Published In: Proc. SPIE 9116, Next-Generation Robots and Systems Date: 4 June, 2014 Human-robot interaction can be made more sophisticated and intuitive if the entire body of a robot is covered with multimodal sensors embedded in artificial skin. In order to efficiently interact with humans in unstructured environments, robotic skin may require sensors such as touch, impact, and proximity. Integration of various types of sensors into robotic skin is challenging due to the topographical nature of skin. Printing is a promising technology that can be explored for sensor integration as it may allow both sensors and interconnects to be directly printed into the skin. We are developing Electrohydrodynamic (EHD) inkjet printing technology in order to co-fabricate various devices onto a single substrate. Using strong applied electrostatic forces, EHD allows the printing of microscale features from a wide array of materials with viscosities ranging from 100 to 1000cP, highly beneficial for multilateral integration
EHD as Sensor Fabrication Technology for Robotic Skins Jeongsik Shin *a, Woo Ho Leeb, Caleb P. Nothnaglea, Muthu B.J. Wijesundaraa
aUniversity of Texas at Arlington Research Institute, 7300 Jack Newell Blvd., S., Fort Worth, TX, USA 76118; bNGS Lab., Dept. of Electrical Engineering, University of Texas at Arlington,
Arlington, TX, USA 76010
ABSTRACT
Human-robot interaction can be made more sophisticated and intuitive if the entire body of a robot is covered with multi-modal sensors embedded in artificial skin. In order to efficiently interact with humans in unstructured environments, robotic skin may require sensors such as touch, impact, and proximity. Integration of various types of sensors into robotic skin is challenging due to the topographical nature of skin. Printing is a promising technology that can be explored for sensor integration as it may allow both sensors and interconnects to be directly printed into the skin. We are developing Electrohydrodynamic (EHD) inkjet printing technology in order to co-fabricate various devices onto a single substrate. Using strong applied electrostatic forces, EHD allows the printing of microscale features from a wide array of materials with viscosities ranging from 100 to 1000cP, highly beneficial for multilateral integration. Thus far we have demonstrated EHD’s capability at printing patterns of Poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) for pressure sensor applications, generating patterns with modified commercial photoresist for mask-less lithography, and obtaining ZnO microstructures for direct device printing. Printed geometries range from a few tens of microns to millimeters. We have used inks with viscosities ranging from 230 to 520cp and from non-conductive to 135µS/cm. These results clearly show that the EHD is a promising multi-material printing platform and would be an enabling technology that can be used to co-fabricate various devices into robotic skin.
1. INTRODUCTION 1.1 Robotic Skin Sensors and Applications
Many robotic platforms are expected to co-exist in the same spaces as humans and perform similar physical assignments. This human-robot co-existence can be successful and safe by developing more sophisticated and intuitive human-robot interaction capabilities. It is widely accepted that if the entire body of a robot is covered with multi-modal sensors embedded in artificial skin for real time feedback, the robot will have the capability to successfully interact with real-world objects and work with humans in unstructured environments. Sensors that enable these sophisticated robotic skins include touch, impact, temperature, and proximity; however, the integration of various types of sensors into robotic skin is challenging due to different sensing modalities, the types of materials that robotic skin and sensors are made of, and the topographical nature of skin.
Much research and developments has been focused on implementing a flexible “sensitive skin” that is composed of an array of multi-modal sensors and electronics. A modular integration approach is proposed by Schmitz et al. who developed a robotic skin called ROBOSKIN which is composed of triangular modules each with 12 capacitive sensors on flexible PCB1. Mittendorfer et al. developed a hexagonal multimodal sensor module called HEX-O-SKIN which comprises of hexagonal PCB equipped with discrete temperature, acceleration and proximity sensors for emulating the human sense of temperature, vibration and touch2. Both ROBOSKIN and HEX-O-SKIN have been applied to some parts of commercially available robotic platforms (i.e. iCUB, Nao, and KASPAR) to demonstrate their viability. Though attractive the integration of these modular sensors into all or part of a robotic skin system is a complex task due to interconnects and topographical issues.
