  The outbreak of COVID-19 exposed the inadequacy of our technical tools for
home health surveillance, and recent studies have shown the potential of
smartphones as a universal optical microscopic imaging platform for such
applications. However, most of them use laboratory-grade optomechanical
components and transmitted illuminations to ensure focus tuning capability and
imaging quality, which keeps the cost of the equipment high. Here we propose an
ultra-low-cost solution for smartphone microscopy. To realize focus tunability,
we designed a seesaw-like structure capable of converting large displacements
on one side into small displacements on the other (reduced to ~9.1%), which
leverages the intrinsic flexibility of 3D printing materials. We achieved a
focus-tuning accuracy of ~5 micron, which is 40 times higher than the machining
accuracy of the 3D-printed lens holder itself. For microscopic imaging, we use
an off-the-shelf smartphone camera lens as the objective and the built-in
flashlight as the illumination. To compensate for the resulting image quality
degradation, we developed a learning-based image enhancement method. We use the
CycleGAN architecture to establish the mapping from smartphone microscope
images to benchtop microscope images without pairing. We verified the imaging
performance on different biomedical samples. Except for the smartphone, we kept
the full costs of the device under 4 USD. We think these efforts to lower the
costs of smartphone microscopes will benefit their applications in various
scenarios, such as point-of-care testing, on-site diagnosis, and home health
surveillance.
