The world’s aging populations are putting national healthcare systems under pressure due to rising hospitalization costs. On the other hand, the current pandemic has induced several countries to promote home-care programs that allow the elderly and patients with chronic conditions to be monitored remotely and are user and environmentally “friendly”, thus making periodic checks more pleasant and timely and providing clinical interventions only when their medical conditions require. For this purpose, in recent years, large investments have been made in both the research and industrial fields to provide innovative wearable devices and technologies able to monitor health conditions [1,2] and/or environmental parameters [3,4,5], resulting in the emergence of an important market segment. In fitness and wellness, the discrete and unobtrusive real-time monitoring of specific physiological, biomedical, and biomechanical parameters of athletes is of great interest in order to improve training compliance and/or competition performance [6]. In the same way, in the medical field, real-time and high-throughput devices for monitoring patients’ physiological parameters are even more desirable [7]. Unfortunately, the continuous monitoring of chronic medical conditions using implantable devices has significant drawbacks, such as the biodegradation of the sensing element and the patient’s immune system response to the host, e.g., changing diffusional barriers [8]. Wearable sensors can monitor physiological parameters in a non-invasive way, thus strongly reducing but not fully avoiding any reactions. They have attracted increasing interest and have been employed in many biomedical applications [9,10]. Moreover, they could potentially monitor both the wearer’s and the environmental parameters, thus acting as an intermediate between the body and surrounding conditions.

The main obstacle to the availability of the full technology, i.e., wearable systems and sensors for continuous health monitoring, is the lack of sensors [11,12]. The research is now focused on this issue since wearable body sensor networks (BSN) are an emerging medical sensor technology for achieving the unobtrusive monitoring of vital parameters in real time.

To minimize system failures when using such sensors, the sensing head must be in contact with the skin surface, and a fixing material (such as an elastic bandage or a tightly fitted undergarment) is used to keep it in place at the skin surface to avoid misplacement and background noise. Unfortunately, the use of a fixing material at the skin surface can cause measurement errors [13]. Another approach for in vivo and remote monitoring is the use of wireless micro-devices for signal transmission [14,15]. They are non-degradable and the electronics must be coated with biocompatible materials. Non-degradable micro-devices must be removed after the monitoring period and are not recommended for the post-operation control of physiological parameters, but rather are more indicated in chronic or long-term disease monitoring. Wearable electronics must be miniaturized, low-powered, and made of biocompatible materials in order to minimize the impact on the daily activities of the wearer. Through the integration of novel technologies, flexible support, such as textiles, can be equipped with information and power transmission capabilities and sensing functions as an infrastructure for embedded wearable microsystems [16,17,18].

Of paramount importance is the monitoring of the elderly and people with chronic conditions participating in “aging in place” programs. Wearable sensors have been used to monitor the recovery of patients after abdominal surgery [19]. The first step to achieving continuous monitoring in everyday life is correctly identifying the activities of daily living (ADL), i.e., walking, sitting, standing, etc. Many approaches have been followed mainly using accelerometers [20], for example, for step counting in patients with Parkinson’s disease [21]. An “in-shoe” pressure and acceleration sensor system [22] was developed to classify an arm movement coupled to the above-mentioned activities.

Several research projects have suggested that the continuous monitoring of ADL activities increases exercise compliance in populations at risk. As an example, the continuous monitoring of physical activities in obese individuals stimulates an active lifestyle, thus reducing clinical interventions [23,24].

Another use of the long-term monitoring of physiological data is for the diagnosis and treatment of cardiovascular diseases. Typical commercial technologies include the long-term monitoring of heart rate, oxygen saturation, blood pressure, body temperature, respiratory rate, and galvanic skin response, but some of these technologies are quite unpleasant. For this reason, clinical studies are currently validating wearable sensor platforms with the aim of improving the clinical management of patients with congestive heart failure [25]. For instance, the MIT Media Laboratory developed LiveNet, a system that measures 3D acceleration, electrocardiogram (ECG), electromyogram (EMG), and galvanic skin conductance for monitoring Parkinson’s symptoms and detecting epileptic seizures [26]. A custom data logger, “LifeGuard”, was developed to monitor the health status of individuals in extreme environments (space and terrestrial) [27].

The system has already been validated in hostile environments with good results. A wearable system to monitor brain activity using a non-invasive approach with fNIRS (functional near-infrared spectroscopy) has been recently developed [28,29] and patented [30] under the EU commission-funded project ASTONISH. The project AMON, again funded by the EU, developed a wrist-worn device capable of monitoring ECG, blood pressure, blood oxygen saturation, and skin temperature to monitor patients with cardio-respiratory problems [31]. Other projects worth mentioning that received grants from the European Commission are [32] MyHeart, WEALTHY, and MagIC. These projects were based on the idea of developing garments with wearable sensors for the health monitoring of people in home and community settings.

Wearable sensing systems can be broadly classified into two main categories: electronic devices fixed in various ways onto the fabric and chemical sensors fully integrated into the textile itself. The second category, certainly challenging but intriguing, allows for the production of a “smart” textile thanks to the full integration of the sensing system. The simplest approach is fabricating colorimetric sensors that cause a detectable color change in the cloth. This is achieved using halochromic dyes and they have been used for employees working with chemicals; the color change immediately alerts the workers of a potentially harmful chemical leak without requiring power sources [33,34,35,36]. Their main drawbacks are the relatively low stability and pH sensitivity, which are quite important at high gas concentrations. To overcome the last limit, most textile sensors have been fabricated with a large surface area, i.e., through electrospinning, thus improving the pH response [37,38,39]. For example, Agarwal et al. [40] developed universal pH sensing nanofibrous sensors using various halochromic dyes with Nylon 6; Pakolpakçıl et al. [41] used natural halochromic dye with sodium alginate and a polyvinyl alcohol mixture to formulate pH-indicating nanofibrous sensors; Guinovart et al. fabricated an electropolymerized polyaniline (PANi)-conducting polymer for the production of a bandage-based wearable potentiometric sensor for monitoring wounds’ pH [42]; Kassal et al. developed a wireless RFID-based smart bandage for the optical determination of pH by embedding covalently modified cellulose particles with a pH indicator dye in a biocompatible hydrogel [43]; Getmeyer et al. [44] detected gaseous NH₃ and HCl with nanofibrous sensors fabricated using electrospinning and sol–gel methods; and Jeevarathinam et al. [45] and Suleymanov et al. [46] used an aggregation-induced emission of dyestuff.

To commercialize smart textiles, the main areas that need to be addressed are service life, washability, productivity, and production costs while maintaining a high detection performance with respect to liquid or gaseous alkalis and acids.

2. Flexible Electronics