Lens-free optical tomographic microscope with a large imaging volume on a chip

Abstract

We present a lens-free optical tomographic microscope, which enables imaging a large volume of approximately 15 mm 3 on a chip, with a spatial resolution of <1 μm× < 1 μm× < 3 μm in x, y and z dimensions, respectively. In this lens-free tomography modality, the sample is placed directly on a digital sensor array with, e.g., ≤4 mm distance to its active area. A partially coherent light source placed approximately 70 mm away from the sensor is employed to record lens-free in-line holograms of the sample from different viewing angles. At each illumination angle, multiple subpixel shifted holograms are also recorded, which are digitally processed using a pixel superresolution technique to create a single high-resolution hologram of each angular projection of the object. These superresolved holograms are digitally reconstructed for an angular range of AE50°, which are then back-projected to compute tomograms of the sample. In order to minimize the artifacts due to limited angular range of tilted illumination, a dual-axis tomography scheme is adopted, where the light source is rotated along two orthogonal axes. Tomographic imaging performance is quantified using microbeads of different dimensions, as well as by imaging wild-type Caenorhabditis elegans. Probing a large volume with a decent 3D spatial resolution, this lens-free optical tomography platform on a chip could provide a powerful tool for high-throughput imaging applications in, e.g., cell and developmental biology. L ight microscopy has been an irreplaceable tool in life sciences for several centuries. The quest to resolve smaller features with better resolution and contrast has improved the capabilities of this important tool at the cost of relatively increasing its size and complexity (1). On the other hand, we have experienced the flourishing of emerging technologies such as microfluidic and lab-on-a-chip systems, which offer fast and efficient handling and processing of biological samples within highly miniaturized architectures (2-7). The optical inspection of the specimen, however, is still being performed by conventional light microscopes, which has in general several orders of magnitude size mismatch compared to the scale of the microfluidic systems. As a result, there is a clear need for alternative compact microscopy modalities toward integration with miniaturized lab-on-a-chip platforms (8).The push for new optical microscopy modalities is not solely driven by the need for miniaturization and microfluidic integration. The fact that high resolution is achieved at the cost of significant field-of-view (FOV) reduction is another fundamental limitation of lens-based imaging. The relatively small FOV of conventional light microscopy brings additional challenges for its application to several important problems such as rare cell imaging or optical phenotyping of model organisms (9-15), where high-throughput microscopy is highly desired.In order to provide complementary solutions to these aforementioned needs, several lens-free digital microscopy techniqu...

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