Soft growing robotics, inspired by plant growth, offer promising solutions for navigating unstructured environments. Existing methods typically rely on external stimuli or energy input, limiting autonomy. Recently, a self-growing liquid crystal elastomer (LCE) was discovered, capable of extending beyond its original length at room temperature without external input. However, this self-growth was limited to freshly prepared samples, necessitating synthesis from monomers for each use – a major obstacle to practical applications. This research addresses this limitation by developing a rejuvenation strategy to restore non-fresh LCE samples to their self-growing initial state. The study's importance lies in paving the way for fully autonomous soft robots, eliminating the need for external energy sources or control systems, and improving the reusability and sustainability of these materials. The potential applications range from exploration in energy-constrained environments to adaptable and reusable soft actuators.

The literature highlights challenges in creating synthetic materials capable of autonomous growth. While shape-shifting polymers offer potential as soft actuators and structures in growing robots, achieving spontaneous growth without external stimuli remains a significant challenge. Prior work demonstrated a self-growing LCE system, but its practical application was hindered by the need to synthesize new samples for each use. The current work builds upon this prior research by focusing on rejuvenating the LCE, addressing a critical limitation.

The researchers synthesized liquid crystal elastomers (LCEs) using a base-catalyzed thiol-Michael addition reaction, employing specific monomers (RM257), chain extender (EDDET), and crosslinker (PETMP). The resulting LCEs contained ester bonds, enabling transesterification. 'Non-fresh' LCE samples (annealed (A-LCE) and grown (G-LCE) samples) were created through annealing and self-growth of the freshly prepared LCEs (F-LCE). Rejuvenation was achieved by swelling the non-fresh LCEs in a dichloromethane (DCM) solution containing a transesterification catalyst (neutralized 1,5,7-triazabicyclo[4.4.0]dec-5-ene, nTBD). The effectiveness of the rejuvenation process was evaluated using various characterization techniques: Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), stress-strain curves, polarizing optical microscopy (POM), X-ray diffraction (XRD), and shear stress relaxation tests. The regrowth and actuation capabilities of the rejuvenated LCEs (R-LCEs) were also assessed. The influence of swelling time, catalyst concentration, and catalyst type (nTBD vs. dipropylamine (DPA)) on the rejuvenation process were investigated. Furthermore, selective rejuvenation and its implications for controlling local growth and creating 3D actuators were explored.

The study successfully demonstrated the rejuvenation of both annealed and grown LCEs. Swelling the non-fresh LCEs in a solution containing the nTBD catalyst led to a restoration of their initial state, enabling subsequent self-growth. The FTIR spectra confirmed the presence of nTBD in the rejuvenated LCEs. DSC and TGA analyses showed that the rejuvenated LCEs exhibited thermal properties similar to the freshly prepared samples. Mechanical property testing indicated that the stress-strain curves of A-LCE and annealed R-LCE were also quite similar. The birefringence behavior of R-LCE and F-LCE were also observed to be exactly the same under POM. The regrown R-LCEs spontaneously grew at room temperature without external stimuli, demonstrating self-growth capabilities analogous to freshly prepared samples. Importantly, the rejuvenation process erased the growth history and actuation modes of the grown LCEs, allowing for reprogramming. The rejuvenation process was repeatable multiple times (over 5 cycles), with consistent actuation strain. The authors also investigated the impact of swelling time, catalyst concentration, and catalyst type on rejuvenation efficiency, finding that nTBD was a more effective catalyst than DPA. A comprehensive investigation into the underlying mechanism indicated that the synergistic actions of solvent-induced disruption of π-π interactions and catalyst-mediated topological rearrangements via transesterification were essential for successful rejuvenation. Furthermore, the authors demonstrated controlled local rejuvenation, achieving both partial and patterned growth, resulting in the creation of complex 3D actuators from initially planar structures.

The findings directly address the research question by providing a practical method for rejuvenating non-fresh self-growing LCEs. The successful rejuvenation and regrowth demonstrate the feasibility of creating reusable and reprogrammable soft actuators. The observation that the growth history and actuation modes can be erased and rewritten has significant implications for designing soft robots with adaptive capabilities. The detailed mechanistic investigation clarifies the importance of both physical (solvent effects) and chemical (dynamic covalent bonds) processes in the rejuvenation. The ability to selectively control the growth process opens avenues for creating sophisticated 3D structures with tailored actuation modes. This research advances the field of soft robotics by providing high-performance, sustainable, and readily reusable materials that can drive the development of fully autonomous soft robots.

This research presents a facile and effective strategy for rejuvenating non-fresh self-growing LCEs, enabling their on-demand reuse and reprogramming. The synergistic effects of solvents and dynamic covalent bonds are crucial for this rejuvenation. The rejuvenated LCEs retain their self-growth and actuation capabilities, exhibiting properties comparable to freshly prepared samples. The ability to erase growth history and control local growth opens up possibilities for creating complex, adaptable, and reusable soft actuators. Future research could focus on exploring different LCE chemistries, investigating alternative rejuvenation methods, and further developing applications in advanced soft robotics and other fields.

While the study demonstrates the effectiveness of the rejuvenation strategy, potential limitations include the specific choice of solvent and catalyst. The long-term stability of the rejuvenated LCEs under various environmental conditions remains to be fully investigated. The scalability of the rejuvenation process for large-scale production of LCE-based soft actuators could also be explored in future work. The study focuses on specific LCE materials; the generality of the approach to other self-growing materials is an area for further research.