Screen-Printed Paper Microbial Fuel Cell Biosensor Comes in Handy for Detecting the Presence of Toxic Compounds in Water

Microbial fuel cell (MFC) technology directly converts chemical energy contained in organic matter into electricity through the metabolic processes of microorganisms and it is shown to be promising to assess the quality of water. The development of a biofilm consisting of an electroactive bacteria on the surface of the anode is capable of transferring electrons generated from the oxidation of organic compounds to the electrode. Thus, the current generated by the MFC can be correlated to the metabolic activity of the anodic bacteria and any possible disturbances to its metabolic pathways, caused by environmental changes, such as organic load, or due to the presence of toxic compound(s), is likely to be reflected in the measured current. In spite of their ability, implementation of MFCs as sensors is limited by device designs and high cost involved in the fabrication process. Researchers at University of Bath, United Kingdom have developed a simple and cost-effective single-component paper-based MFC (pMFC) sensor device by screen printing carbon-based electrodes onto a single sheet of paper (Jon Chouler et al., Biosensors and Bioelectronics, 102 (2018) 49-56).

The paper-based MFCs (pMFC) sensor was fabricated by screen-printing. The conductive ink contains a solution mixture with 20 mg α-cellulose dissolved in 1-ethyl-3-methylimidazolium and dimethyl sulfoxide with 92:8% w/w ratio in which 40 mg carbon nanofibers and 40 mg graphite powder were thoroughly dispersed. Three layers of the conductive ink were screen-printed (43–80 μm mesh) onto the paper to form the electrodes (Fig. 1). Since the paper substrate itself acts as the separator between the two electrodes, the cellulose fibers within the paper were cross-linked with glyoxal (0-24% w/v at 20 °C for 3 h), which helped to increase robustness and operational lifetime of the sensor device. The electrochemical performance of the paper-MFC sensor for detecting formaldehyde, a potential toxic compound, was evaluated. Two pMFCs were folded back-to-back to fabricate fpMFC which enriched the performance of the sensor device. Portability, facile use, and biodegradability are the unique advantages of this paper-based MFC sensor.

Fig. 1 (a) Schematic of the pMFC and electrical connection; (b) Photograph of the actual pMFC, showing size; (c) Principle of operation of the pMFC; and (d) Assembly of the fpMFC by folding two pMFCs back-to-back (1), with parallel electrical connection (2).

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Jon Chouler et al., Biosensors and Bioelectronics, 102 (2018) 49-56

Flexible sensors for real-time monitoring of temperature, pressure and flow

Development of strain/stress sensors for human-motion detection and implantable devices for human health monitoring assumed significance in recent years. Multi-functionality, poor interface stability and flexibility of such sensors still remain as a major challenge. Researchers at State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, and Engineering Research Center of Advanced Glasses Manufacturing Technology, College of Materials Science and Engineering, Donghua University, China have reported fabrication of a mechanically strong and flexible vessel-like sensor that is capable of continuously monitoring applied temperature, strain, and the frequency, pressure, and temperature of the in-vessel flow.

Braided cotton yarn served as the base and it was coated with single-walled carbon nanotubes (SWCNTs) by dip-coating (cotton@SWNTs) to function as a current collector/electrode. ZnO nanoseeds were nucleated on the surface of cotton@SWNTs to promote the growth of ZnO nanorods, which could serve as temperature sensing arrays. A polyvinylidene fluoride (PVDF) film was formed over the cotton@SWCNTs/ZnO through dip-coating. A PVDF fibrous membrane was further prepared on the PVDF surface through electrospinning. The PVDF fibrous membrane was immersed in ethyl acetate for 12 h to obtain a compact film. The PVDF membrane could serve as piezoelectric nanofibers. Finally, silver paste was uniformly coated on the surface of the PVDF membrane. The various stages involved in the fabrication are shown in Fig. 1.

Fig. 1 various stages involved in the fabrication of flexible sensor

The morphological features of cotton@SWNTs indicate a uniform distribution of SWCNTs on the surface of cotton yarn (Fig. 2(a)). Similarly, the ZnO nanorods arrays@PVDF are also uniformly developed on the surface of cotton@SWCNTs (Figs. 2(b) and 2(c)).

