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This paper provides a tutorial introduction to recently devised technologies for transducers embedded within the organic coatings of metallic (or, more generally, of electrically conductive) mechanical components, such as organically coated flat sheet steel, and three-dimensional machine parts, such as bearing supports and mold jaws. The integration of sensors and actuators into the surface coating of these components is of particular interest: first, it allows assessment of physical parameters at locally defined positions that are not accessible with usual discrete transducers; and second, a functionalized component equipped with sensory intelligence adds value for producer and customer. However, realization of embedded transducers entails challenges posed by (i) any potential preexisting protective organic coating and (ii) the shape and surface roughness of the underlying metallic substrate. This tutorial addresses the major issues that arise when sensors or actuators are to be integrated into the organic coating of an electrically conductive machine part, and provides hints and guidelines for successful realization based on recent research.

Microfluidic co-flow viscometers require the measurement of the interface position between two fluids (one sample and one reference fluid) flowing in parallel inside a microfluidic channel. From the interface position, the flow rates of both fluids and the known viscosity of the reference fluid, the viscosity of the sample fluid can be calculated. The detection of the interface position is usually done by optical means, therefore requiring optical access, and often coloring of the liquids or suspension of tracer particles to achieve optical contrast. In this contribution we present a device that is capable of determining the interface position in an alternative manner by capacitive measurements. The fully polymeric microfluidic chip is fabricated by simple processes requiring no specialized equipment. The capacitive sensor is fabricated by screen-printing. In order to enhance sensitivity, an insulation coating made from PMMA and barium titanate is applied. To assess the interface detection by capacitive means, measurements using different water/glycerol mixtures are performed and compared to optical images of the interface position, establishing a relationship between capacitance and interface position. The capacitive measurement does not require coloring of the liquids or suspension of tracer particles, if there is a dielectric contrast at the measurement frequency.

For keeping control of processes in lab-on-chip applications, monitoring the flow velocity inside channels is an important task. One popular approach to this is to consider heat transfer inside the channel as a flow velocity dependent effect, which can be measured by relatively easy means. Additionally, these thermal measurement methods can be designed and applied in a way that the sensors and actuators do not disturb the flow profile. A fabrication technique suitable to fabricate such sensors is screen printing, which combines cost efficient and fast processing with layers thin enough to not alter the fluid flow. In this work, we report on a devised sensor that is fabricated by screen printing and embedded into a 3D printed channel. The sensor layout enables measuring the flow velocity in a microchannel while enabling the user to compensate for the change in thermal conductivity when switching to a different fluid. This design requires calibration only for one fluid, since a simple empirical model is presented to extrapolate the calibration data to fluids with varying thermal properties. The sensor was tested for one calibration fluid and two fluids with differing thermal properties. The flow velocities of these fluids were measured with an error below 5%.

The field of machine health and system monitoring has gained interest in different application fields, e.g., monitoring of lubricants, vibrations, surroundings, environmental conditions or the temperature of critical machine parts. This work presents two different condition monitoring devices, which are fabricated in a low cost printing process directly on the non-planar surface of the object to be monitored. The first system is used for thermal condition monitoring, where temperature monitoring is performed by 24 thermocouples resulting in a 24 pixel temperature image of the sample. The thermocouples are printed in an array arrangement, which enables the reduction of the required electrical connections from 48 to 25. The second device is intended for the use in aerodynamic systems, e.g., airplane wings, rotor blades of wind turbines, and helicopters where icing is a critical issue. The aerodynamic lift of aircraft is critically influenced by deposits on the wing or rotor blade surface. Also in other rotor blade systems, e.g., wind turbines the efficiency is drastically dropped and in worst case the icing can cause the failure of the system. The aim is the fabrication of an active monitoring system which is able to detect and also remove the ice from the monitored surface. To this end the system is directly fabricated on the 3D shaped substrate like a wing or rotor blade and the icing is detected using a capacitive sensor and a thermocouple sensor array. In case of icing a heater removes the critical ice deposit.

