɛ-caprolactone) (PCL) sensors including machined PCL microfiber sheet, machined PCL sheet, no machined PCL microfiber sheet, and no machined PCL sheet. These sensors are fabricated for measure their conductivity properties such as electrical current resistance after gold particles were deposited. Our stretchable sensor can be attached on the desired substrate and could potentially be controlled by applied extensional force or strain. To our knowledge, we are the first to investigate the fabrication of stretchable force sensor through the use of melt electrospining Keywords: Force & Strain sensor, Melt electrospinning, Microfibers, Laser machining. 1. Introduction As force sensors could not be actually stretched therefore it would be tough and challenging to attach such sensor for physiological conditions such as human skin. For instance, in the case of control glove, the glove itself can be wearable but it can’t attach on human hand because it's not stretchable. Our auxetic stretchable sensor can be attached on skin and is controlled by force or strain. The objective of this study is how we could make stretchable sensors fabricated by melt electrospinning. Melt electrospinning is a relatively under-studied polymer processing technique which provides polymeric fibers with controllable and desired physical, mechanical and topographical properties [1-4]. Compared to solution electrospinning, the technique of melt electrospinning would lead into highly controllable products with an excellent degree of reproducibility for different kind of applications. These applications are various, and provide a broad set of challenges to researchers involved in electrospinning. Poly-єcaprolactone (PCL) as a biodegradable, saturated polyester has been used in this study as our polymer of choice. The superior rheological and viscoelastic properties within the group of aliphatic polymers makes PCL the polymer of choice to be applied during the development of microfiber and sheet based sensors based on melt extrusion processes. It is tailorable in its mechanical properties, rate of biodegradation, and mechanism of controlled release, solubility and crystallinity. PCL has a very low melting point (65˚C) and glass transition temperature (-60˚C). PCL is a hydrophobic, biocompatible polymer and is introduced as one of the most commonly used synthetic polymers for engineering applications. Coupled with relatively inexpensive production routes, this technique of melt electrospinning provides a capable fabrication stand
for the production of force sensors which may be manipulated physically, chemically and mechanically to possess tailorable properties. Here, we propose that PCL and microfiber machined sheet act as a force sensor, which transduce force into mechanical extension and thereby measuring the resistance. Key characteristics of the stretched sensors are very important to be considered when designing such sensors [5]. Virtues include high elasticity, large strain, low hysteresis and easy to fabricate and direct measurements while they should be also easy to mount and have an excellent degree of reproducibility [6, 7]. Our stretchable sensor can be attached on the desired substrate and could potentially be controlled by applied extensional force or strain. We have developed four sets of samples, including no machined PCL sheet (NPS), no machined PCL microfiber sheet (NMS), machined PCL sheet (MMS) and machined PCL microfiber sheet (MPS). All samples were transferred to gold coating device in order to deposit gold nanoparticle to fabricate extended force sensors. Samples are experimentally characterized in our lab to measure their conductivity properties such as electrical current resistance. 2. Material and Methods 2.1. Material and Melt electrospinning Poly (ε-caprolactone) (PCL) (Mn ~45,000) was purchased from (Sigma Aldrich, USA) with a melting point of 60 ˚C. A custom-made melt electrospinning apparatus (Fig. 1A) consisted of a computer numerical controlled (CNC) machine (K2 CNC Inc., USA), a custom-made chamber press, a syringe pump (New Era Pump Systems Inc., USA), a heating band (Heterwerks Inc., USA), a custom-made machined melting chamber, and custom-made nozzles that could be interchanged. The flat tipped
nozzles used to extrude the melt were fabricated from aluminum 6061 with internal diameters 200 μm (~10 μm)[1]. Fig. 1B is PCL microfiber sheet manufactured by melt electrospinning.
Fig. 2. (A) A custom-made mold and (B) PCL sheet.
2.2. Laser machining
Fig. 1. (A) Schematic of custom-built melt electrospinning device and (B) PCL microfiber sheet
During the melt electrospinning process, 20kV was applied to the molten PCL using a high applied voltage supply (Gamma High Voltage Research Inc., USA) with the working distance, 5cm, between the nozzle and aluminum foil collector. The PCL granules were dispensed into melting chamber with the nozzle attached and then heated to the desired temperature. The zero shear rate viscosity of PCL was measured using an ARES-G2 rheometer (TA Instruments, USA) as 291.5 Pa.sec. at 80°C using 25mm parallel plates geometry with the gap of 25mm [1]. For the collection of mesh looped microfibers, a wood plate covered with aluminum foil served as the counter electrode.
A schematic diagram of the femtosecond laser system in order to create the structure on PCL and microfiber sheets is shown in Fig. 3. Ti: Sapphire femtosecond laser which consists of Millenia VsTsunami (Spectra Physics, USA) and SpitfireEmpower (Spectra Physics, USA) with an approximately 120fs pulse duration, approximately 0.35 W output power and 1 kHz repetition rate at a central wavelength of 800nm was used to irradiate the materials in the experiment. The laser beam was guided into a microscope and focused by using a 20X objective lenses (Mitutoyo Co., Japan) with 0.42 numerical apertures. An electronic shutter controlled by computer was used to timely turn on/off the laser beam at desired location on the samples.
