Abstract
Novel textile temperature sensors based on strain-relieved braiding constructions offer attractive monitoring possibilities for numerous application fields involving e-textiles in general, and medical textiles in particular. Thus, the research work presented in this paper focused on theoretical foundations, manufacturing, and procedural, mechanical as well as thermal testing of these newly developed, textile-based sensors for temperature measurements. The median basic resistance of the scalable sensor yarns using a helical stainless steel wire is about 0.81 Ω/mm. Within the temperature range of 22 to 40℃, the developed sensor yarn has a mean linearity deviation of 0.028 K. The measured temperature coefficient is 0.68 × 10−3 K−1, which correlates with the properties of the stainless steel wire.
As a fundamental variable of physics, temperature is one of the most important parameters to be sensorially measured in technical and also medical applications. Flexible sensors, which are bendable, stretchable and stable under mechanical loads such as pressure, can be used in various applications. In addition to wound monitoring, 1 which the developed sensors described herein are designed and mostly suited for, potential medical uses of these flexible sensors include the monitoring of prostheses with smart skin, 2 e-textiles in assisted physical rehabilitation scenarios, 3 or sports medicine.4–6 Furthermore, flexible/stretchable electronics and especially sensors are of great interest for other areas, such as infrastructure, 7 soft robotics, 8 and many more.9,10 Despite their enormous economic importance, flexible strain-relieved temperature sensors, which can be integrated directly into textiles (e.g., wound dressings), are currently not available. Therefore, we will present a newly developed textile-based strain-relieved temperature sensor in this paper.
Stretchable temperature sensors can be widely used in different application scenarios. With respect to wound monitoring, temperature has a decisive effect on wound healing, particularly in the case of chronic wounds, such as ulcus cruris or decubitus. Optimal healing of these wounds is possible near normal body temperature. Increased body temperature commonly indicates infection and, thus, disturbance of the wound healing process. Decreased temperature delays the healing process due to slower biochemical (enzymatic) reactions. 11 Low wound temperatures occur most frequently in the extremities. Therefore, they are not a definite sign of wound healing deficits and must be compared with the local ambient temperature. The temperature profile is another parameter specific to wound healing. A temperature increase in the area of the wound indicates inflammation of the wound. For example, an increase between 2.8 K and 3.6 K is typical for an inflammation reaction on a diabetic foot.12,13
Throughout the industry, a wide range of temperature sensors is available. The basic principles, however, differ significantly, depending on the application in the cases of temperature range, accuracy, as well as environmental or chemical influences.
Apart from conventional liquid thermometers (e.g., mercury), whose structure and measuring principle make them unsuitable for a flexible, non-stationary use needed in medical applications, 12 other established principles also present obstacles, further proving the urgency of developing more flexible systems.
Thermo-elements that comprise two different contacting metals exploit the thermoelectric effect (Seebeck effect), creating a change in electric potential (voltage) during temperature changes. These reliable and long-term stable sensors, usually designed for temperature measurements in industrial applications (up to 1000℃), only provide limited accuracy, as compensating leads and/or reference circuits are necessary.14,15
Optical temperature sensors usually work in the IR (infrared) spectrum, hence, they are often employed by medical staff to quickly check skin and wound temperature, and they are frequently part of sports medical studies. 16 Such IR temperature sensors, which are mainly used as handheld devices, allow for contactless, indirect temperature measurements based on the reflectivity of the respective surface. As the emissivity of the body must be known for this method, measurement results may be flawed compared with direct systems.