Most current sensor fabrication and integration relies on semiconductor manufacturing methods and equipment. In most cases, these conventional fabrication methods are not suited for realizing sensor arrays on large area substrates like robotics skins. Also conventional manufacturing techniques have limitations on fabricating sensors on flexible and uneven substrates. Printing is considered to be a promising technology that can be explored for sensor integration onto large areas substrates. Printing can accommodate an array of materials and substrates with various sizes, shapes, and
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complex topographies. Among many printing techniques, inkjet printing can be used at various stages of micro/nano systems manufacturing; for instance, direct device printing (gas sensors, organic LEDs, and thin film transistors), mask-less lithography (front-end fabrication as well as PCB production for back-end applications), and packaging (dispensing epoxies and printing interconnects). The ability to integrate several disparate materials into one fabrication platform is a key for multi-modal sensor fabrication on robotic skin. It is also drawing attention due to its cost effectiveness as printing sensors does not require a large amount of ink materials and does not generate waste.
In this paper, we explore Electrohydrodynamic (EHD) printing as a versatile printing method that can be used for sensor integration onto robotic skin. The main benefit in using our EHD printing for sensor fabrication is its flexibility in printing various liquids and metals onto conductive and even potentially nonconductive substrates of differing topographies. As robotic skin would require a fusion of materials, a system with the capability to dispense nearly any material with minimal reworking is very advantageous. We will discuss here the printability of disparate materials (conductive, non-conductive, and colloidal) with experimental results. We will also highlight the applicability of these materials for sensor integration onto robotic skins.
1.2 Electrohydrodynamic Printing (EHD)
EHD printing uses an electric field to eject fluids. The applied electric field enables fine jetting which creates smaller features by an order of magnitude below the nozzle size3. In EHD, a pressure is applied to the ink chamber until a fluid meniscus is formed at the end the nozzle tip before applying an electric field across the nozzle and substrate for jetting. Figure 1(a) shows a generic configuration of a typical EHD setup. The fine jetting feature is possible due to the pulling and focusing effect from the electric field, in comparison to pushing through applied pressure as in the case in piezo-driven jetting. Electric field-driven ejection enables creating smaller features by an order of magnitude below the nozzle size thereby vastly improving the resolution as shown in Figure 1(b) and (c). Applying a DC electric field produces stable and continuous jetting which is suitable for printing uniform lines4,5. Drop-on-demand printing can be achieved by using external pulsating voltage. This mode is very useful in printing complicated patterns such as those required for sensor fabrication. Micro-dripping is another jetting mode that uses pulsating voltage to deliver droplets to a substrate without a spreading effect. It has been demonstrated by making uniform patterns out of micro/nano sized droplets and would be highly beneficial for fabrication of a sensor array with minimal sensor to sensor variation 6.
Figure 1. (a) Generic printer configuration of EHD printing. (b) Nozzle tip with fluid meniscus before applying electric field, and (c) jetting characteristics after applying electric field.
One advantage of using strong electric fields for fluid ejection is the ability to print materials with viscosities as high as 1000cP. This expands the choice of available materials and is highly beneficial for the fabrication of various types of sensors. In comparison piezo-jet based printing can only handle inks with viscosities of up to 50cP. The printability of high viscosity materials enables the printing of thicker microstructures; a key feature that sets EHD apart from other inkjet printing methods which typically use multiple superimposed print runs to achieve this. Despite these apparent key advantages, one of the drawbacks of EHD is the requirement of a conductive substrate surface for generating an electrical potential. The requirement of a conductive substrate can be eliminated by using a ring electrode for electric field generation, which is an active research area for EHD printing.
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2. EXPERIMENTAL 2.1 EHD Setup
The EHD system used in this experiment consists of an ink chamber, interchangeable nozzle tips, a high-voltage power supply, a function generator, a precision pressure regulator, and an automated positioning stage. Figure 2 shows the current configuration of the EHD set up used in this work. A high DC voltage is maintained between the nozzle tip and the substrate to create continuous jetting while a pulsating voltage generated through a function generator is used for drop-on-demand printing. Voltage settings applied in the experiments vary for different inks. The distance between the nozzle tip and substrate varies based on the size of the nozzle tip and the voltage applied. The use of precision staging includes four degrees of freedom (X, Y, Z, and Theta) allowing the system to scan non-planer surfaces with micron precision and maintain a constant distance between nozzle and substrate for EHD printing.
Figure 2. The current configuration of the EHD setup (a) CAD model showing gantry system with x-y-z-theta stages (b) A photograph of the current EHD setup (high voltage amplifier, function generator and precision pressure regulator are not shown in this photograph)
2.2 Printing Experiments
Three different types of custom inks with different characteristics were used. The first type of ink was used to demonstrate the mask-less lithography capability of EHD. Commercial KMPR 1010 photoresist7, a non-conductive polymeric solution, was modified using N-Methylpyrrolidone (NMP) and OMNOVA PolyFox Fluorosurfactants to achieve the desired properties for printing. The measured viscosity of the modified KMPR 1010 was 520cP. Gold coated silicon, silicon oxide, and Kapton were used as substrates. Drop-on demand experiments were performed with a square wave of 600Hz with a 20% duty cycle. The peak of the square wave was 600V with a bias of 400V. A gold coated 10µm glass nozzle was used with a gap of 160µm between the nozzle and the substrate. After printing gold was selectively etched and KMPR was removed from the substrate to form patterned gold traces.
