Monolithic joints and linkages for low-cost microscopy

Abstract:
Standard microscopes designed for the direct view through oculars have a rather large footprint due to standardised distance between objective and ocular. Most parts in microscopes are made from cast aluminium and brass for reasons of mechanical strength and vibration damping. In the future, microscopes could be designed more compact if solely digital imaging is used as the optical path could be shorter and less optical elements would be needed. One crucial mechanical part of a microscope is the mechanism to adjust the relative position of objective and probe. This work focusses on the deployment of linkages with monolithic joints to provide high-precision linear motion. As a demonstrator, the prototype of a fully automatic compact light sheet microscope is presented. The Λ-mechanism by Chebyshev has been selected as it allows a complete straight part of the coupler curve. By arranging two such mechanisms in parallel and two of such arrangements in two parallel planes, a three-dimensional linear guide with a vertical axis of motion is created. To allow easy manufacturing and precise motion, monolithic joints with low friction and backlash are used. Using the described mechanism, a prototype of a light-sheet microscope was built using fused-deposition modelling 3Dprinting (FDM) of ABS plastics. The device consists of three subgroups: the probe conveyer, the laser adjustment unit, and the actual microscope. The probe conveyor holds 26 specimens in transparent tubes and positions each tube under the lens. This is then moved vertically by the described monolithic mechanism by a stepper motor. Image acquisition is done using a Raspberry V2.1 camera (8 megapixels) with an additional 2.1 mm lens forming a 4f-optics arrangement and a Raspberry Pi3 Model-B computer. The latter is also used to control the mechanics of the microscope. For validation a pappus of dandelion seed was assessed. It was possible to easily visualize the geometrical structure in different horizontal planes. Structures down to ~1.2 µm could be differentiated. It became clear that the vertical stiffness of the monolithic mechanism is sufficient as is the alignment of the axes. The tolerances of the FDM printing process lead to only a very small lateral and angle displacement. However, lateral stiffness of the mechanism is not yet sufficient. Future development will provide higher optical resolution by deploying higher quality lenses and interchangeable fluorescence filters for greater flexibility.