Temperature sensors based on changes in resistance of the electrical conductor, so called resistance temperature detectors (RTDs), enable very precise measurements. Conventional temperature sensors, such as the industrial-type Pt100 or type Pt1000 elements, are based on platinum as conductive material. 17 Apart from these metallic conductors, sintered and doped metal oxides or ceramics are applied, which are commonly referred to as thermistors. These can have either a negative or a positive temperature coefficient and are therefore referred to as NTC (negative temperature coefficient) or PTC (positive temperature coefficient) thermistors. In the medical field, these very precise, but rather expensive sensors are used for point measurements in cerebral or pulmonary environments. 18
None of the abovementioned measuring principles are entirely suitable for the integration of sensors into a bandage, a Band Aid, or a piece of clothing due to their material, measuring method, and/or high requirements of accuracy and flexibility under mechanical stresses and large-area applications. Flexible approaches, for example, printed sensors based on foils, can be stretched and bent, but are either very complex in structure or too costly due to the materials used. Furthermore, their foil characteristics do not allow for sufficient removal of sweat and wound exudate, which is essential in terms of long-term monitoring.19–21
For textile-based temperature sensors, a fabric is typically used as base material for printing an electrically conductive ink.22,23 These sensors for temperature measurement heavily react to mechanical stresses due to their plain construction and, therefore, lack of strain-relief. In contrast, yarn-based sensors are appropriate for the measurement of strain stresses, but cannot be employed for the monitoring of physiological parameters, such as temperature. 24 Fiber-based sensors offer high flexibility under bending stresses, however, the effect of strain stress is not given. 25
Generally, it can be summarized that all mentioned systems have disadvantages with regard to temperature measuring in monitoring applications for wound healing, either due to inflexible design, unsuitability for integration into textiles, or inappropriate metrological behavior under mechanical stresses.
Materials and methods
Concept
Apart from the requirements mentioned above, the main objective of the research efforts presented in this paper was to ensure the usability of the sensor in the physiologically relevant temperature range (22 to 40℃). The coverage of the entire detection field (the wound margin) is a rather challenging task. By means of braiding, which allows for the processing of fine yarns and thin metallic wire materials, complex, multilayered yarn constructions are manufactured efficiently and reproducibly. A metal conductor (wire) was selected as sensor material. Thus, the yarn sensor is based on the RTD principle. So far, there have been no suitable material combinations (commercially available conductors with insulation layers) that fulfill the medical requirements of biocompatibility while encompassing favorable mechanical strengths and high sensitivities. It was also determined that an oxide layer, for example consisting of titanium, offers an insufficient isolation effect regarding the interference factors present in the wound environment of chronic wounds. However, particularly in the case of resistance measurements being an essential aspect for the application of a temperature sensor, the lack of an electrical isolation layer would greatly distort measured values.
Furthermore, this wire has to fulfill textile-technological requirements. They relate primarily to mechanical strengths for textile processability. In terms of tensile properties, the yarns and wires to be used must have a minimum strength of approximately 100 cN due to the geometrical conditions and given parameters of the braiding machine employed for the manufacturing of the sensor yarn.
Considering the requirements of flexibility and mechanical stability, fine textile materials and thin wires were taken into consideration for these developments. During braiding, the textile material is not only exposed to tension, but to bending and shear loads due to the process-inherent crossing of the braiding yarns. Therefore, brittle materials are not suited for such requirements.
Based on these principal demands, conductor materials, suitable isolation layer materials and coating methods were evaluated. Titanium and stainless steel have high specific electric resistances, whereas stainless steel displays a greater strength and better biocompatibility than for example copper, paired with lower material costs than gold or platinum. Thus, stainless steel was selected as conductor material for this research work.
Calculated values for the length-related electrical resistance R L and breaking force F R of stainless steel wires of type 1.4401
Even though the tensile strength R m increased with decreasing diameter due to the production process of the wire drawing, a tensile strength Rm of 600 MPa was assumed for the sake of comparability, which matches the legal minimum tensile strengths of solution-annealed steels for wire production (type 1.4401) according to DIN EN 10088-3:2014. 26 The specific electrical resistance ρ of this type of steel is listed as 0.70 Ω mm2/m.
All mentioned wire diameters fulfill the requirements of minimal tensile strength in braiding (100 cN). Therefore, a wire diameter of 50 µm was selected for its high base resistance.