The second type of ink made of PEDOT:PSS, Poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate), a widely used pressure sensor fabrication material was tested for the development of an EHD printed sensor fabrication process. This test used 5.0 wt. % conductive screen printable paste from Orgacon™ EL-P-5015, PEDOT:PSS, with a resistance and viscosity of 50-150Ω/sq and 50,000-90,000cP, respectively. The material was diluted with NMP to reduce the viscosity and the initial experiments were performed using the ink with viscosity at 230cP. A 100µm stainless steel nozzle was used and a DC voltage of 1.6kV with an 800 µm gap between the nozzle and the substrate were used during printing.
The last set of experiments were performed using highly conductive zinc ion (Zn2+) ink to demonstrate the EHD capability of direct device printing. The viscosity of the ink was 455cP, made using zinc acetate (precursor), ethylene glycol (solvent), and polyvinylpyrrolidone (binding matrix). A continuous jetting mode with a DC voltage of 850V was used. A gold coated 10 µm glass nozzle was used with printing speeds (substrate travel) of 600, 1000, 1400, and 1800 mm/min. Printing was followed by a soft bake step (100ºC for one minute) to evaporate the solvents. Finally, a sintering process was performed above 400ºC for transforming printed structures into polycrystalline Zinc Oxide (ZnO). .
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3. RESULTS AND DISCUSSION 3.1 Mask-less lithography
The objective of using mask-less lithography for patterning metal traces on various substrates is to demonstrate the feasibility of EHD printing as a pattern transfer technique that is well suited for sensor integration onto robotic skin. Figure 3 shows printed KMPR patterns on gold coated silicon substrates. Modified KMPR was used as an EHD ink because it was designed for a similar kind of lithographic processes. Drop-on-demand printing was performed at a frequency of 600Hz with substrate speeds changing from 200 to 1000 mm/min. Data shows that the transition from dots to uniform lines occurs around a 600 mm/min stage speed. Further reducing the stage speed, the linewidth was increased. The linewidth as a function of stage speed is shown in figure 4. These particular printing conditions resulted in a line with a maximum height of 3 microns with a parabolic cross sectional shape. The parabolic line shape was expected due to surface tension driven restructuring. The process has been repeated on gold coated silicon oxide and Kapton substrates (data not shown) with similar results
Figure 3. Variation of line shapes with respect to speeds for 600Hz drop-on demand KMPR printing. Printing speed was changed from 1000mm/min to 200mm/min while other conditions remained the same.
Figure 4. The linewidth variation as a function of stage speed for drop-on demand KMPR printing at 600Hz.
The viability of pattern transfer using EHD printed lines was evaluated using typical etching and resist removal processes. Figure 5 displays images of patterned gold lines on (a) silicon, (b) silicon oxide, and (c) Kapton clearly showing pattern transferability on various substrate materials. Most robotic skins are fabricated using flexible substrates
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(a) (b) 150µm
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of a size, shape, and topography that are not suited for conventional lithography. This data encourages replacing photolithography processes with direct EHD printing of photoresist for mask-less lithography-based patterning of metal interconnects and other materials for sensor integration onto robotic skin
In recent years, PEDOT:PSS has been the sensing material used in touch screen technology. PEDOT:PSS is a transparent, conjugated conducting polymer that is highly ductile and stretchable along with having good environmental stability. Several researchers have fabricated and characterized tactile sensors using PEDOT:PSS8,9,10,11. The resistance of ink-jet printed PEDOT:PSS is about a few kilo ohms. It has elongation of 10% or more. The gauge factor of PEDOT:PSS is 5 to 20, whereas the gauge factor of a conventional metal film is 2. Calvert et al. fabricated piezoresistive sensors using PEDOT:PSS inkjet printing for smart textiles8. That tactile sensor showed a resistance of 5 kilo Ohms and a gauge factor of 5 after 500 cycles of testing. Here we report the results of a first attempt at printing PEDOT:PSS in order to fabricate pressure and strain sensors as well as interconnects on robotic skin. Figure 6 shows the printing results for an array of interconnected square pads printed on a gold coated glass substrate. By examining the results as printed, it is apparent that solvent pooling and surface tension-driven restructuring are occurring. Currently, optimization of the PEDOT:PSS ink and printing process is progressing to mitigate this issue. Future plans include characterization of the printed material and implementation of the printing process to tactile sensor fabrication on skin like substrates.