2.2. Pressure moulding In order to fabricate PCL sheet, a custom-made molds are machined by Aluminum 6061 as shown in Fig. 2A. The gap between male and female molds is 0.3 mm. PCL polymer is placed on a female mold after heating up to 80°C and then presses the polymer down with a male mold. After completely cool down, PCL sheet is removed from the molds as shown in Fig. 2B.
Fig. 3. Schematic of femtosecond laser machining
2.4. Gold coating The sensor samples were coated by the Anatech Hummer VI which is a metal sputter coater setup with a Gold Palladium target. Hummer VI parameters were 75 mTorr Argon and 10 mA current with 2minute run times. The PCL and microfiber samples are coated with 15nm thickness of gold particles. 2.5. Tensile and Resistance test setup As shown in Fig. 4, ALIO Vertical micro milling machine stage was used for stretching the samples continuously. The simultaneous tensile force was measured during the stretching operation using a Kistler table dynamometer (MiniDyn 9256C1). The resistance was also measured by Agilent multimeter (Agilent 34411A) during the operation. The samples were clamped by custom made clamper on center of dynamometer. Force data and Resistance data are simultaneously acquired using the setup.
Fig. 5. Geometrical design and deformation mechanism of an auxetic sensor
3.2. Fabrication of PCL auxetic sheet In the fabrication process the samples of PCL and microfiber sheets of 17.92mm (width) X 32.64mm (length) X 0.3mm (thickness) were fixed on glass slides by the double sided tape on a CNC 3 axis stage for machining. The speed of machining was fixed at 0.3 mm/s to prevent the polymer from collapsing and repeat 3 times with 0.1 mm depth increment each to confirm the samples to machine completely. The fabricate sheets with auxetic geometry are shown in Fig. 6.
Fig. 4. Setup for tensile and resistance test
3. Sensor design and fabrication 3.1. Design of the auxetic structure In the development of the stretchable force sensor, stretchable behavior is achieved through geometrical design and unique deformation mechanism of auxetic structures. For a brief description auxetic materials become wider when stretched and thinner when compressed [8-10]. It has been demonstrated through studies and experiences the same equipartition of mechanical stresses and the property of a material to resume its original shape or position after being bent, stretched, or compressed have recommended auxetic materials for applications in biomedical industry such as in implant and prostheses [11, 12]. Therefore, PCL and microfiber sheets were tailored with an angled solid squares geometry in which each unit cell contains four squares, each square contains four vertices, and two vertices correspond to one hinge using laser machining after designed in SolidWorks engineering software as illustrated in Fig. 5. The arrangement of involving repeating squares connected together at their vertices by hinges has given the rise to stretchable behavior to the sensor.
Fig. 6. Machined and coated samples: (A) Machined Microfiber sheet (MMS), (B) Machined PCL sheet (MPS), (C) No machined PCL microfiber sheet (NMS) and (D) No machined PCL sheet (NPS).
4. Experimental results 4.1. Tensile test All four different sensors including machined microfiber sheet (MMS), machined PCL sheet (MPS), no microfiber sheet (NMS), and no machined PCL sheet (NPS) samples characterized mechanically. Force-strain curves are shown in Fig. 7. Fig. 7A and C illustrate the mechanical behavior of microfiber sheet. As MMS and NMS can be respectively seen by increasing the strain to 3.71 mm and 0.99 mm, samples showed a linear force strain behaviors which show its linear elastic property. They showed approximately 3.7 times tensile distance because of differences between machined and no machined. Similarly, Figures 6B and D show the behavior of PCL sheet. MPS and NPS sheets respectively increased up to 1.7mm and 0.57. The tensile distance of MPS is approximately 3.5 times larger than NPS. Machined samples (MMS, MPS) required more force than no machined samples (NMS, NPS).
Figure 8 shows average of force test (Machined samples n=6, No machined samples n=3). The ratios of tensile force against tensile distance in average are respectively 0.13 N/mm from 0 to 2.74 mm, 1.52 N/mm from 0 to 1.44 mm, 5.82 N/mm from 0 to 0.89 mm, and 15.39 N/mm from 0 to 0.51mm for MMS, MPS, NMS, and NPS.
Fig. 8. Average of force test (Machined samples n=6, No machined samples n=3)
4.2. Resistance test Resistance vs. strain for all four various force sensors including MMS, MPS, NMS, and NPS samples are shown in Fig. 9. Fig. 9A and C illustrate the change in resistance when strain is applied for MMS and NMS. As they can be observed by increasing the strain, samples respectively showed significant increase from approximately 2000 Ω up to 17000 Ω and 10000 Ω in the resistance values when the tensile distance was increased up to 2.28 mm and 0.89 mm. Fig. 9B and D illustrate the effect of strain on resistance for MPS and NPS. As we expected from the structure, the linear behavior is observed and samples showed partially linear behavior since by increasing the strain, the sheets showed the increase up to 10000 Ω in the resistance when the tensile distance was increased up to 1.62 mm and 0.55 mm. Fig. 10 shows average of resistance test (Machined samples n=6, No machined samples n=3). Ratios of resistance against tensile distance in average are respectively 1973.7 Ω/mm from 0 to 2.28 mm, 1925.3 Ω/mm from 0 to 1.12 mm, 1988.4 Ω/mm from 0 to 0.79 mm, and 2101.7 Ω/mm from 0 to 0.45 mm for MMS, MPS, NMS, and NPS.