Thermoplastic polymers are excellent isolation layer materials as they have a high chemical stability. Furthermore, they can be applied as isolation layers onto the wire in various ways. One possibility is to braid them around a stainless steel wire core. For this method, the required thermoplastic isolation material has to be available in textile form, that is, as fiber or yarn, and can be melted later in the process. Several biocompatible thermoplastic fiber materials are used as bandaging or suture material in medical technology. These materials include polyethylene (PE), polypropylene (PP), Parylene, or polyether ether ketone (PEEK). They can be melted by hot air (convection) or application of electrical current to the core wire. Special types of Parylene can be used to coat the wire by means of chemical vapor deposition (CVD). Here, a chemical reaction is exploited to separate a solid component from a gaseous phase on a heated substrate. Excellent isolation effects can be achieved on stainless steel resulting in layer thicknesses of circa 8 µm. 27
The advantage of this CVD-method in comparison to an isolation layer wound onto the core is the separation of homogeneously thick layers that cover the entirety of the surface, regardless of the substrate geometry. Therefore, this method is often made use of for the insulation of electronic components, such as circuit boards. Parylene-C, which was applied in this case, is often employed for the coating of medical implements and implants. 28
For textile processing via braiding, a yarn material was selected that maintains a mechanically stable but also flexible structure. Thus, viscose was used as it offers good tensile strength and chemical stability while meeting all relevant medical demands.
A helically shaped wire was integrated into the yarn construction developed for this research project. This arrangement had several advantages in comparison to a stretched wire. Mechanical stability is greatly improved as a properly integrated yarn acting as a carrier structures decreases tension load. Furthermore, compared with stretched conductors, any elongation of the yarn construction has a significantly smaller influence on electrical resistance. This increases the total resistance as well, causing higher changes in absolute resistance at different temperatures and resulting in a metrological advantage, as global measurement uncertainties caused by line resistances are considerably smaller. The theoretical concept and feasible total winding length of the wire (Figure 1) were calculated based on the Pythagorean theorem (Equation 3).
Theoretical sensor yarn construction as a helix.
At known height z0, one winding and a known circumference of the cylinder diameter as the sum of radii r0 of core yarn and sensor wire, multiplied by 2π, the length of a winding l0 can be determined. The known number of windings set by machine parameters (drive and take-off speeds), a core yarn of 0.5 mm diameter (6 × 16 tex viscose, braided), and a wire diameter of 50 µm will result in a 2.2-fold length integration in comparison to a stretched wire. The basic resistance of the wire increases by the same degree. A theoretical model of the helical structure is shown in Figure 1
Materials
Airspun viscose yarn of 16 tex was manufactured with an Air-Jet Spinning Tester J20 (Rieter Holding AG, Switzerland). A laboratory yarn twisting machine DirecTwist (AGTEKS Ltd. Sti. Turkey) was used for twisting the yarn. The stainless steel wire type 1.4401 with a diameter of 50 µm was purchased from Goodfellow Cambridge Ltd. (UK).
Methods
The viscose yarn of 16 tex was twisted, resulting in a 2-ply (2 × 16 tex), which was used for the production of the braid. An RU 2/12-80 braiding machine by August Herzog Maschinenfabrik GmbH & Co. KG, Germany, was employed for the production of the braiding yarn. In the first production step of the strain-relieving core yarn, six viscose yarns (2 × 16 tex) were produced with a braiding density of 11.54 braids per 10 mm and a machine speed of 400 rpm. The second production step involved the integration of wire material by overbraiding and was carried out with five viscose yarns (2 × 16 tex) and one Parylene-coated stainless steel wire with a braiding density of 46.16 braids per 10 mm and a machine speed of 100 rpm. For both braiding process steps, a braiding pattern of 1:1-1 was used. The layers of Parylene-C were applied to the stainless steel wire by means of CVD by the company FSP GmbH, Dresden, Germany.
An SGY 0200-650D TFP machine (ZSK Stickmaschinen GmbH, Germany) was selected for stitching with the tailored fiber placement (TFP) technology. This stitching machine enables the textile application of the yarn with a precisely guided path. A zigzag stitch with a stitch length of 3.5 mm, a stroke of 9.0 mm and a stroke (Pantograph) of 3.5 mm was used. Minimum bending radii of 2 cm and bending angles of 120° were used for the test. During pressure tests, the stitched-on sensor networks were loaded with defined weights of 1, 2, and 4 kg. The mass was distributed by means of an acrylic glass plate (mass 75 g, area 100 cm2). Thus, a maximum pressure of about 1, 2, and 4 KPa can be simulated, which represents pressures typically occurring in a pressure bandage for chronic wounds. The layer thicknesses of Parylene were examined by an optical microscope Axiotech 100 (Carl Zeiss AG, Germany). The reference sensors Pt100 and Pt1000 were purchased from JUMO GmbH & Co. AG, Germany.
