Flexible compact system for wearable health monitoring applications
Graphical abstract
Introduction
The growth of flexible electronics has received a cumulative attraction in recent years due to their widespread applications such as smart wearable products [1], [2], [3], [4] especially in medical field [5,6]. The elderly population growth is expected to increase 56 percent in 2030 and to be doubled in 2050 [7]. In the upcoming generations, the human beings are likely to carry the small wearable flexible devices comprises of sensors and signal processing units which will communicate with the body and send information about their health to the hand-held devices. In order to support the current upcoming trends, the components used in such technology should also be flexible as well. This motivates the researchers to develop low cost, innovative technological solutions which would transform the current health care systems to more efficient, reliable, and cost-effective.
Flexible and wearable antennas play a major role in wearable and remote health care applications [8]. Fig. 1 shows the schematic block diagram of the wireless wearable health monitoring system. In this, RF front end module is the key component which organise remote data communication using Wi-Fi antennas. Among them, multimodal wide bandwidth antenna with low profile high gain antennas were preferred for Wi-Fi applications [9], [10], [11]. The antenna used in this system should be flexible to make the entire system as wearable. The major issue to be addressed in this area is the selection of flexible substrate, which has a significant impact in enhancing the efficiency and gain of the flexible antenna. Flexible substrate used for the fabrication of flexible antenna especially for the biomedical applications should be biodegradable, inert, biocompatibility, and non-toxic. Candidates such as papers [12], [13], [14], textiles [15], [16], [17] and polymers [18,19] were used as flexible substrates for the device fabrication. Flexible antennas fabricated using paper and textiles possess excellent properties such as high gain, wide bandwidth and flexible, but it also has significant drawback such as poor thermal stability. Poor thermal stability restricts the use of antennas in out-door communications, whereas the polymer such as Polyetherimide (PEI) or Polyethylene terephthalate (PET), Liquid Crystal Polymers (LCP) based antennas can be used. However, low thermal stability, high gas permeability, and toxic nature of LCP limit their use in wearable applications.
A compact dual-band frequency-reconfigurable textile antenna [20], flexible polyamide substrate based antennas for laptop computers [21], ultra-wide band flexible antennas for medical applications [22], flexible antenna using PDMS substrate [23], and compact wearable textile antennas for wearable applications [24] have been reported. Substantial works have been reported in the flexible antenna using different flexible substrates for different applications. Amongst them, Polydimethylsiloxane (PDMS) based flexible substrate is considered as one of the best substrates for the flexible wearable devices which has the capability to replace all the existing flexible substrate by its unique characteristics and revolutionize the entire flexible, stretchable and wearable domain.
Flexible PDMS substrate with high-quality properties was reported in our previous work for the first time to the best of our knowledge [25]. PDMS substrate has a high thermal stability, low gas permeability, high tensile strength, and resistance to solvents. It has a contact angle of about 120° which states that it has an excellent resistance to water, thus enabling long-term stable devices [25]. It also exhibits tunable RF characteristics such as dielectric constant (ɛr) and loss tangent (tan(δ)). PDMS substrates are biodegradable and have high conformability over the skin that results in reduced skin contact impedance. The skin contact impedance is one of the important parameters to be considered while designing a wearable antenna. Characteristics such as chemically inert, permeable to water vapor and gasses, simple to handle, thermally stable and isotropic and homogeneous properties [26] initiated the researchers to focus their research on flexible PDMS substrates. It can also be easily mixed with other composites to form a composite based flexible substrate because of its flexible nature and to alter its dielectric properties.
Though studies reported on the progress of the wearable flexible antennas as discussed above, still, enhancement in the flexible wearable antenna for the health care applications is much needed. In this regard, a flexible antenna for the ISM bands (2.4 – 2.49 GHz) using PDMS flexible substrate is reported. The proposed antenna can be easily integrated into the garments and used for the wireless health monitoring and remote data transfer applications. The ISM band of 2.4 – 2.5 GHz is utilized by the IEEE standards such as 802.11b, 802.11g, and 802.11n. The flexible antenna has high performance in terms of return loss, VSWR, efficiency, and gain. The paper is organised as follow: Section 2 and 3 give a brief on the antenna design and discussions of results, respectively. The optimization of feed position, patch length and ground width are also reported in Section 3. Section 4 concludes the paper.
