Short communication Tekscan pressure sensor output changes in the presence of liquid exposure Kyle S. Jansson a, n , Max P. Michalski a , Sean D. Smith a , Robert F. LaPrade a, b , Coen A. Wijdicks a a Department of BioMedical Engineering, Steadman Philippon Research Institute (SPRI), 181 West Meadow Drive, Suite 1000, Vail, CO 81657, USA b The Steadman Clinic, Vail, CO, USA article info Article history: Accepted 29 September 2012 Keywords: Pressure Sensing Film Tekscan Knee Orthopaedics Sports Medicine abstract The purpose of the study was to evaluate the load output of a pressure sensor in the presence of liquid saturation in a controlled environment. We hypothesized that a calibrated pressure sensor would provide diminishing load outputs over time in controlled environments of both humidified air and while submerged in saline and the sensors would reach a steady state output once saturated. A consistent compressive load was repeatedly applied to pressure sensors over time (Model 4000, Tekscan, Inc., South Boston, MA) with a tensile testing machine (Instron ElectroPuls E10000, Norwood, MA). All sensors were initially calibrated in a dry environment and were tested in three groups: humid air, submerged in 0.9% saline solution, and dry. Linear regression of load output over time for the pressure sensors exposed to humidity and submerged showed a 4.6% and 4.7% decline in load output each hour for the initial 6 h, respectively (b ¼ 0.046, 95% CI: [ 0.053 to 0.039]; p o0.001) (b ¼0.047, 95% CI: [ 0.053 to 0.042; p o0.001). Tests after 72 h of exposure had linear regression decline in load output over time of 0.40% and 0.47% per hour for humidified and submerged sensors, respectively (b ¼0.004, 95% CI: [ 0.006 to 0.003]; p o0.001) (b ¼0.047, 95% CI: [  0.053 to 0.042]; p o0.001). Because outcomes in biomedical research can affect clinical practices and treatments, the diminishing load output of the sensor in the presence of liquids should be accounted for. We recommend soaking sensors for more than 48 h prior to testing in a moist environment. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Real-time pressure sensing film has become increasingly used to quantify contact area, pressure, and forces in biomedical studies (Becher et al., 2008; Elguizaoui et al., 2012; Flanigan et al., 2010; Hofer et al., 2012; Kock et al., 2008; Lee et al., 2006; Ode et al., 2012; Ostermeier et al., 2007; Prisk et al., 2010; Seo et al., 2009; Verlinden et al., 2010; von Lewinski et al., 2006). It is important to properly utilize this technology, as outcomes in biomedical research can affect clinical practice and patient care. This study reviews a new technique to properly utilize a pressure sensing film for use in moist environments, such as in cadaveric joints. In a recent in-house study examining cadaveric tibiofemoral contact mechanics, loads were transmitted within the knee and measured with pressure sensors. The pressure sensors were implanted on the tibial plateau and underwent consistent and repeatable axial loading over 6 h. During testing, the sensors were exposed to fluids from the cadaveric specimens and sprayed with saline to prevent tissue desiccation. It was observed over time that the cumulative load output from the sensors was diminishing in the presence of the same load. Due to the open-cell material of the flexible sensors, the manufacturer states that they are susceptible to absorbing moisture which can alter the electrical output signal. The purpose of this study was to evaluate the load output of a pressure sensor in controlled environments similar to those found in cadaveric testing. We hypothesized that the load output of a calibrated, non-vented pressure sensor would diminish over time when enclosed in humid air and when submerged in saline. These load outputs would reach a steady-state saturation limit and stabilize in a manner similar to testing in a dry environment. 2. Materials and methods A test protocol was created to mimic the loading, timing, and environment for the aforementioned cadaveric knee testing. The pressure sensors were calibrated with a single point method mimicking our test setup, as recommended by the manufac- turer’s applications engineer for this test protocol (Model 4000, Tekscan, South Boston, MA). The calibrations were performed by ramping a compressive load for 12 s up to 450 N and maintaining this for 30 s. Compressive loads were applied to the sensors with a tensile testing machine in a custom jig designed with size and material mimicking human knee cartilage stiffness and thickness (ElectroPuls E10000, Instron Systems, Norwood, MA). Data was acquired using I-Scan 6.10 software (Tekscan, South Boston, MA) and analyzed with Microsoft Excel (Microsoft, Redmond, WA). 2.1. Control testing A single sensor with two separate sencell tabs was used to test if the sensors would have a consistent output over time in a dry environment. Following storage Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics 0021-9290/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jbiomech.2012.09.033 n Corresponding author. Tel.: þ1 970 479 9797; fax: þ1 970 479 9753. E-mail address: kjansson@sprivail.org (K.S. Jansson). Journal of Biomechanics 46 (2013) 612–614