**Abstract**

Recent technological advances of soft functional materials and their assembly into wearable (i.e., on-skin) biosensors lead to the development of ground-breaking biomedical applications ranging from wearable health monitoring to drug delivery and to human-robot interactions. These wearable biosensors are capable of unobtrusively interfacing with the human skin and enabling long-term reliable monitoring of clinically useful biosignals associated with health and other conditions affecting well-being. Scalable assembly of diverse wearable biosensors has been realized through the elaborate combination of intrinsically stretchable materials including organic polymers or/and low-dimensional inorganic nanomaterials. In this Chapter, we review various types of wearable biosensors within the context of human health monitoring with a focus of their constituent materials, mechanics designs, and large-scale assembly strategies. In addition, we discuss the current challenges and potential future research directions at the end of this chapter.

**Keywords:** advanced functional materials, wearable biosensors, health monitoring systems, stretchable electronics, sensor technology

## **1. Introduction**

Wearable sensor technology has evolved from traditionally fundamental measurement technology across science, engineering, and industry, and is now increasingly significant as a core method to advance human healthcare. To meet the requirements allowing for preventative health monitoring, diagnosis, and treatment, various types of wearable biosensors have been developed to capture the physical and electrophysiological biosignals (e.g., temperature, heart rate, respiration rate, electrodermal activity, and body motion) or biochemical responses (e.g., biomarkers in biofluids). Many of these wearable biosensors are required to remain in contact with the human skin for a prolonged period throughout the continuous monitoring of the biosignals. In this field, the largest continuing challenge is that rigid or semi-flexible forms of biosensors, particularly when integrated with wireless communication unit, are not compatible with the soft and irregular skin surface [1]. This mechanical mismatch results in discomfort to users as well as considerable noise signals during data collection. Recent advances in soft functional materials and assembly techniques have led to the development of mechanically stretchable and flexible biosensors that can be unobtrusively integrated into the human skin in

a manner that complies with the natural motion of the wearer [2, 3]. The thin and flexible nature of these biosensors allows their conformal, seamless contact to the skin while simultaneously providing (i) excellent breathability and deformability for user comfort and (ii) durability to allow repeated attachment and detachment to the skin without irritating the wearer and damaging the devices. These aspects play a critical role in achieving high-fidelity recording of biosignals during long-term use in many clinical applications [4, 5].

According to the report by Grand View Research, Inc, the global market for wearable (i.e., on-skin) biosensors is anticipated to reach USD 2.86 billion by 2025 [6] at a phenomenal Compound annual growth rate (CAGR) of 38.8% during the forecast period (**Figure 1**) [6]. The wearable biosensors are a key component of electronic medical platform systems used by consumers as interest in real-time motion detection activity tracking grows. Furthermore, the wearable biosensors are emerging as a promising revolution that captures clinically important parameters form a distance and thereby reduces the patient's overall hospital cost. The manufacturers have incorporated contextual information and data to determine motion detection activities. Additionally, this analysis provides users with results that can be used to define their health and fitness goals [6]. This rapid interest over this market is likely to drive industry growth over the forecast period into advanced stages. The market of wearable biosensors in the field of health and fitness monitoring and diagnosis is gaining more attention due to their potential number of applications.

The wearable biosensors demand the following requirements. First, they require the ability to interface with the human skin with high compatibility, durability, and abrasion-resistant for the recording of biosignals with high-fidelity. Therefore, the conventionally-used semiconducting materials (e.g., silicon) remain impracticable. Second, they require an accurately operatable sensing system ('selectivity') that can differentiate between various environmental stimuli including mechanics, temperature, humidity, and various mechanical components such as atmospheric pressure, lateral deformation, shear, flexion, torsion, and vibration. Lastly, they require an affordable 'sensitivity' to detect the tiny biological signals through an appropriate amplification.

In this Book chapter, we overviewed various types of wearable biosensors tailored for the monitoring of mechanical, optical, and biochemical responses for human healthcare. We categorized the wearable biosensors according to their formfactors. In Section 3, we overviewed the functional materials used for these

**Figure 1.**

*A schematic graph that depicts continuous growth of wearable sensor market size. Adapted from Ref. [6].*

*Advanced Materials and Assembly Strategies for Wearable Biosensors: A Review DOI: http://dx.doi.org/10.5772/intechopen.94451*

biosensors, such as carbon-based nanomaterials and inorganic nanostructured materials, that provide tailored mechanical, electrical, or/and electrochemical properties. We also described the basic sensing mechanism (e.g. piezoresistive, piezocapacitive, iontronic, and piezoelectric sensing) of the wearable biosensors. In addition, we discussed organic field-effect transistor type of wearable biosensors, which can amplify small biological signals into large-signal information. In Section 4, we described various transduction systems according to their sensing mechanisms (e.g., electromechanical, optoelectrical and chemical sensing). In Section 5, we reviewed recently-reported assembly strategies to construct the wearable biosensors in a cost-effective manner. In Section 6, we discussed about future opportunity to further facilitate the commercialization of these wearable biosensors at a wider scale.