Section snippets
Wearable Antenna Design
Flexible PDMS substrate is fabricated using PDMS and Tetraethylorthosilicate (TEOS). The PDMS of molecular weight 110,000 and 4,200 are mixed in equal weight ratio and dissolved in a solvent toluene to form a uniform mixture. TEOS of weight ratio 50% to the total concentration of PDMS is added and sonicated for few minutes. Then the curing agent ‘di-n-butyltin dilaurate’ is added to initiate the polymerization reaction and sonicated for few minutes. Finally, the jelly like precipitate is formed
Results and Discussion
Fig. 5 shows the prototype of the fabricated flexible patch antenna. The optimization of microstrip feed and dimensions of ground plane were also carried out. The designed antenna has a maximum return loss of around -46.97 dB at 2.42 GHz when the feed is positioned exactly at the center (x, y) = (0,0) with respect to the dimensions of the patch. Fig. 6(a) and 6(b) show the changes in return loss with respect to the feed position in left and right axes, respectively. From the figure, it is noted
Conclusion
Flexible compact antenna is presented with the detailed simulated and measured results. The presented flexible antenna has advantages such as high radiation efficiency, board impedance matching, high gain, and compact size with high thermal and mechanical stability. The flexible antenna is omnidirectional which covers a broad range in both E-plane and H-plane. It shows similar response under the bend condition which implies it can be used for flexible applications. In addition to this, the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors (DSS and Marcos) would like to thank the Conicyt FONDECYT (Fondo Nacional de Desarrollo Científico y Tecnológico) Project No. 3180089 and Millennium Nucleus MULTIMAT for funding and support.
Dr. Shanmuga Sundar Dhanabalan is a research fellow at Faculty of Physical and Mathematical Sciences, University of Chile, Chile. Currently, he is a visiting research fellow at Functional Materials and Microsystems Research Group at RMIT University, Australia. He completed his Ph.D. from Anna University, Chennai, India. He undertakes research in flexible electronics and materials, biosensors, optics, and photonics.
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Dr. Shanmuga Sundar Dhanabalan is a research fellow at Faculty of Physical and Mathematical Sciences, University of Chile, Chile. Currently, he is a visiting research fellow at Functional Materials and Microsystems Research Group at RMIT University, Australia. He completed his Ph.D. from Anna University, Chennai, India. He undertakes research in flexible electronics and materials, biosensors, optics, and photonics.
Dr. Sitharthan R is working as an Assistant professor in the School of Electrical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India. He has completed research funded project as a principal investigator under the ECRA scheme, SERB, DST, India. His research interests include renewable energy systems, artificial intelligence, cloud platform, IoT, soft computing techniques, and piezoelectric materials.
Dr. Madurakavi Karthikeyan is working as an Assistant Professor Senior Grade in School of Electronics, VIT, Vellore. He received his PhD in Department of Electronics and Communication Engineering from Pondicherry University, Puducherry, India. He published papers in reputed journals. His current research area includes MIMO receiver design, Biomedical antennas and metamaterial antenna design.
Dr. Arun Thirumurugan is a Researcher at the University of ATACAMA working on the development of magnetic nanocomposite for energy storage and biological applications. His research interests are synthesizing magnetic nanoparticles, surface modification of nanomaterials for the potential applications in detoxification, photocatalyst, energy storage, and biomedical applications.
Dr. Rajesh M is an Associate professor in Sanjivani college of engineering, India and a research associate in raga academic solutions, Chennai, India. He has published over 90 papers in international refereed journals like IEEE, Springer, and Elsevier. His main research interests include IoT, blockchain techniques, Cyber physical system, Industry 4.0, e-health technologies, IoT and soft computing techniques.
Prof. A. Sivanantha Raja is working as Professor in Alagappa Chettiar Government College of Engineering and Technology, Karaikudi with 31 years of teaching and research experience. Currently, he is guiding 5 Ph.D. scholars. 9 Ph.D. scholars, 2 M.S scholars and more than 150 M.E scholars have completed their respective degrees under his guidance in the field of optical communication, flexible electronics and biomedical applications.
Prof. Marcos Flores is working as Professor in Physics Department, Faculty of Physics and Mathematics Science, Universidad de Chile, with 10 years of teaching and 15 years of research experience. He has published around 60 papers in the reputed international journals. He has presented his work in more than 50 national and international conferences. He has been awarded around 10 national grants.