Figure 6. The results of an array of interconnected square pads printed on gold coated glass substrate (a) as printed and (b) after room temperature drying for three hours.
3.3 Printing Metal Oxide Microstructures
As multifunctional materials, metal oxides are poised to play a critical role in many advanced technologies that include flat panel displays, printed electronics, sensors, and actuators. Although printing and fabrication methods described here may not lead to fabrication of these devices directly onto robotic skin due a high temperature processing step, EHD
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printing can be used to integrate metal oxide-based miniature gas sensors, thin film transistors, and energy harvesters to other parts of a robotic platform. This investigation was focused on printing ZnO from a highly conductive (135µS/cm) zinc ion containing ink. The printing results discussed in this section not only demonstrate the formation of ZnO microstructures but also show the capability of EHD as a versatile printing method by printing non-conductive KMPR ink to highly conductive Zinc ion containing ink. After printing on a silicon oxide substrate, printed microstructures were subjected to a sintering process for creating ZnO microstructures. X-Ray Diffraction (XRD) analysis performed on the printed ZnO films sintered at various temperatures confirmed that the ink produces polycrystalline ZnO upon heating above 400ºC. Figure 7(a) displays the representative XRD data showing the polycrystalline nature of the sintered film. Figure 7(b) shows the SEM image of formed ZnO lines with no detectable cracks or pinholes. Further investigation is needed to better understand the effect of matrix loading and sintering on grain characteristics and film stress.
Figure 7. (a) the representative XRD data of the sintered ZnO film at 400ºC and (b) the SEM image of ZnO microstructure.
Figure 8 (a) shows the linewidth of the printed structures based on printing speed using at an jetting voltage of 850V. After the sintering process, one of the key aspects to note is the shrinkage of the structure due to the removal of matrix materials. It is therefore essential to build a correlation between printed and final microstructure geometry in order to obtain desired device geometry. Figure 8 (b) shows the representative surface profile of ZnO ink printed at 850V with a printing speed of 1000 mm/min before and after sintering. The lateral and thickness shrinkage percentage for this particular ink were determined to be 3% and 92% respectively. With the negligible lateral shrinkage, this printing has resulted in ZnO microstructures with linewidths ranging from 18 to 65µm and thicknesses ranging from 33 to 62nm. The capability of printing a wide range of thickness by changing process parameters is encouraging as this can lead to different types of device using EHD.
Figure 8. (a) Linewidth of printed structures as a function of printing speed using a jetting voltage of 850V and (b) surface profile data on ZnO ink printed at 850V with a printing speed of 1000 mm/min before and after sintering.
4. CONCLUSIONS AND FUTURE DIRECTIONS Based on the preliminary experimental work performed here, it is evident that EHD printing could be an attractive method for integrating sensors into robotic skins for human-robot interactions. We have shown the printing of high
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viscosity materials with varying conductivity and the ability to achieve geometries ranging from a few tens of microns to millimeters. This clearly shows that the EHD is an enabling technology that can be used to co-fabricate various devices onto a single substrate. Although many technological advancements are necessary to fully tap the potential of EHD, the experimental results discussed here evidently show the possibility of EHD as a manufacturing platform for integrating sensors into robotic skin.
Our immediate future research is focused on two main areas: (1) further development of the EHD printer technology, and (2) fabricating an array of pressure sensors onto flexible substrates by printing PEDOT:PSS. The primary goal of EHD technology development is to integrate a ring electrode to the nozzle head so that printing can be performed regardless of substrate conductivity. Though several attempts have been made by a few research groups to integrate ring electrodes into EHD printers, this is a major technological barrier that hampers the application of EHD printing technology to a wide array of substrates. In terms of sensors for robotic skins, we plan to fabricate an array of pressure sensors onto a flexible substrate by using EHD printing. The work will starts with characterization of printed materials and ends with the fabrication of an entire sensor array by printing PEDOT:PSS and interconnects using EHD.
5. ACKNOWLEDGMENTS This work was supported in part by the Office of Naval Research Grants #N00014-08-C-0390, #N00014-11-C-0391, and the National Science Foundation NRI Grant #IIS-1208623. We wish to thank Raminderdeep Sidhu, Lester Corwin, and the UTA Research Institute staff for their help with the experimental work presented in this paper.
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