Hydrostatic pressure provides an efficient way to tune and optimize the properties of solid materials without changing their composition. In this work, we investigate the electronic, optical, and mechanical properties of antiperovskite X₃NP (X²⁺ = Ca, Mg) upon compression by first-principles calculations. Our results reveal that the system is anisotropic, and the lattice constant a of X₃NP exhibits the fastest rate of decrease upon compression among the three directions, which is different from the typical Pnma phase of halide and chalcogenide perovskites. Meanwhile, Ca₃NP has higher compressibility than Mg₃NP due to its small bulk modulus. The electronic and optical properties of Mg₃NP show small fluctuations upon compression, but those of Ca₃NP are more sensitive to pressure due to its higher compressibility and lower unoccupied 3d orbital energy. For example, the band gap, lattice dielectric constant, and exciton binding energy of Ca₃NP decrease rapidly as the pressure increases. In addition, the increase in pressure significantly improves the optical absorption and theoretical conversion efficiency of Ca₃NP. Finally, the mechanical properties of X₃NP are also increased upon compression due to the reduction in bond length, while inducing a brittle-to-ductile transition. Our research provides theoretical guidance and insights for future experimental tuning of the physical properties of antiperovskite semiconductors by pressure.

Microrobots-assisted drug delivery and surgery have been always in the spotlight and are highly anticipated to solve the challenges of cancer in situ treatment. These versatile small biomedical robots are expected to realize direct access to the tumor or disease site for precise treatment, which requires real-time and high-resolution in vivo tracking as feedback for the microrobots’ actuation and control. Among current biomedical imaging methods, photoacoustic imaging (PAI) is presenting its outstanding performances in the tracking of microrobots in the human body derived from its great advantages of excellent imaging resolution and contrast in deep tissue. In this review, we summarize the PAI techniques, imaging systems, and their biomedical applications in microrobots tracking in vitro and in vivo. From a robotic tracking perspective, we also provide some insight into the future of PAI technology in clinical applications.

Distinguishing early-stage tumors from normal tissues is of great importance in cancer diagnosis. We report fiber-based confocal visible/near-infrared (NIR) optical-resolution photoacoustic microscopy that can image tumor microvasculature, oxygen saturation, and nanoprobes in a single scanning. We develop a cost-efficient single laser source that provides 532, 558, and 1064 nm pulsed light with sub-microseconds wavelength switching time. Via dual-fiber illumination, we can focus the three beams to the same point. The optical and acoustic foci are confocally aligned to optimize the sensitivity. The visible and NIR wavelengths enable simultaneous tumor imaging with three different contrast modes. Results show obvious angiogenesis, significantly elevated oxygen saturation, and accumulated nanoparticles in the tumor regions, which offer comprehensive information to detect the tumor. This approach also allows us to identify feeding and draining vessels of the tumor and thus to determine local oxygen extraction fraction. In the tumor region, the oxygen extraction fraction significantly decreases along with tumor growth, which can also assist in tumor detection and staging. Fiber-based confocal visible/NIR photoacoustic microscopy offers a new tool for early detection of cancer.