# Healable, Flexible Supercapacitors Based on Shape Memory Polymers
## 1 Introduction  
Self-healable polymers, as one of the most important bio-mimetic materials, have been widely exploited for several decades. The shape memory-assisted self-healing (SMASH) approach proposed in 2009 enabled externally visible cracks to be healed, prolonging the life of polymeric materials [1]. For the traditional self-healing polymers, mechanisms include self-healing effects were achieved via different approaches such as micro-capsules and Diels-Alder reactions. Compared to the traditional self-healing polymers, the SMASH acts as a novel mechanism, including healing reagents and shape recovery to make the cracks interface close up and become more adhesive. Due to the excellent performance, SMASH has been widely reported [2; 3; 4; 5].  
With the development of wearable and portable electronics, flexibility has played an important role in energy storage devices [6; 7; 8]. Compared to batteries and conventional capacitors, supercapacitors (SCs) as novel and wearable storage devices can make up for the shortcomings of traditional devices and provide a higher power density than that of batteries and a higher energy density than that of conventional capacitors. Thus, flexible SCs have become important energy storage devices. Furthermore, with the demand for increasing storage, lots of different flexible SCs have been invented. However, most SCs are vulnerable to external physical damage and could not be repaired. Therefore,the remarkable reliability of epidermal electronics and intelligent textiles is required in case of mechanical damage. Smart energy-storage devices such as self-healable supercapacitors [9; 10; 11; 12; 13], having excellent healable electrochemical performance, are believed to well meet the requirement. However, most of the self-healing was achieved by hydrogen bonding [9; 10; 11], magnetism [12], or coordination [13] and some of these are the reversible interaction processes.  
In comparison, the shape memory-assisted self-healable polymers could restore larger cracks and physical damage. Furthermore, there is a simple method to fabricate a self-healable composite. Inspired by these facts, herein we fabricated a self-healable, flexible and conductive composite based on shape memory polymers. According to our previous reports on the nano carbon-containing shape memory polymer composite [14; 15; 16], the functional flexible electronics exhibiting electro-trigger shape memory behaviors and temperature/water stimuli sensing peculiarity were explored. The SMP is quite attractive as a mechanical support and current collector because of its superior properties such as softness and repeated deformations [1]. Additionally, carbon nanotubes with large specific surface area and special hollow structure are loaded with MnO\({}_{2}\), which acts as an excellent active material in supercapacitors [17; 18; 19; 20]. The prepared MnO\({}_{2}\)/CNT/PCL/SMPU composite is denoted as MCPxSy, where x and y are the mass percent of PCL and SMPU, respectively. In our work, we found that the MCP40560 showed the best flexibility and good healing efficiency. Electrochemical measurements demonstrated that the MCP40560 composite exhibited a good capacitance of 27.33 mF cm\({}^{-1}\) and good cycle performance (only 4.08% capacitance loss after 1500 cycles). Importantly, the MCP40560 retained 93% capacitance after the composite was repaired. The composite not only had excellent capacitance but also was a flexible and healable smart material.