To measure the resistance, a precision ohmmeter 8846 A by Fluke, USA, was used. This ohmmeter has a precision of 1 mΩ in the selected range of up to 1 KΩ. It has to be noted that even this precision ohmmeter has a limited detection accuracy for the developed sensor. Given a base resistance of 150 Ω, the detection limit of the temperature is about 0.1 K.
The temperature characterization was based on DIN EN 12470-129 using a water bath and thermostatic control to achieve a temperature range of 22 to 40℃. The strain tests were conducted on a Z2.5 by Zwick GmbH & Co. KG. This machine was set to a load cell with a range of 500 N and a precision of 0.07% with a relative repeatability of 0.01%. For testing the long-term stability, the electrical resistances of the temperature sensors were measured within 3 to 7 days. The developed sensor yarns were kept in the water bath for the duration of this time.
The test setup for the measurement of sensitivity by means of resistance change (sensor output) for varying temperatures (sensor input) is shown in Figure 2.
Testing of temperature sensitivity with thermostat and precision ohmmeter.
The long- term stability of the sensor was tested for a duration of 7 days by submerging the sensor completely into the tank of the thermostat while being connected to the measuring cables.
For strain testing, the sensor yarn of about 20 cm was placed between two clamping jaws of flat Vulkollan in an unbent position, which held the yarn with a pressure of 5 bar. The clamping length was 40 mm, whereas each clamp has a thickness of about 30 mm. The tests were conducted with speeds of 10 mm/min and the yarn was elongated during five cycles with 2, 4, 6, 8, and 10% tension. Each cycle consisted of 10 strain loads followed by the next cycle. The test setup is shown in Figure 3.
Setup for strain testing of the sensor yarn (white).
A test setup (Figure 4) specifically developed for the bending tests allowed multi-cycle bending with a defined bending radius and angle by providing an adjustable piston rod and bevel gears. With a bending radius of 2 cm and a bending angle of 120°, eight yarns were examined regarding the influence of bending on metrological properties. The electrical resistances of the samples were measured prior to insertion into the clamp, as well as before and after loading (15 min, which corresponds to ca. 300 bending cycles).
Test setup for bending tests.
The applied Parylene-C layers deposited on steel wires were examined with an optical microscope. The cross section of a single stainless steel wire is presented in Figure 5.
Cross section of a Parylene-coated steel wire.
The coating thickness of Parylene ranged from 7 to 9 µm. Subsequently, the wire was integrated into a yarn by braiding onto a strain-relieving core yarn. The final sensor yarn is shown in Figure 6. The linear density of the sensor yarn was 0.42 g/m, which corresponds to a fineness of 420 tex.
Braided sensor yarn with integrated steel wire.
Results and discussion
Sensitivity and long-term stability
The measurement results for the water bath tests of the reference sensors (Pt100 and Pt1000) and three identical temperature sensor yarns TS1, TS2, and TS3 are given in Figure 7.
Change in resistance of three identical sensor yarns TS 1 to TS3 in comparison with resistive reference temperature sensors.
As expected, the resistances of the three examined sensor yarns depended linearly on temperature. For the reference sensors Pt100 and Pt1000, linearity deviations of the temperature values were determined in the range of 0.013 K and 0.008 K. The mean linearity deviation of the developed sensor yarn type was 0.028 K, therefore slightly above the reference sensors. The median basic resistance in all three tested sensor yarns was about 146 Ω at a sensor length of 180 mm. Therefore, the transmission factor amounts to 99.4 mΩ/K, which results in a temperature coefficient α of 0.68 × 10−3 K−1
During the long-term stability test of the developed sensor yarns, the basic resistance changed by an average 0.005% (7.4 mΩ) over a duration of 7 days. The maximum deviation in measuring accuracy equals a temperature difference of 0.074 K.
Influence of cyclic strain stress
As one of the main features of the strain-relieved temperature yarns, a strain test was conducted on a tensile testing machine. The applied strain (red curve) and the corresponding electrical resistance (blue curve) are illustrated in Figure 8.
Applied strain (red curve) versus corresponding electrical resistance of a yarn-based sensor (blue curve).
