Pekka Iso-Ketola

Screen-Printing Fabrication and Characterization of Stretchable Electronics.

This article focuses on the fabrication and characterization of stretchable interconnects for wearable electronics applications. Interconnects were screen-printed with a stretchable silver-polymer composite ink on 50-μm thick thermoplastic polyurethane. The initial sheet resistances of the manufactured interconnects were an average of 36.2 mΩ/◽, and half the manufactured samples withstood single strains of up to 74%. The strain proportionality of resistance is discussed, and a regression model is introduced. Cycling strain increased resistance. However, the resistances here were almost fully reversible, and this recovery was time-dependent. Normalized resistances to 10%, 15%, and 20% cyclic strains stabilized at 1.3, 1.4, and 1.7. We also tested the validity of our model for radio-frequency applications through characterization of a stretchable radio-frequency identification tag.

Dynamic bending test analysis of inkjet-printed conductors on flexible substrates

The need to optimize space in electronic devices has made flexible electronics an attractive option for manufacturing electronics. Techniques to fabricate flexible circuits have become more and more common and the processes increasingly more efficient. Printed electronics is a potential technique for manufacturing electronic patterns on flexible substrates. In particular, inkjet printing is an effective way to produce fine, thin, conductive structures without touching the substrate material. This study concentrated on dynamic bending analysis of inkjet-printed silver conductors on a polymer substrate. Because printed electronics is a relatively new manufacturing method, not much research is yet available on mechanical endurance of printed structures. By default, thin layers of inkjet-printed traces may just prove to have good tolerance against bending. However, factors such as adhesion between ink layer and substrate and the effect of the porous structure of sintered nanoparticle ink must be studied. This paper evaluates the capability of the inkjet technique on a flexible substrate and benchmarks the results on conventional flexible copper circuit boards. Measurements were made in real time of the resistance of conductors while bending the sample along two different radii. Results showed that printed conductors were superior in endurance over etched copper circuits.

Development of a Lower Extremity Rehabilitation Aid Utilizing an Insole-Integrated Load Sensor Matrix and a Sole-Embedded Measurement Node

Following an individual exercise program after a hip or a knee operation is crucially important as it enables the muscles of the operated leg to regain their strength and thereby support the recovery of the operated joint. Usually the patients have no assisting devices at home to help determine how much weight they put on the operated leg. Training with a scale helps, but in general, humans can not accurately estimate the amount of weight they put on the leg. To address this problem we have designed a lower extremity rehabilitation aid, which utilizes an insole-integrated capacitive load sensor matrix and a sole-embedded read-out electronics with wireless communication link. In this paper we present in detail the manufacturing process of the insole sensor. The placement of the electrodes, calibration, and user interfaces are also discussed. The system is found to be suitable for detecting too light and too heavy steps.

Electrode Comparison for Textile-Integrated Electrocardiogram and Impedance Pneumography Measurement

Wearable electronics is a quickly broadening category in sports, wellbeing and entertainment products. Also, fully textile-integrated electronics is used increasingly to improve user experience. Medical industry is interested in exploiting, especially the latter sub-category of wearable electronics in long-term home care. In this study, we report a textile-integrated electrocardiography (ECG) and impedance pneumography (IP) measurement system. The performance of the system is evaluated by comparing the measurement accuracy for heart rate and respiration rate obtained with different electrode types and different measurement methods. Three electrode types: disposable, textile, and printed electrodes, are investigated and both, bipolar and tetrapolar measurement methods are compared by using a modified commercial evaluation board. Disposable electrodes provide the least noisy signal and the most stable results. However, the skin irritation caused by these electrodes prevents their use in long-term monitoring. The textile and printed electrodes did not seem to cause similar skin irritation. From the two measuring techniques, tetrapolar measuring method had higher noise levels, but heart rate and breathing were estimated with better accuracy compared to bipolar measuring method.

Screen-Printed Stretchable Interconnects

This article focuses on characterization of screen printed electrical interconnects for wearable applications. The interconnects are screen printed with stretchable silver-based ink on a thermoplastic polyurethane film. By optimizing the process parameters, we were able to fabricate 200 μm line widths with spacing of 200 μm. The average sheet resistance is 36.2 mΩ/□, however a significant lot-to-lot deviation was noted. In the strain tests, half of the samples have lost conductivity when 74.1% strain is reached. The normalized resistances of the samples rise linearly to approximately 30 % strain, after which the growth rate and deviation between samples increases significantly. It was also discovered that no cracking can be found from the traces while subjected to strains under 30%.