Fig. 2 Morphological features of (a) SWCNTs coated cotton yarn; (b, c) cotton@SWCNTs/ZnO@PVDF; (c) magnified image of ZnO nanorods

Structural representation of the tubular sensor used for monitoring temperature, motions and liquid flow is depicted in Fig 3(a). The thermosensitivity of ZnO grown between the fibers of SWCNTs helps monitoring the temperature variation, which is detected through resistance measurement of the tubular inner electrode (Fig. 3(a)). The I-V curves measured using the functionalized inner electrode of the sensor at different temperatures indicate the change in resistance with variation in temperature (Fig. 3(b)).

Fig. 3 (a) Schematic representation of the sensor used for monitoring temperature, motions and liquid flow; (b) Linear I-V curves of the sensor measured under different fluid temperature.

The sensor is also capable of monitoring human body motions by sensing both bending and pressing. When a force is applied on the surface of the PVDF nanofibers, they get polarized. The equivalent bound charges accumulated on the surface of PVDF nanofibers are measured as electrical signals with the help of bonding wires between the two electrodes – cotton@SWCNTs/ZnO layer and the silver paste layer. The output voltage is increased from 0.038V to 0.1V with an increase in bending angle from 1.4 rad to 2.1 rad (Figs. 4(a) and 4(c)). Similarly, an increase in output voltage is observed with an increase in frequency during compression (Figs. 4(b) and 4(d)). These inferences suggest that human motions such as bending and compression can be directly monitored without any external power supply.

Fig. 4 (a, b) Demonstration of the bending and pressing motion; (c) output voltage generated by the sensor under different bending angles; and (d) output voltage generated by the sensor under compression at different frequencies.

The ability of the sensor to monitor blood temperature and blood pressure is also evaluated. Fig. 5(a) shows the experimental set-up used to monitor liquid flow. The temperature of the test fluid (deionized water to mimic blood flow) that flows into the sensor is accurately controlled using microfluidic channels heated by sample heater. The variation in the temperature of the fluid under flow from 35 to 40 °C is clearly reflected by the change in resistance measured using the sensor (Fig. 5(b)). The sensor could also detect the variation in the input pressure and frequency of the pulsed flow. The increase in the output voltage with an increase in applied pressure indicates the ability of the sensor to measure the variation in pressure under flow conditions (Fig. 5(c)). Similarly, the variation in the output current is sufficient enough to qualify the sensor for its ability to measure the pulse frequency of fluid (Fig. 5(d)).

Fig. 5 (a) Experimental set-up used to monitor liquid flow using the sensor; (b) I-V curves of the sensor measured under a narrow fluid temperature range; (c) output voltage generated by the sensor under different fluid input pressure; and (d) output current generated by the sensor under different pulse frequencies.

The cotton@SWCNTs/ZnO@PVDF sensor is flexible and durable. The output signal of the sensor remains steady during 1920 circles of the bending circle test. The sensor facilities detection and continuous monitoring of the applied temperature, strain, and the frequency, pressure, and temperature of pulsed fluids. It can be used for implantable physical sensing applications and monitoring of human health.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Wei Zhang et al., A strong and flexible electronic vessel for real-time monitoring temperature, motions and flow, Nanoscale, 2017, DOI: 10.1039/C7NR05575G

Ultrathin Epidermal Piezoelectric Sensors for Real-Time Pulse Monitoring

Real-time monitoring of heart rate, blood pressure, and respiration rate assume significance since these data can provide the status of one’s personal health and early warning about deterioration of health so that necessary therapeutic treatments can be given to the patient. In spite of the numerous developments of sensors for real-time monitoring, meeting their requirements such as flexibility, sensitivity, response time and mechanical stability and biocompatibility remains to be really challenging. Researchers at Korea Advanced Institute of Science and Technology (KAIST), Republic of Korea, Severance Cardiovascular Hospital, Yonsei University College of Medicine, Republic of Korea, KAIST Institute for NanoCentury (KINC), Republic of Korea, The Pennsylvania State University, USA and ROBOPRINT Co., Ltd., Republic of Korea have demonstrated a self-powered flexible piezoelectric pulse sensor based on PZT thin film for real-time healthcare monitoring.