Devising a cost effective way to fabricate sensors onto non-planar surfaces is of particular interest in many industrial applications such as condition monitoring or process controlling. This paper presents a low cost and low tech alternative approach for the fabrication of sensors on non-planar substrates in a two-step process. Screen printing is used to create a 2D pattern of the sensor on a water transfer foil which, in a second step, is transferred on a non-planar substrate using water transfer printing. We demonstrate the feasibility of this water transfer technique by fabricating a strain as well as a temperature sensor, which are both characterized and compared with state of the art screen printed sensors on planar substrates.

We present a fully spray processed polymer based temperature sensor which can be embedded in the organic coating of metallic machine components. The main aim of the sensor is to measure the temperature in such coatings. This could be used, e.g., in friction bearings for condition monitoring. For the present demonstrator, the sensor element is embedded on a steel substrate. Here, the sensing element is a thermocouple, which is made out of two conductive paints with carbon black and silver as organic or metallic filler particles, respectively. The used carbon black paint is custom-made and uses polyamide-imide as polymer binder which serves also as the polymer backbone of the insulation and the encapsulation layer. The commercially available silver paint is based on polyimide which yields the desired bond strength with respect to the insulation and encapsulation layer. The investigated thermocouples are characterized on a temperature test rig with and without top coating up to a junction temperature of 200 °C. Subsequently, the influence of prolonged heat treatment (in total 16.5 days at 200 °C) on the thermocouple sensitivity is investigated. Finally, the thermocouple cross-sensitivity study of the pressure influence on the temperature sensitivity is performed. First a test was made with pure pressure load of 40 MPa without a temperature gradient between junction and terminal. The final cross-sensitivity measurement was performed in a climatic chamber including a pressure test rig which is able to apply a load of 200 MPa at a maximum temperature of 141.5 °C.

In this work we present a condition monitoring device which is fabricated in a low cost printing process directly on the 3D shaped substrate. The temperature monitoring is performed by 24 thermocouples resulting in a 24 pixel temperature image of the sample. The thermocouples are printed in an array arrangement, which enables the reduction of the required connections from 48 to 25. The printing is performed in a two-step process in which first the sensor pattern is screen printed onto a transfer foil and then transferred in a second step onto the target substrate by water transfer printing.

In this contribution we present an approach towards programmable shape memory transformations utilizing the intrinsic physical properties of a ternary polymer-based composite. The underlying idea is to use an open porous elastic skeleton as the base, where the pores are filled with a component that can alter its elasticity abruptly upon an external stimulus. In this study we use PDMS as the base network, and Paraffin as the pore filler, which causes a huge change in elasticity of the composite when undergoing the solid to liquid phase change at elevated temperature. This binary composite, already featuring shape memory characteristics, is then additionally made controllable by adding nano-particles with specific stimulus responsive properties. We demonstrate the programmable shape transformation capability of the devised ternary composite by the example of carbon black and ferromagnetic iron oxide nano-particles.

In this contribution we present a minimum effort approach to sensors for physical quantities like, e.g., force, pressure, strain, and temperature without the need for cost-intensive lab equipment. Only adhesive tape, carbon black, some kind of cutting tool, and a heat source are needed to fabricate sensors that can easily be affixed also to unusual and curved surfaces. We demonstrate the ease of fabrication and usage at the example of a rather generic sensor structure that can be used to measure strain, force/pressure, and temperature at virtually any chosen point of interest.

The concept and suitable technologies leading to an electrostatic energy harvester based on the motion of an electrolyte acting as a liquid electrode are presented. It is designed to exploit the mechanical energy associated with a 90° rotation by simultaneously changing the area of two antiparallel capacitors. In order to increase their capacitance, a custom made dielectric coating with enhanced relative permittivity is produced. Tests concerning the mobility of the electrolyte are presented, which can be potentially improved by adding small amounts of surfactant. Experimental validation shows promising results for the proposed energy harvester design being especially suitable for operation at low frequencies.

A highly parallelizable approach for the precise dosing of homogenous liquids is presented. Droplets of the dosing liquid rest on a superhydrophobic plate featuring a 3x3 array of holes, which evaporate with time and therefore reduce in size. If the size of the droplets is small enough, they are able to pass through the holes, which can be subsequently used for further applications. Furthermore, microheaters are screen-printed around the holes of the superhydrophobic plate, in order to accelerate the evaporation process. Experimental data regarding temperature distribution and the improvement due to the addition of the microheaters to the setup is provided.

