Our work began with a previously reported K⁺ ion exchange experiment (Angew. Chem. Int. Ed. 2019, 58, 4169) on a crystal with a Li⁺ ion channel composed of [18]crown-6 (Li salt; Figures 1a and b). When Li salt crystals are immersed in an aqueous solution, the Li⁺ ions in the ion channels are exchanged for K⁺ ions in the aqueous solution, maintaining a crystalline state. However, present Ca²⁺ ion exchange experiments have confirmed that some [18]crown-6 molecules in the crystal are desorbed during the ion exchange (Li→Ca salt, Figure 1c and d). Furthermore, the lattice volume of the crystals decreases by approximately 32% after desorption. This value is much larger than those of other single-crystal materials and comparable to the contraction rate of the skeletal muscles of the body. This discovery has led to the evolution of the original inspiration for mimicking ion channels in living organisms and the development of functional materials that reproduce the movements of living muscles.

Further development of this research was triggered by the discovery made by a question from the first author, Dr. Manabe. In the presence of Ca²⁺ ions, Li salt undergoes a Ca²⁺ ion exchange reaction involving the desorption of [18]crown-6 from the crystal, resulting in volume contraction. He then wondered what would happen to the crystals in an environment where [18]crown-6 was present. Indeed, ion-exchange experiments in aqueous solutions containing equal amounts of [18]crown-6 and Ca²⁺ have shown that the ion-exchange reaction is suppressed. This implies that the shrinkage of the crystals can be artificially controlled. This discovery inspired us to use the muscle-like stretching and contraction exhibited by Li salt as a chemical logic gate that can be controlled by external chemical species. Muscles contract when Ca²⁺ ions and ATP are present and relax when they are absent. From a computer science perspective, muscles function as biological logic gates (Figure 2a). Logic gates in materials chemistry have been constructed in the past; however, most studies have focused on microscopic- and molecular-level phenomena because of their potential to record information in molecules. However, muscle stretching and contraction are macroscopic phenomena involving interlocking cells, which contrast with logic gates at the molecular level. Single crystals are revolutionary materials that can undergo macroscopic structural changes.

Our Li salt can switch between the contracting and relaxing states depending on the environment of the aqueous solution (Figure 2b). In the presence of Ca²⁺ ions, the Ca²⁺ ion exchange reaction proceeds with the desorption of [18]crown-6 from the crystal, resulting in volume contraction. In environments containing excess [18]crown-6, the ion-exchange reaction is inhibited; this reaction is described as an INHIBIT gate. Furthermore, this ion-exchange reaction is reversible and returns the original structure of Li salt (relaxed state) in the presence of [18]crown-6 and Li⁺. Of course, this reaction cannot proceed with either [18]crown-6 or Li⁺ alone; therefore, it acts as an AND gate. These reactions also depend on whether the crystals immersed in the aqueous solution are in a contracted or relaxed state. Therefore, the sequence of reactions is integrated as an RS flip-flop circuit, with the outputs of the INHIBIT and AND gates fed into the inputs. These reversible changes are analogous to the repeated contraction and relaxation behaviours of muscles, and their logic gates are similar. In addition, the crystal-derived magnetism and conductivity also change significantly with structural changes; therefore, the above reactions serve as logic gates that can control the macroscopic structure and physical properties. This logic gate can also control the transformation of physical properties corresponding to reversible structural changes, such as those in muscles, and is expected to have more applications in more advanced material development beyond living organisms.