The fabrication process of self-powered flexible pressure sensor on an ultrathin polyethylene terephthalate (PET) substrate is described schematically in Fig. 1. A high-quality PZT thin film was deposited on sapphire using a 0.6 M sol-gel PZT solution by spin coating at 2000 rpm for 30 s, annealed at 700 °C for 2 h (Fig. 1(a)), followed by attachment of a thermal release tape (Fig. 1(b)). Subsequently, the PZT thin film was exfoliated by an inorganic-based laser lift-off (ILLO) technique, which involves irradiation at the backside of the sapphire substrate using a XeCl Excimer laser (Fig. 1(c)). The exfoliated PZT thin film was then transferred to an ultrathin PET substrate using an UV-cured adhesive polymer and the transfer medium was detached by thermal treatment (Fig. 1(d)). Photolithography and wet etching process were employed to develop gold interdigitated electrodes (IDEs) (Fig. 1(e)) followed by deposition of a photocurable epoxy passivation layer on the PZT thin film. The ultrathin piezoelectric sensor is highly flexible, which enables a conformal contact of the sensor to the human skin topography and improves the sensing ability of tiny pressure arising near the surface region of epidermis. The epoxy passivation of the piezoelectric sensor offers negligible cytotoxicity.

Fig. 1 Schematic illustration of the various stages involved in the fabrication of self-powered flexible pressure sensor on an ultrathin PET substrate

The flexible ultrathin piezoelectric sensor can be conformally attached on human wrist (Fig. 2(a)) and carotid artery (Fig. 2(b)) using a biocompatible liquid bandage spray. Besides, it can be integrated with the medical mask (Fig. 2(c)).

Fig. 2 (a-c) Photographs of piezoelectric pulse sensor conformally attached on (a) human wrist; and (b) carotid artery position (top) and the middle of the throat (bottom) using a biocompatible liquid bandage; (c) sensor integrated with the medical mask

The variation in the radial artery pulse signals before (red line) and after (blue line) physical exercise (running for 10 min) (Fig. 3(a)) indicate the effectiveness of the sensor to respond to blood vessel movements. Similarly, the flexible ultrathin piezoelectric sensor can be used to monitor carotid artery pulse and muscle movements (Fig. 3(b)) and respiratory activities (Fig. 3(c)).

Fig. 3 (a) Radial artery pulse signals showing different heart rates and generated output voltages before and after physical exercise; (b) output voltage in response to carotid arterial pressure (top) and saliva swallowing actions (bottom); and (c) output voltage response of the pressure sensor to normal (right bottom, blue) and deep oral breathing (right top, red) during periodic oral breathing.

The applicability of the self-powered flexible pulse sensor for real-time pulse monitoring is validated by integrating signal amplification, frequency filtering,  signal processing circuit to identify arterial pulse signals, decision making module and a microcontroller unit for wireless transmission of the signal to a smart phone, in the assembly (Fig. 4). The outputs from the system confirm that the self-powered pulse sensor can be effectively utilized for continuous real-time monitoring.

Fig. 4 (a) Photograph of the LED and speaker unit operated synchronously corresponding to the radial artery pulse (inset: output voltage from the first (bottom, red) and second (top, blue) amplifier stage; (b) Photograph of wireless transmission of the pulse to a smart phone, showing capability for a real-time arterial pulse monitoring system.

The sensitivity (0.018 kPa⁻¹), fast response time (60 ms), good mechanical stability (5000 pushing cycles), response to lower frequency vibrations (0.2 to 5.0 Hz) and higher frequency sound waves (240 Hz), ability to conformally attach on human epidermis of the flexible piezoelectric sensor helps effective  monitoring of radial/carotid artery pulse, respiratory activities, and trachea movements. Development of such sensors is believed to make significant impact in medical diagnosis towards achieving a better human health care.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Dae Yong Park et al., Self-Powered Real-Time Arterial Pulse Monitoring Using Ultrathin Epidermal Piezoelectric Sensors, Adv. Mater. 2017, 1702308, DOI: 10.1002/adma.201702308