A new microscopy technique promises high-quality imaging and analysis of biological samples without the need for lenses and could transform access to healthcare worldwide. Lensless holographic microscopy (LHM) is achievable with small, cheap devices that are well-suited for point-of-care disease diagnosis, microfluidic assessment, and many other important diagnostic processes. That means that these tests can be performed at the point of care in underdeveloped and remote areas, without the need to access advanced laboratory facilities.
Presentation of the lensless holographic microscope. (a) Assembly diagram of a central cross-section of the microscope including the DIHM scheme. (b) Perspective view of the microscope once the top cover is taken off. (c) Global view of the microscope with external dimensions. Image Credit: Picazo-Bueno, J. A. et al.
Making Microscopy More Accessible
Biomedical assessments use optical microscopy more than any other technique. This is because it can image microscopic specimens in real-time and under a non-invasive inspection principle. Modern medical laboratories use relatively bulky and expensive optical compound microscopes to perform medical imaging, whose high prices are often determined by the need for complex and precise lens systems.
Lensless microscopy, by contract, can be achieved with smaller, inexpensive devices combined with (sometimes open source) digital imaging software. LHM offers a miniaturized solution that is ready for widespread adoption.
Lensless microscopes tend to be cost-effective, lightweight, compact, and portable. They could improve diagnosis rates and expected outcomes for people around the world by facilitating point-of-care diagnosis, eliminating the need for repeat visits to a clinic or cold sample storage and transportation from remote areas to distant laboratories.
Cheap and portable microscopy would also enable a higher resolution of global healthcare monitoring. With artificial intelligence now able to analyze and operate vast amounts of health data, a global input network based on accessible devices such as lensless microscopes needs to provide that data to realize the benefits of machine learning in public health.
Holography: Dennis Gabor’s “New Microscopic Principle”
LHM is achieved by implementing Gabor holography with digital means. Dennis Gabor, a Hungarian-British physicist, invented holographic microscopy in 1948 (receiving the Nobel Prize in Physics for this work in 1971.) He proposed a “new microscopic principle”, originally to correct spherical aberrations in electron microscopy.
In the end, electron microscopy did not make use of Gabor’s new holographic method, but it opened up a new field in optics instead. Using Gabor holography, researchers found that they could store phase information optically.
Scientists film (or digitally record) the intensity of a field that emerges at a set distance from the subject. The object can then be reconstructed by applying a light source onto the film (or digitally manipulating the image data.)
The recreated image produced in this technique – a “holograph” – was quickly taken up by public attention, with holograph messages becoming something of a science-fiction trope.
Making Holographic Microscopes
An LHM device is made from a small number of obtainable components in an extremely simple layout. A coherent point (spherical) light source illuminates the sample, and light is scattered onto the sensor area. Then, a digital Gabor hologram is recorded. Imaging is performed by a computer on the hologram model with numerical diffraction equations and digital image processing tools.
Researchers have identified several methods for achieving a spherical point source, for example:
- Using a laser beam and pinhole combination
- An objective lens with an LED
- An LED by itself or illuminated through fiber optics, or through gradient-index (GRIN) optics
- Through a pulsed laser radiation
- With superluminescent diodes (SLEDs)
- With terahertz lasers
- With laser diodes or a laser diode combined with a tunable lens
LHM has two opposing methods, employed for different purposes.
Digital in-line holographic microscopy (DIHM) illuminates the sample at close range to the light source but relatively farther away from the digital image sensor. This technique geometrically projects a hologram at five to 20 times magnification, comparable to conventional (expensive) medical imaging techniques.
On-chip microscopy, on the other hand, induces magnification with geometry performed on a combined sensor and processor chip. This results in a field-of-view (FOV) that is roughly equal to the size of the sensor area. On-chip microscopy’s magnification is limited by pixel geometric constraints rather than optical diffraction and can theoretically be advanced with pixel superresolution digital imaging techniques.
Latest in LHM
A team of researchers working at the Optics and Optometry and Vision Science laboratory at the University of Valencia, Spain, recently presented a new high-performance, compact, low-cost LHM device in a paper in a 2022 special issue of the journal Sensors dedicated to 3D biophotonics sensing for biomedical research.
The team’s DIHM microscope included all components in a cube-like structure about the size of a kitchen toaster, which could be easily transported in a backpack. The device obtained lateral resolution up to 1.65 μm in real-time without using lenses. It was also possible to modify magnification and FOV by simply changing the sample’s position in the device.
Engineers from Hangzhou Dianzi University, China, proposed another high-precision LHM device using the on-chip method. Their work, published in Sensors in 2021, used an iterative phase recovery algorithm to evaluate sample information and machine learning to identify and count mixed biological samples with high degrees of accuracy.
Another recent paper, by physicists from the National University of Colombia at Medellín and published in the journal Applied Optics in 2021, characterizes an open-source LHM method that can be readily deployed to remote and underdeveloped regions to improve access to healthcare.
Using 3D printing and off-the-shelf materials and components, the team produced a DIHM device that cost just $52.82. Open-source software was developed to process data from the device, further driving up accessibility in global healthcare.
References and Further Reading
Huang, X. et al. (2021) High-Precision Lensless Microscope on a Chip Based on In-Line Holographic Imaging. Sensors. 21(3), 720. https://doi.org/10.3390/s21030720.
Picazo-Bueno, J. A., K. Trindade, M. Sanz, and V. Micó (2022) Design, Calibration, and Application of a Robust, Cost-Effective, and High-Resolution Lensless Holographic Microscope. Sensors. 22(2), 553. https://doi.org/10.3390/s22020553.
Tobon-Maya, H., S. Zapata-Valencia, E. Zora-Guzmán, C. Buitrago-Duque, and J. Garcia-Sucerquia (2021) Applied Optics. Vol. 60, Issue 4, pp. A205-A214. https://doi.org/10.1364/AO.405605.
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