Skills learned
Image processing (MATLab)
Product design for high-throughput systems
Protocol writing
Motivation
Biological research always benefits from the existence of high complexity in vitro models – the better the in vitro model is, the greater the scientists ability to experiment in more accurate conditions. Organoids are one such model, these three dimensional, multicellular, self organizing masses are the basis for a still rapidly expanding field. In essence, stem cells are guided through the differentiation process to produce complex tissues such as the following:
Brain/eye organoids (Gabriel et al. 2021):
These cells self organized into a mass of brain tissue, and later grew two light sensitive orbs, the precursors to eyes.
Lung organoids (Leko et al. 2023):
These cells self organize into lung buds, the embryonic stage of lung tissue
As is clear in these images, these tissues introduce a new imaging problem, we cannot see inside them. There are many ways to work around this limitation, some labs opt for sacrificing their samples, others choose to focus on exterior traits, and some have looked to optical coherence tomography (OCT) to provide 3D imaging. However, some of these tissues are inherently load bearing or structural tissues such as cartilage, and require mechanical analysis, which OCT cannot provide. This, again, results in the need to sacrifice the sample, slice into thin segments, then subject them to traditional shear or torsional testing machinery.
Our proposition is to utilize Optical Coherence Elastography to measure the shear modulus of 3D tissues nondestructively.
Optical Coherence Elastography (OCE)
OCE uses OCT to visualize the motion of shear waves across a material. In essence, a high frequency shear wave is excited in the tissue of interest, then we scan twice over each location and compare the measured waves (light) to each other, and use the shift to calculate the distance that portion of tissue has moved. This can then be used to calculate the speed of the tissues vibration. For a more math forward explanation feel free to look at this paper, which my lab published recently and is the basis of the processing pipeline we currently use.
System Design
Phase 1: Design requirements for <2mm highly stiff tissues
The initial hurdle to this project was determining if it is even possible to excite shear waves that are high enough in frequency and amplitude to observe one and a half or so periods within the tissue. This difficulty arises from the nature of shear wave propagation:
As the stiffness of the tissue increases, for the same density (which we generally estimate to equal that of water) we see an increase in shear wave speed.
As the shear wave speed increases as a result of stiffer tissues, and wave length must decrease as a result of smaller tissues, we are required to push frequency higher.
For similar applications in the eye we need a minimum of 1-2kHz of excitation, for the cartilage organoids we will be working with , we need a minimum of 6-7 kHz of excitation. These high frequency waves will attenuate much more quickly than their low frequency counterparts, introducing the need for higher amplitude excitation as well.
This leads to the design requirements:
- compatibility with various sample shapes and sizes
- compatible with convenient imaging mediums for high throughput (non disruptive)
- piezoelectric device must be protected from small amounts of liquid
Unfortunately this is all I can share until we complete our patent submission! Hopefully more coming soon.