This work presents a screen printed sensor that integrates the monitoring of flow velocity and thermal conductivity of a fluid in a microchannel. A modified anemometer principle is utilized to determine the flow velocity, which accounts for spurious heat transfer paths, e.g., in the substrate or channel walls. The sensor is fabricated by screen printing and tested using three different fluids at varying flow velocities. It is shown that the devised sensor setup is feasible for sensing both, flow velocity and thermal conductivity of the fluid.

In this work we demonstrate the realization of a simple and low cost NO 2 gas sensor on a flexible polymer substrate. The sensor was fabricated using inkjet printing and hydrothermal growth and consists of a printed interdigital structure on which a sensitive zinc oxide layer was grown. The chosen growing process does not require an additional seed layer, which allows to directly grow the zinc oxide layer on the substrate. Furthermore, the operating temperature was intrinsically achieved with an inkjet printed heater, which is located at the backside of the sensor.

We present a fully screen printed sensor for measuring the flow velocity inside microfluidic chips based on a modified anemometer principle. In contrast to previously reported sensor concepts, this device is designed to automatically compensate for the spurious heat propagation in the substrate and heat conduction in the liquid of interest. Using this approach it is possible to measure the flow velocity of any liquid in a channel directly by means of a single voltage without the need to correct for thermal side effects in a post-processing step. Additionally, fabrication by screen printing allows for integration of the sensor without disturbing the fluid flow.

This work deals with the realization of a fully printed thermocouple-array on a flexible substrate for condition monitoring applications. The thermocouple-array consisting of carbon black and silver was fabricated on PET-foil in a screen printing process and characterized up to a junction temperature of 150 °C. To ensure that no spurious voltages of the thermocouple occur due to deformations of the flexible substrate, the cross-sensitivity to deformation of the foil was investigated as well. Finally, as a test case, the temperature gradient of a plastic bar heated on a single end was measured with the thermocouple-array.

Embedding functional sensor layers directly into mechanical systems in heavy-duty surroundings facilitate the real-time monitoring of the system&rsquo, s state. This work presents a fully-spray coated pressure sensor that is suitable for applications in the high pressure range. It is embedded into functionalized organic coatings that additionally act as a dielectric for the capacitive sensing mechanism. The sensitivity of the sensor, as well as its long-time stability, has been determined. Additionally, testing has been performed at elevated temperatures to determine the temperature dependent sensitivity that arises from the temperature dependence of the Young&rsquo, s moduli.

In this work we present the realization of a fully spray processed embedded temperature sensor. The sensing element is a thermocouple made of conductive materials which are high temperature stable polymer based paints with organic or metallic filler particles. The thermocouple is embedded in an organic paint layer which shows, besides good high temperature properties up to 250 °C, excellent mechanical and chemical stability. The manufactured thermocouples are tested up to a junction temperature of 250 °C, first without and afterwards with an encapsulation layer. The measured output voltage for a terminal temperature of 25 °C and a junction temperature of 250 °C is in the region of 4.23 mV before and after top coating. Finally, a layer analysis of the encapsulated device is made using a cross-section polish.

Increasing the relative permittivity of dielectric materials can be useful in many applications, including capacitive sensing in microfluidics. In order to be able to efficiently integrate capacitive sensors with high sensitivity into all-polymer microfluidic devices, polymeric layers with high dielectric constants are required. In this contribution, a dielectric coating made from a polymeric base with mixed in ceramic particles, which exhibits enhanced dielectric properties compared to the polymer itself, is presented. Poly(methyl methacrylate) is chosen as a polymeric base material due to its processing properties. Its relative permittivity is increased by mixing in barium titanate particles at concentrations ranging from 0 vol% to 90 vol%. The dielectric properties of each fabricated mixture are investigated and the results are compared to theoretical values derived from standard mixing rules. To demonstrate the sensitivity enhancement of sensors due to the use of the fabricated dielectric coating, a microfluidic device is presented featuring a capacitive sensor for detection of fluids with different dielectric constants. It is shown that the sensitivity of the capacitive sensor is significantly increased by using the custom dielectric mixture when compared to pure poly(methyl methacrylate).