Tough, transparent, biocompatible and stretchable thermoplastic copolymer with high stability and processability for soft electronics

Abstract

Materials with a variety of required properties, including stretchability, toughness, optical transparency, biocompatibility, processability, and recyclability are in great demand for soft electronics. Creation of such materials, however, has been hampered due to the challenges of balancing these properties. Herein, we report intrinsically stretchable thermoplastic copolymers with a random sequence of hard and soft domains in the polyimide backbone in which their superior traits are harnessed to enable the properties of the copolymer tunable and balanced. We obtained a tough and stretchable copolymer with high optical transparency, biocompatibility, thermal and chemical stability, and low water vapor transmission rate. In addition, the polymer is recyclable and shows excellent processability, allowing for the fabrication of fibers, thin films, and molded parts for soft electronics. Furthermore, the utility of the copolymer was successfully demonstrated for a wearable temperature sensor on the stretchable copolymer and a copolymer-based fully stretchable sweat collection patch, suggesting that they have great potential in soft electronics.

Introduction

In the past decade, studies on development of intrinsically stretchable materials for stretchable electronics have been conducted with target applications associated with health monitoring devices, electronic skins, soft robotics, human–machine interfaces, and implantable devices [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. In line with great demand for new materials appropriate for stretchable electronics, intrinsically stretchable materials ranging from conductors to semiconductors to dielectrics have been developed to fabricate a myriad of stretchable devices. Additionally, they are rapidly merging with biomedicine with the increased prevalence of soft bio-integrated electronics [19], [20], [21].

A key feature of stretchable electronics is their mechanical stretchability, which enables skin conformality to enhance user comfort while maintaining its original functionality and performance level even when a stretchable device is subjected to a large mechanical deformation [22], [23], [24]. Other desirable attributes required for soft electronics include mechanical toughness, optical transparency, thermal stability, chemical stability, low water absorption rate, and good processability [25], [26], [27]. Since high toughness of intrinsically stretchable materials is difficult to be reconcilable with high stretchability or low stiffness, there have been some investigations to circumvent the trade-off between stiffness and toughness [28], [29], [30], [31]. Another important aspect of soft electronics is its biocompatibility allowing seamless integration of stretchable materials into applications for human health monitoring [32]. The lack of biocompatibility in a skin-mountable sensor, for example, can result in complications such as skin irritation, skin sensitization, or even chronic skin inflammation. For future soft electronics, the environmental friendliness of materials is also another important consideration [33]. In this respect, the recyclability of stretchable materials is of great interest. However, achieving all the key features in one system is challenging because materials with desirable mechanical properties are at odds with other essential features. To achieve all the key properties, it is highly desirable to develop an intrinsically stretchable and tough material balanced with other physical and chemical properties.

Intrinsically stretchable polymeric materials that have well-balanced properties for use as substrates, encapsulants, or functional layers are still limited. Commercially available, intrinsically stretchable materials, including silicone elastomers such as polydimethylsiloxane (PDMS) and Ecoflex® with a very low elastic modulus, have been widely used as stretchable materials for soft electronics [24], [34], [35]. However, these highly stretchable elastomers suffer from limited formability [36], [37] and are not tough enough to endure the device fabrication processes. Other organic materials for stretchable electronics include stretchable hydrogels [38]. Stretchable hydrogels are also very soft due to the high content of water inside the polymer networks but suffer from poor mechanical toughness and instability under ambient conditions. Another class of commercial intrinsically stretchable materials is the elastomeric copolymers with tunability in their properties, such as polyurethane (PU), poly(styrene–butadiene–styrene) (SBS), and polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS). These copolymers have high stretchability, low elastic modulus, and good processability [39], [40], [41]. Commercial tough thermoplastic PU and SEBS were reported to have very high toughness [40], [41]. However, there are still a few limitations associated with their use in wearable soft electronics due to difficulty in obtaining high decomposition temperature (T_d) (above 200 °C), chemical and thermal stability, and long-term prevention of moisture penetration [39].

As an approach to overcome the limits of those elastomeric copolymer materials, we propose a stretchable thermoplastic copolymer with a backbone of polyimide (PI), which can be a suitable candidate with balanced properties for soft electronics. Exceptional chemical, mechanical and thermal stability, good biocompatibility, and high barrier properties of PIs [42], [43], [44] can provide a great advantage in developing targeted materials. A few previous works have shown that flexibility of the copolymers with PI backbone could be increased by incorporating a soft segment like polydimethylsiloxane (PDMS) in the PI backbone [45], [46], [47], [48]. However, their applications in soft electronics have so far been limited because of their poor processability in common organic solvents [44], high glass transition (T_g) or melting temperatures (which precludes molding and melt processing) [44], deep coloration [43], and limited stretchability [43].

Copolymers are broadly classified into block and random copolymers. Block copolymers require a complicated multi-step synthesis procedure, which is often tedious and time-consuming, thus limiting their scalability. On the other hand, random copolymers allow facile one-pot synthetic preparation making them promising candidates for large-scale applications [49], [50]. Here, we have designed and synthesized a stretchable and tough thermoplastic copolymer from hard and soft blocks which are randomly arranged within the PI backbone. This random copolymer offers synergetic advantages of the processability and stretchability of siloxane and the high toughness, thermal stability, and chemical resistance of PI. By controlling the ratio of the soft and hard blocks, stretchability and mechanical toughness could be compromised, which makes it possible to overcome low toughness of PDMS and other elastomeric copolymers while retaining thermal stability with high T_d and chemical resistance. A stretchable copolymer with a maximum elongation of ∼333% with 15% recoverable stretchability, high fracture toughness of ∼24 kJ·m⁻², an optical transparency of 88%, a high T_d of 396.2 °C, good chemical resistance in acidic and basic solutions, and excellent in vitro biocompatibility was obtained. Furthermore, its solubility in common organic solvents and the thermoplastic nature of the stretchable copolymer provides excellent processability which allows it to be formed into many different shapes through solution processing such as spinning and spin-coating and a replication molding process for facile fabrication of microstructures on its surface. The copolymer could also be recycled successfully. We demonstrated a stretchable temperature sensor on the copolymer substrate and a stretchable sweat collection patch using our copolymer, thus proving its utilization in soft electronics. By alleviating the issues of conventional stretchable polymeric materials, our stretchable thermoplastic copolymer with well-balanced properties has great potential for a variety of applications in future stretchable electronics.