Flexible, Stretchable Skin Sensors

for Two-Dimensional Position

Tracking in Medical Simulators1

Jason LuDepartment of Mechanical Engineering,

University of Minnesota,

Minneapolis, MN 55455

Timothy M. KowalewskiDepartment of Mechanical Engineering,

University of Minnesota,

Minneapolis, MN 55455

1 Background

We introduce a flexible smart skin sensor that can conform tothe surface of a mannequin, prosthetic, or medical device. Itsenses contact and localizes contact position continuously over itsentire surface. This can allow, for example, medical mannequinsimulators to assess accurate placement of fingers or tools duringprocedures such as localizing anatomical landmarks or correctlylocalizing breast tumors during exams.

Prior art has also considered smart skins: flexible materialsembedded with force or position sensors, which can be molded todifferent geometries. Several attempts at smart skins have utilizedarrays of rigid sensors embedded in flexible media. Typicalapproaches use piezoresistive sensors [1] or capacitive force sen-sors [2]. Rogers et al. implement novel microfabrication techni-ques in silicones (e.g., Ref. [3]) for stretchable circuits. Theseapproaches have one or more drawbacks. Most are cost prohibi-tive for medical training applications. All use arrays of discretesensing elements, limiting data collection to finite locations sepa-rated by dead zones. The sensing elements are rigid, creating arti-facts in the material properties of the simulated tissue. Even assensors get smaller, sensing is limited to discrete positions in the2D plane. We exploit a novel carbon nanotube (CNT)-doped poly-dimethylsiloxane (PDMS) elastomer sheet to overcome suchdrawbacks and implement a 2D potentiometer similar to the workinitially proposed by Walz et al. as a low-cost surgical tissue sim-ulation tool [4]. We document accurate mapping between sensorvoltages and position via a polynomial surface fit.

2 Methods

Our proposed smart skin sensor body consists exclusively ofstretchable and flexible materials. The sensor was 15� 15 cm2

and consists of three layers: an Ag-Nylon conductive cloth (Med-tex, Statex Productions, Bremen, Germany); a dividing noncon-ductive perforated layer (powermesh fabric, 99% polyester, and1% spandex); and a 100 lm layer consisting of CNT–PDMSbonded onto a 1.5 mm PDMS substrate (7-SIGMA Inc., Minneap-olis, MN) (Fig. 1).

The electrodes were machined from brass and clamped onto thebottom PDMS silicone substrate (Fig. 2). The electrodes were19.0 mm in diameter with a 1.30 mm machined quarter circleindent. A single wire is then soldered onto each node and attachedto a custom circuit.

The circuit (Fig. 3(a)) consisted of a quad multiplexer (MAX-394) coupled to a microcontroller (ATmega328) programmed torapidly change each opposing pair of nodes to either 5 V orground in a specific sequence: down-top nodes high, bottom low;left: left nodes high, right low, etc. Contact was manually applied(approximately 15 N over a 5 mm diameter area) at points in a1 cm grid (Fig. 3(b)) by puncturing grid crossings on graph paperoverlaid and anchored on the sensor (Fig. 3(c)). Each grid pointwas held at approximately constant contact force for approxi-mately 1 s. Since the CNT near the edges of the pad were not asuniform, data from the edges were not collected to avoid boundaryartifacts. We replicated the simulation approach of [4] for thespatial voltage distribution on a permutation of voltage input.MATLAB’s CFtool was used to fit a polynomial surface to the data(Mathworks Inc.).

3 Results

A contour plot of raw voltages over position (with interpolatedsurfaces) indicates close agreement with simulation (see Ref. [4])with a few measurement artifacts (Fig. 4). Data from both conju-gate configurations (e.g., down and up) were aligned and aver-aged. A polynomial surface of minimal order was fit to thisaverage consisting of order 3 to match the odd function behaviorand order 2 to match the even function behavior R2¼ 0.9956(Fig. 5). The root mean squared error (RMSE) between actual x-yposition and the mapping to the fit polynomial surface was0.47 mm.

