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BIT Has Made New Progress in Key Technologies of Optical 3D Microscopic Imaging Probes

  Recently, Prof. Xie Huikai from School of Information and Electronics, Lecturer Li Jianhua from School of Mechatronical Engineering as well as the doctor student Zhou Liang, the visiting scholar Yu Xiaomin and Prof. Philip Feng have developed MEMS (Micro-electro-mechanicalsystem) lens with high filling rate, high optical quality and large zoom range. It is a key device for making two-photon microscopic imaging miniature optical probe, which can be used to realize head-mounted real-time brain imaging of free moving small animals, and better study the relationship between brain neuron activity and animal behavior. The study result is published in Optics Express, a top journal in the field of Optics, with the title of A MEMS Lens Scanner Based on Serpentine Electrothermal Bimorph Actuators for Large Axial Tuning (Impact Factors: 3.669).

  
Fig. 1. (a) Freely movable small animals in optical imaging, with a miniature fiber optic probe fixed on its head
(b) 3D scanning light path inside the miniature probe

  

  Two-photon microscopy (TPM) has high resolution and large imaging depth, and has become a prominent figure in the field of brain imaging in vivo. Many research groups have used TPM achieve long-term observation of neuronal activity in the brain of mice, which provides an important way to explore and study the brain. But at present, because the TPM probe is too heavy, the mice and TPM probe can only be fixed relatively. Therefore, in order to truly study the mice in free movement, this study proposes a kind of head-mounted optical fiber TPM miniature probe as shown in Fig. 1A. The TPM probe must be able to perform 3D scanning due to the brain neurons are three-dimensionally interlaced. As shown in Fig. 1B, the x/y two-dimensional scanning can be achieved by MEMS scanning micro-mirrors, but the depth scanning in the z-direction usually requires a motor to make completion, and the volume and weight are still difficult for mice to carry. Although it has been proposed to use MEMS zoom lens to reduce the volume and weight, the optical quality of the lens and the required focusing displacement cannot be solved. Therefore, a novel MEMS actuator unit (Fig. 2A) is proposed to drive the lens with high optical quality directly to realize the MEMS zoom lens with large optical aperture, large stroke and low driving voltage (Fig. 2C).

  
Fig. 2. (A) Electrothermal MEMS Actuator SEM
(B) MEMS-driven micro-platform chip
(C) MEMS micro lens photo
(D) USAF resolution card laminated micrographs of MEMS micro-lens under different voltages

  

  The entire MEMS-driven micro-platform chip (Figure 2B) is circular (4.4 mm in diameter) to maximize the optical aperture (1.8 mm in diameter) with a given probe diameter. It is composed of 16 groups of the same MEMS actuators in parallel to enhance its load capacity, so it can directly drive the precision-machined high optical quality lens. In order to increase the displacement, the MEMS actuator (Fig. 2A) is cascaded by three-stage bi-material layer electrothermal driving structure in series, and the maximum displacement can reach 200μm at 10V driving voltage. Fig. 2C shows the assembled MEMS micro-lens with a mass of only 8mg. Figures 2D-F are in-depth scanning micrographs of the USAF resolution card stack under 0V, 5V, and 8V driving voltages obtained through this MEMS micro-lens, and the corresponding lens displacements are 0μm, 77μm, and 151μm, which shows that G7E6 resolution units (2.2μm line width) at different depths are clearly distinguishable by using voltage-controlled MEMS micro-lens. Next, the MEMS lens will be integrated into the two-photon probe, so that the whole TPM probe weighs less than 2g, and then be connected to the light source and detection system (Fig. 1A) through a thin flexible wire containing optical fiber and copper wire, so as to conduct real-time 3D imaging of brain neurons in free-moving mice to truly establish a reliable model between neuron activity and animal behavior, and help explore the brain and carry out neurological diseases treatment. Besides, The MEMS lens can also be widely used in optical microscopic endoscopic images.

  Paper link: https://doi.org/10.1364/OE.400363