Powerful Plastic Microscope Brings Diagnostic Care to Rural Areas

A low-cost, miniature achromatic microscope has been fabricated for the identification of lymphocytes, monocytes, and granulocytes in samples of whole blood stained with acridine orange.

The white blood cell (WBC) differential is one component of a complete blood count (CBC), an evaluation performed for standard blood work-ups. The WBC differential count is essential at the point-of-care when screening for quantitative abnormalities in otherwise morphologically normal white blood cell populations, a condition which may occur with certain infectious diseases.

Bioengineers at Rice University (Houston, TX, USA) developed an achromatic miniature microscope, optimized for the peak emission maxima of acridine orange bound to DNA and ribonucleic acid (RNA) at 525 nm and 650 nm, respectively. It was specifically designed for the observation and classification of white blood cells in undiluted whole blood samples stained with acridine orange. By optimizing a microscope for these emission peaks, the scientists were then able to quantify the white blood cells in a sample consisting only of 20 µL of dye, 20 µL of whole blood, and a glass slide with a coverslip. The custom microscope objective was fabricated out of plastic via single point diamond turning for rapid prototyping purposes. Single point diamond turning produces plastic lenses with optical quality surface roughness.

Plastic substrates for the custom fabricated lenses were carefully selected to minimize axial chromatic aberration. The plastic objective was integrated into all-plastic, 3D-printed housing. Once the objective was initially adjusted to the proper working distance using the 3D-printed optomechanics, the system required no further manual adjustments to refocus the optics between different samples. As the sample consists of fluorescently stained undiluted blood, only one field of view was necessary to capture statistically significant information of more than 100 WBCs/field of view regarding quantities of various white blood cell types. The digital microscope classifies WBC types (lymphocytes, monocytes, granulocytes) based on the ratio of red to green intensity within each cell, rather than morphology.

The prototype microscope, which also includes an LED light source, power supply, control unit, optical system, and image sensor, cost less than USD 3,000 to construct. At production levels upwards of 10,000 units, the scientists estimate that this price would fall to around USD 600 for each unit, with a per-test cost of a few cents. Tomasz Tkaczyk, PhD, associate professor and senior author of the study, said, “One of the driving aspects of the project is the cost of the sample or sample preparation. Many systems which work for point-of-care applications use quite expensive cartridges. The goal of this study is to make it possible for those in impoverished areas to be able to get the testing they need at a manageable price point.” The study was presented on at the Frontiers in Optics 2015 meeting held during October 18–22, 2015, in San Jose (CA, USA). 

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Rice University