Fig. 7. Results of force test
Finally, results of applied force and measured resistance for all sets of force sensor including MMS, MPS, NMS, and NPS samples have been provided and results are shown in Fig. 11. All four samples may be electrically disconnected but mechanically connected. For example, MMS sample was mechanically disconnected in tensile distance 2.74 mm but the sample was electrically disconnected in 2.28 mm. In order to match tensile distance between mechanical and electrical results, we assume all samples are electrically and mechanically disconnected when the samples show the electrical disconnection. MMS sample shows significant increase in resistance. In other words, it is easily stretched with small tensile force because of characterization of auxetic design and PCL microfiber. The sensitivity of MMS is 14448.15 Ω/N. MPS presents a linear behavior between 0 and 1.8 N but it shows exponential curve after 1.8N. The sensitivity of MPS is 409.04 Ω/N. NMS and NPS are not machined so they require much more force than MMS and MPS in order to getting stretched. NMS shows a lot of fluctuation at constant tensile force because gold particles are moved among microfibers but it overall shows increase. The sensitivity of NMS is 195.35 Ω/N. NPS is the toughest sample to stretch as shown in Figure 9D. The variance of resistance is less than 1000 Ω because the sample is stretched within 0.5 mm. The sensitivity of NPS is 175.62 Ω/N.
Fig. 9. Results of resistance test
Fig. 10. Results of Force & Resistance test
5. Conclusions
Fig.10. Average of resistance test (Machined samples: n=6, No machined samples: n=3)
4.3. Force & Resistance test
In this study, the fabrication and characterization of PCL and auxetic mechanism based force stretchable sensor was investigated through the use of melt electrospinning technique. We were able to fabricate and characterize the mechanical and electrical properties of such sensors including novel machined microfiber sheet (MMS), machined PCL sheet (MPS), no machined microfiber sheet (NMS)
and no machined PCL sheet (NPS). We can also successfully study the effect of applied force and strain on the measured resistance of such stretchable novel force sensors fabricated based on auxetic geometry via combination of micromachining and melt electro spinning techniques. Accordingly the sensitivities of such sensors are reported as 14448.15 Ω/N, 409.04 Ω/N, 195.35 Ω/N, and 175.62 Ω/N for MMS, MPS, NMS, and NPS respectively. MMS sensor is appropriate for requiring highly sensitivity. In our follow up studies, we will optimize four kinds of sensors and test them on human body. Acknowledgements The authors would like to acknowledge support from Natural Sciences and Engineering Research Council (NSERC) Discovery Grants. We are thankful to Nima Khadem Mohtaram, PhD Candidate of the Department of Mechanical Engineering, and the University of Victoria for helping our works. References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Ko, J., et al., "Fabrication of poly (caprolactone) microfiber scaffolds with varying topography and mechanical properties for stem cell-based tissue engineering applications," Journal of biomaterials science. Polymer edition, 2013. 25(1): p. 1-17. Dalton, P.D., et al., "Electrospinning of polymer melts: Phenomenological observations," Polymer, 2007. 48(23): p. 6823-6833. Deng, R.J., et al., "Melt Electrospinning of Low-Density Polyethylene Having a LowMelt Flow Index," Journal of Applied Polymer Science, 2009. 114(1): p. 166-175. Lyons, J., et al., "Melt-electrospinning part I: processing parameters and geometric properties," Polymer, 2004. 45(22): p. 75977603. Noda, K., K. et al., "Stretchable Force Sensor Array Using Conductive Liquid," 26th IEEE Int. Conf. on Micro Electro Mechanical Systems (Mems 2013), 2013: p. 681-684. Tok, J.B.H., et al., "Recent advances in flexible and stretchable electronics, sensors and power sources," Science ChinaChemistry, 2012. 55(5): p. 718-725. Ma, S.D., et al., "Intrinsically conducting polymer-based fabric strain sensors," Polymer International, 2013. 62(7): p. 983990. Lakes, R., "Foam Structures with a Negative Poissons Ratio. Science," 1987, 235(4792): p. 1038-1040. Caddock, B.D., et al., "Negative Poisson’s Ratios I: Microstructure and Mechanical Properties," Journal of Physics D: Applied Physics, 1989. 22: p. 1877-1882. Grima, J.N., et al., "Do zeolites have negative Poisson's ratios?," Advanced Materials, 2000. 12(24): p. 1912-+. Bhullar, S.K., et al., "Influence of Negative Poisson's Ratio on Stent Applications,"
12.
Advanced in Materials, 2013: p. 42-47. Scarpa, F., "Auxetic materials for bioprostheses," IEEE Signal Processing Magazine, 2008: p. 125-128.