4 Interpretation

We demonstrate the feasibility of a flexible smart skin thattracks two-dimensional position contact with minimal electronicsand a RMSE value of 0.47 mm in resolving 2D position of con-tact. The sensor body remains stretchable and the sensing surfaceis continuous.

There are several limitations in this work. Though the skin wasstretched multiple times before recording data, the effects ofstretch are not evaluated. Our manual testing likely introducederror. The anomalous readings in Fig. 4 were likely due to incom-plete penetration of the graph paper. A refined, automated testprotocol is required to address these issues. Our approach onlysenses single contact points. Also, PDMC–CNT material isopaque black; this limits its applications in medical simulators tosubcutaneous implementations. Despite these limitations, thiswork demonstrates that such a simple, stretchable skin sensor isfeasible. The favorable results in accuracy motivate further

Fig. 1 Side view of CNT skin sensor

Fig. 2 (a) PDMS substrate with partial CNT strips showsstretchability, (b) perforated layer detail, (c) brass node clamp,and (d) brass clamp contact node attached to the CNT–PDMSpad

1Accepted and presented at The Design of Medical Devices Conference(DMD2015), April 13-16, 2015, Minneapolis, MN, USA.

DOI: 10.1115/1.4030138Manuscript received March 3, 2015; final manuscript received March 16, 2015;

published online April 24, 2015. Editor: Arthur Erdman.

Journal of Medical Devices JUNE 2015, Vol. 9 / 020927-1Copyright VC 2015 by ASME

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Fig. 3 (a) Circuit, (b) 1 cm grid test grid, and (c) sensor CNT surface

Fig. 4 Raw voltage (color) versus x-y contact position; “up” and “right” plots are flipped toillustrate similarity

020927-2 / Vol. 9, JUNE 2015 Transactions of the ASME

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investigation. Future work will employ more rigorous testing,evaluate the effects of stretch and contact forces.

Acknowledgment

7-SIGMA Inc. (Minneapolis, MN) donated the PDMS–CNTconformable sensor technology.

References[1] Shan, J. H., Mei, T., Sun, L., Kong, D. Y., Zhang, Z. Y., Ni, L., Meng, M., and

Chu, J. R., 2005, “The Design and Fabrication of a Flexible Three-DimensionalForce Sensor Skin,” IEEE International Conference on Intelligent Robots andSystems (IROS 2005), Edmonton, Canada, Aug. 2–6, pp. 1818–1823.

[2] Ulmen, J., and Cutkosky, M., 2010, “A Robust, Low-Cost and Low-Noise Artifi-cial Skin for Human-Friendly Robots,” IEEE International Conference onRobotics and Automation (ICRA), Anchorage, AK, May 3–7, pp. 4836–4841.

[3] Kim, D.-H., Ghaffari, R., Lu, N., Wang, S., Lee, S. P., Keum, H., D’Angelo, R.,Klinker, L., Su, Y., Lu, C., Kim, Y.-S., Ameen, A., Li, Y., Zhang, Y., de Graff,B., Hsu, Y.-Y., Liu, Z., Ruskin, J., Xu, L., Lu, C., Omenetto, F. G., Huang, Y.,Mansour, M., Slepian, M. J., and Rogers, J. A., 2012, “Electronic Sensor andActuator Webs for Large-Area Complex Geometry Cardiac Mapping andTherapy,” Proc. Natl. Acad. Sci., 109(49), pp. 19910–19915.

[4] Walz, R., Meier, Z., Winek, M., and Kowalewski, T. M., 2014, “Medical Simula-tors for Developing Countries Via Low-Cost Two-Dimensional PositionTracking,” ASME J. Med. Devices, 8(3), p. 030949.

Fig. 5 Polynomial surface fit (order 3 3 2) to averaged data in“down” configuration; R2 5 0.9956, RMSE 5 0.47 mm

Journal of Medical Devices JUNE 2015, Vol. 9 / 020927-3

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