Satu Rajala

Film-Type Sensor Materials PVDF and EMFi in Measurement of Cardiorespiratory Signals— A Review

In this paper, some recent results obtained with film-type sensor materials polyvinylidenefluoride (PVDF) and electromechanical film (EMFi) are presented. The materials generate a voltage when they are mechanically deformed, however, because of their capacitive nature only the change of an external force can be measured. Hence, the sensors made of these materials are useful especially in the measurements of physiological pulsatile signals (e.g., heart rate and respiration). The entire measurement process with the PVDF and EMFi sensors is considered here; from the design and construction of the sensors and measurement electronics to the analysis of the measured data. The paper also presents some examples of the physiological signals (heart rate, respiration, and heart sounds) measured with the PVDF and EMFi sensors. Flexible and thin sensor materials are useful especially in physiological applications where the sensor is integrated into clothing or into daily life objects (e.g., a chair or a bed). The sensor attachments to human can be minimized and the measurement systems can be designed to be unobtrusive and comfortable for the user.

Flexible Piezoelectric Energy Harvesting Circuit With Printable Supercapacitor and Diodes

We report a flexible energy harvesting circuit fabricated by roll-to-roll compatible, solution-processable methods. The circuit incorporates a supercapacitor fabricated from a viscous carbon nanotube dispersion, printed Schottky diodes, and a piezoelectric element. Used low-temperature materials enabled component integration on poly(ethylene terephthalate) substrate. The supercapacitor was built with a paper separator and an aqueous NaCl electrolyte. Together with carbon-based electrodes, these materials translated into a disposable and environmentally safe electronic device. The energy harvested from mechanical movement was used to drive a commercial electrochromic display.

Plantar shear stress measurements - A review.

BACKGROUND Mechanical stress at the plantar surface has two components, pressure acting normal to the surface and shear stress acting tangential to the surface. Typically only pressure is measured and reported. However, plantar shear stress also plays a major role, especially in diabetic ulceration. METHODS During the last few decades, a variety of methods have been developed for the measurement of plantar shear stress. This paper reviews the technologies used in plantar shear stress measurements. FINDINGS Several technologies have been used, e.g. magneto-resistors, strain gauges, optical methods, piezoelectric materials and capacitive sensors. Examples of plantar shear stress values measured with the developed devices are also collected here and the relationship between sensor characteristics and the measured plantar shear stress distribution is discussed. INTERPRETATION Even with the limitations of current plantar shear stress measurement technologies, they can provide useful information on the plantar stress distribution.

Characteristics of Piezoelectric Polymer Film Sensors With Solution-Processable Graphene-Based Electrode Materials

The sensor characteristics of piezoelectric polyvinylidenefluoride (PVDF) sensors with solution-processable electrode materials were studied. The electrodes were solutionprocessed on 28-μm-thick PVDF film. Two graphene-based printable inks, ink-jet, and screen formulated ink, were used. Sensors with evaporated metal electrodes were used as a reference to compare the properties of novel sensor structures. The sensor characteristics studied here were sensitivity, nonlinearity, hysteresis, and the effects of frequency and temperature. The sensor sensitivity measurements revealed mean sensitivities of (31.1 ± 1.4) pC/N for the reference sensors and (26.2 ± 2.2) and (21.4 ± 1.3) pC/N for the sensors with graphene-based ink-jet and screen formulated ink electrodes, respectively. The sensor characteristics of the novel sensors were found to be similar to those of the reference sensors. The new sensors are linear, hysteresis error is negligible, and the operation under changing frequency (up to 100 Hz) is rather stable. Change in ambient temperature somewhat affects the sensor sensitivities. The sensors presented here can be used in several sensing applications, e.g., in plantar pressure distribution measurements.

Engineering and Characterization of Bacterial Nanocellulose Films as Low Cost and Flexible Sensor Material

Some bacterial strains such as Komagataeibacter xylinus are able to produce cellulose as an extracellular matrix. In comparison to wood-based cellulose, bacterial cellulose (BC) holds interesting properties such as biodegradability, high purity, water-holding capacity, and superior mechanical and structural properties. Aiming toward improvement in BC production titer and tailored alterations to the BC film, we engineered K. xylinus to overexpress partial and complete bacterial cellulose synthase operon that encodes activities for BC production. The changes in cell growth, end metabolite, and BC production titers from the engineered strains were compared with the wild-type K. xylinus. Although there were no significant differences between the growth of wild-type and engineered strains, the engineered K. xylinus strains demonstrated faster BC production, generating 2-4-fold higher production titer (the highest observed titer was obtained with K. xylinus-bcsABCD strain producing 4.3 ± 0.46 g/L BC in 4 days). The mechanical and structural characteristics of cellulose produced from the wild-type and engineered K. xylinus strains were analyzed with a stylus profilometer, in-house built tensile strength measurement system, a scanning electron microscope, and an X-ray diffractometer. Results from the profilometer indicated that the engineered K. xylinus strains produced thicker BC films (wild type, 5.1 μm, and engineered K. xylinus strains, 6.2-10.2 μm). Scanning electron microscope revealed no principal differences in the structure of the different type BC films. The crystallinity index of all films was high (from 88.6 to 97.5%). All BC films showed significant piezoelectric response (5.0-20 pC/N), indicating BC as a promising sensor material.