For typical application-specific load scenarios involving strain cycles of up to 4%, the sensor almost completely recovers to its base resistance. Only at levels above 6%, was a slight drift of the mean resistance values observed. After a strain of 10%, the sensor showed a deviation of about 0.7 K. The corresponding gauge factors (k-factors) of each strain level are given in Figure 9. Based on these values, it can be concluded that the resulting cross sensitivity toward elongation was very low. In comparison to typical metallic bulk materials or strain gauge sensors, the mean k-factor was reduced to about 2.5%.
Applied strain levels and corresponding k-factors of the developed yarn-based temperature sensor.
Influence of bending stress
Prior to the bending test, the temperature sensorś resistance increased by 0.154% ± 0.028% as a result of the clamping, which equals a temperature change of 2.25 K ± 0.41 K. After the test, the resistance change was 0.052% ± 0.001% compared with the base resistance prior to the test, which equals 0.76 K ± 0.02 K. This shows that the test-related clamping with a screw caused a much higher increase in resistance than the bending load itself. Therefore, the temperature sensors were placed on a textile fabric to achieve quantifiable results. For this more practical approach, the sensor yarns were stitched onto a polyester non-crimp fabric structure. The TFP stitching machine allows a textile application of the yarn with a precisely guided path. The stitched-on yarns were tested under bending and pressure loads. To determine the effect of the stitchable radii, the sensor yarns were applied as shown in Figure 10.
Stitched temperature sensor yarn.
Even with the smallest radius of 1 mm, the yarn could be processed without any constraints. In comparison to unprocessed yarns, an increase in resistance (30 mΩ, which corresponds to a temperature change of 0.3 K) was found after processing. As the sensor is calibrated afterwards, this small increase is neglectable.
For subsequent tests under bending and pressure loads, a meandering sample geometry was used based on the dimensions of a commonly used bandage (75 × 180 mm).
During bending tests, which were conducted under the same properties as mentioned before, no mechanical degradation could be found after 15 min (300 bending cycles). The changes in resistance of the temperature sensors were all below 0.005% (0.073 K). Therefore, it can be concluded that the sensor has a high stability under bending stresses.
Influence of pressure
Resistance changes of the stitched yarn sensors in pressure test
It must be noted that the plate is pressing directly onto the sensors, whereas in the case of normal applications, the pressure is distributed more evenly. Hence, these measurements reveal that the yarn-based temperature sensor offers sufficiently good sensor stability under pressure loads.
Conclusions
The reproducible fabrication of temperature sensor yarns by braiding has been proven feasible. The developed textile-based and strain-relieved temperature sensors are highly flexible and metrologically stable under mechanical loads such as bending and pressure. These excellent properties demonstrate their textile processability by stitching and allow their application for the monitoring of chronic wounds.
Further development efforts will include the usage of platinum wires as resistive material. Although more costly than stainless steel, this material offers a better temperature coefficient of resistance, thus creating an even higher thermal sensitivity. The usage of tailored braiding bobbins also enables the manufacturing of thinner wires and finer textile materials.
Furthermore, the sensor, here specifically manufactured for medical applications, can be used in many additional fields like sports medicine and e-textiles. In sports medicine, temperature measured on the skin allows conclusions regarding muscular activity. Especially in high-performance sports, for instance bicycle racing, relative changes in skin temperature of the thigh area (quadriceps fermoris) indicate the athletes' maximum capacity. In combination with heart rate, the same is true for runners. In the growing field of e-textiles, monitoring of internal (body) or external (ambient) temperatures is the most important parameter besides humidity and heart rate.
Footnotes
Acknowledgements
We would like to thank the Institute for Solid State Electronics (IFE), TU Dresden, for the support in the measurement of the wires and FSP GmbH in Dresden, Germany, for coating the Parylene layer.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The IGF research projects 17826 BR/1 and 18640 BR/1 of the Forschungsvereinigung Forschungskuratorium Textil e. V. in cooperation with the Forschungsgesellschaft für Messtechnik, Sensorik und Medizintechnik e. V. are funded by the AiF within the program for supporting the “Industrielle Gemeinschaftsforschung (IGF)” from funds of the Federal Ministry of Economics and Energy (BMWi) by a resolution of the German Bundestag.