Mechanoluminescence (ML), also called triboluminescence (TL), refers to the phenomenon/process that materials could emit light under mechanical stimuli, e.g., friction, stretch, compression, impact, etc. The ML materials could utilize the ubiquitous mechanical energy in daily life to generate light emissions, avoiding the requirement of an artificial photon- or electron-excitation source as that in photoluminescence (PL) or electroluminescence (EL). Therefore, ML materials show great advantages in energy saving and environmental protection. 

For practical applications, ML crystals or powders are required to composite with bulk matrices to generate structural non-destructive ML. Among the fabricated ML composites, elastomer-based ones have attracted increasing attention owing to the rising requirement of incorporating stress sensing characteristic into flexible/wearable devices. The present ML elastomer composites mainly employ transition metal ion doped sulfides (TM-sulfides) as the luminescent components because of their intense ML intensity. However, the TM-sulfides usually have poor chemical stability and may cause severe environmental pollution as well as lack of rich emission color.

Theoretically, rare earth doped oxides (RE-oxides) are promising alternatives because of their high chemical stability, nontoxicity, and abundant energy levels. It is essential to develop efficient and ideally multicolored ML of RE-oxide based elastomer composites, so that flexible devices may possess remarkable and environmentally friendly mechanical responsive optical characteristics. 

The present applications of ML elastomer composites are mainly focused on the mechanical strength-dependent luminescence or the forms of mechanical behaviors (such as wind, magnetostriction, and electrostriction), while the intrinsic characteristic of ML, that is, the ML can only be generated under a dynamic stress state (such as dynamic stretching), is commonly ignored for developing possible revolutionary applications.

In the latest work, Zhaofeng Wang and his co-workers from Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS), Lanzhou University (LZU), and University of Connecticut (UConn) reported an environmentally-friendly and novel oxide-based ML material, Sr₃Al₂O₆: Eu³⁺ (SAOE), serving as the alternative for the widely-used but environmentally-hazardous transition metal-doped sulfides.

The results suggested that SAOE could simultaneously emit bright PL and ML as embedded in a flexible polydimethylsiloxane (PDMS) matrix. The emitting color can be readily manipulated by controlling the reduction degree of Eu³⁺. Based on the wavelength selectivity of photoluminescence and dynamic stress response of ML of the SAOE/PDMS elastomer, two types of intriguing flexible devices were further designed and fabricated, i.e., a photon/mechanics activated dual-responsive anticounterfeiting device, and a comprehensive stretching/strain sensor to sense both strain level and stretching states.

Because both the anti-counterfeiting device and the stretching/strain sensor were designed by utilizing the unique dynamic stress response characteristic of ML materials, the as-fabricated devices represent the state of the art in their own fields. Compared to the conventional anticounterfeiting device made by PL materials responding to only one stimulus, this intriguing dual-responsive anticounterfeiting device activated by both UV radiation and mechanical strain is expected to further elevate the security level of anti-counterfeiting technology. Furthermore, the multi-mode stretching/strain sensor capable of sensing both strain level and stretching states breaks the limit of static strain sensing in present researches.

We also discussed the underlying ML mechanisms for SAOE/PDMS elastomer and confirmed that the release of the trapped carriers in SAOE should be intrinsically responsible for the generated ML. The as-fabricated stretching/strain sensor based on ML are promising for applications in mechanical failure monitoring, intelligent artificial skin, and other related fields.

These findings are described in the article entitled Efficient Mechanoluminescent Elastomers for Dual-Responsive Anticounterfeiting Device and Stretching/Strain Sensor with Multi-Mode Sensibility, recently published in the journal Advanced Functional Materials. The corresponding authors are Prof. Zhaofeng Wang at LICP, Prof. Jiachi Zhang at LZU, and Prof. Luyi Sun at UConn. Chen Wu at LICP and Songshan Zeng at UConn are the co-first authors of this work. 

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