Structural and Electrical Characterization of Solution-Processed Electrodes for Piezoelectric Polymer Film Sensors

Solution-processable graphene and carbon nanotube-based electrode materials were used here to provide electrodes on flexible piezoelectric polyvinylidenefluoride sensors. Piezoelectric sensitivity measurements, image-based analysis, adhesion tests, and sheet resistance measurements were applied to these printable sensors to rigorously analyze their performance and structure. The printable sensors showed electrical performance similar to metallized sensors, whereas the adhesion of the solution-processed materials to the substrate is not as high as that of the evaporated metal films. This also affects the measured sensor sensitivity values. The measurements based on optical images were found to be a promising method to capture detailed information about the electrode surface structure.

Correlation approach for the detection of the heartbeat intervals using force sensors placed under the bed posts

Abstract This study proposes a method for detecting the heartbeat intervals of a person lying on a bed from ballistocardiographic signals recorded unobtrusively with four dynamic force sensors located under the bed posts. The method does not recognize individual heartbeats, but the intervals where the correlation between two consecutive signal segments maximizes. This study evaluated the performance of the method with nine subjects in 1-h long recordings and achieved 91% beat-to-beat interval (BBI) recognition coverage; 98.6% of the detected BBIs differed less than 50 ms from the values calculated from a reference electrocardiogram signal. This study also evaluated the reliability of two parameters of heart rate variability that have been used in sleep quality assessment in several studies and are usually calculated for 30 s epochs. The results suggest that the method is able to provide sufficient reliability for using the data in evaluation of sleep quality.

A survey of printable piezoelectric sensors

Availability of solution-processable piezoelectric sensor and electrode materials enable low-cost and high-throughput fabrication of fully printable piezoelectric sensors. Results obtained with piezoelectric polymer (polyvinylidenefluoride, PVDF), cellulose nanoflbril (CNF) and cellulose nanocrystal (CNC) films as sensor materials are presented here. These sensor materials can be processed in solution and used in combination with printed electrodes to obtain full printability of the sensors. A commercial PVDF film and in-house fabricated CNF and CNC film are used as sensor materials. In addition, conducting polymer, graphene and carbon nanotube (CNT) based inks are used as solution-processable electrode materials in the sensors, whereas conventional metallic electrodes are used as reference electrode material. The sensor operation of the fabricated sensors is evaluated through piezoelectric sensitivity measurements. The sensor sensitivity measurements revealed mean sensitivities from 2 pC/N to 42 pC/N in transverse direction, depending on set of the sensor and electrode materials used.

Testing and comparing of film-type sensor materials in measurement of plantar pressure distribution

Simple in-shoe sensors based on film-type sensor materials were developed in this study. Three sensor materials were tested: polyvinylidenefluoride (PVDF), cellulose nanofibrils (CNF) and ElectroMechanical Film (EMFi). Plantar pressure distributions of a subject were measured with the developed in-shoe sensors; each consisting of three sensor channels (lateral and medial metatarsal heads and heel). In addition, piezoelectric sensor sensitivities and crosstalk between the sensor channels were determined. Differences between the tested film-type materials and measured plantar pressure distribution signals were studied.Simple in-shoe sensors based on film-type sensor materials were developed in this study. Three sensor materials were tested: polyvinylidenefluoride (PVDF), cellulose nanofibrils (CNF) and ElectroMechanical Film (EMFi). Plantar pressure distributions of a subject were measured with the developed in-shoe sensors; each consisting of three sensor channels (lateral and medial metatarsal heads and heel). In addition, piezoelectric sensor sensitivities and crosstalk between the sensor channels were determined. Differences between the tested film-type materials and measured plantar pressure distribution signals were studied.

Comparison of photoplethysmogram measured from wrist and finger and the effect of measurement location on pulse arrival time

OBJECTIVE The aim of this paper was to compare photoplethysmogram (PPG) signals measured from the wrist and finger and to evaluate if wrist PPG signal could be used to calculate pulse arrival time (PAT), the time delay between electrocardiogram (ECG) R peak and a feature (e.g. peak, foot, first derivative peak) in the PPG signal. Further, the correlation between pulse wave velocity (rePWV, defined as PWV from ECG R peak to extremity) and systolic blood pressure was studied. APPROACH Thirty subjects were measured at rest by a trained research nurse. For reference measurement, chest ECG and finger PPG were measured using commercial sensors. Wrist PPG and arm ECG were measured with a custom-made setup, where the PPG sensor was located at the back surface of the forearm. MAIN RESULTS Reference finger and wrist PPG signals were found to differ in shape and also in amplitude. The PPG foot or first derivative peak detection methods seemed to be the most suitable methods for wrist PAT calculation. The Pearson correlation coefficient between blood pressure and rePWV was found to be 0.44 for the reference finger measurement and 0.37 for the wrist measurement. SIGNIFICANCE Wrist PPG signal is widely used in optical heart rate monitors. Based on the results obtained in this study, wrist PPG signal may be used also for PAT calculation. The use of PAT for blood pressure estimation still has challenges, but PAT as such could be used as an interesting indicator of vascular health.