Results in Engineering 22 (2024) 102323 Available online 1 June 2024 2590-1230/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Review article Review of fabrication of SAW sensors on flexible substrates: Challenges and future Saima Qureshi a, * , Mehwish Hanif b , Varun Jeoti a , Goran M. Stojanovi´c a , Muhammad T. Khan c a Faculty of Technical Sciences, University of Novi Sad, Trg Dositeja Obradovi´ca 6, 21000, Novi Sad, Serbia b Tyndall National Institute, University College Cork, Lee Maltings, Cork, T12 R5CP, Cork, Ireland c Department of Electronic Science and Engineering (ESE), University of Electronic Science and Technology of China (UESTC), PR China A R T I C L E INFO Keywords: Surface acoustic waves Flexible substrates Flexible sensors Wearable sensors Piezoelectric SAW ABSTRACT Surface acoustic waves sensors are based on piezoelectric materials. These sensors are the best choice for sensing purposes due to their unprecedented accuracy. SAW sensors are often used in biochemical sensors, drug pro- duction, chemicals agents and gas analytes as sensing devices. These systems are extremely competitive due to their compact size, high thermal efficiency, stability, and capacity to integrate wirelessly. In the past two de- cades, considerable research has been conducted on the manufacturing of SAW on rigid substrates. These sub- strates of SAW sensors preclude their use in wearable technologies. The review will cover the research conducted on flexible saw sensors to highlight limitations in this research area for future applications sensors in wearable technology. In the end, future directions and strategies have been discussed to overcome the current challenges. 1. Introduction The special electromechanical link in piezoelectric (PE) crystals gives them extraordinarily strong materials. One specific usage of piezo electric is to produce surface acoustic waves, known as SAWs, and these SAWs have amazing properties, similar to the material from which they come. In general, SAWs are mechanical waves that are confined on the elastic layer with a deterioration in the bulk of the object. In 1965, White and Voltmer’s research into the development of SAWs utilizing piezo- electric substrate was a technological advance that provided a new di- rection to tools based on SAW [1]. Thereafter, components used in most electronic systems have been used extensively [2,3]. SAW tools are used in a broad variety of products, including wireless radio transmission as filters, spectrum duplexers, and radio frequency identification tags (RFIDs), biological sensors, medical research, biomedical, engineering, and biological sciences as microsensors for the measurement of physical parameters and biochemical detection [4,5] and as micro-actuators for microfluidic devices [6,7] and application for lab-on-a-chip [7,8]. Typically, SAW sensors are designed on conventional surfaces such as silicon (Si) or sapphire piezoelectric (PE) substances coated with PE materials like zinc oxide (ZnO) or aluminum nitride (AlN). Interdigitated electrodes (IDE) are often constructed with gold (Au) and aluminum (Al). Because of the rigid piezoelectric surfaces and metallic electrodes, these rigid SAW substrates are not applicable in the field of stretchable and transparent electronics [9]. As a result, their imple- mentations in modular and translucent electronics are constrained [10]. Stretchable electronics have various benefits over traditional solid-state electronics due to their flexibility. Transparent electronics are an inno- vative technology of significant application opportunities and creativity. Numerous new flexible devices and microelectronics, such as power conditioning displays [11], electronic skin [12], and others, have been published recently. It would be enormously beneficial if SAW systems could be built inexpensively on a lightweight and flexible substrate, such as plastics that are either removable or recycled materials [13]. The development of flexile substrates and advanced functional materials has made it possible to combine SAW sensors with flexible technology and new applications of flexible saw sensors in wearable technology has become the recent topic of research. The main aim of writing the current review is to discuss the flexible substrates and fabrication challenges of SAW sensors on flexible sub- strates Recent review papers on SAW sensors have emphasized the use of sensing layers, nanomaterials, and the application of these SAW sensors in a variety of areas but have paid less attention to the manufacturing * Corresponding author. E-mail addresses: saima.qureshi@uns.ac.rs (S. Qureshi), mehwishhanif1991@gmail.com (M. Hanif), varunjeoti@uns.ac.rs (V. Jeoti), sgoran@uns.ac.rs (G.M. Stojanovi´c). Contents lists available at ScienceDirect Results in Engineering journal homepage: www.sciencedirect.com/journal/results-in-engineering https://doi.org/10.1016/j.rineng.2024.102323 Received 11 October 2023; Received in revised form 22 May 2024; Accepted 26 May 2024