New Hydrogel Expands Capabilities of Expansion Microscopy

As a result of the development of a new, uniform-configuration hydrogel, MIT engineers say they can now image biological samples with accuracy at the scale of 10 nanometers. The new methodology, described in a Nature Nanotechnology paper published today, builds on expansion microscopy, an approach that involves embedding biological samples in a hydrogel and then expanding them before imaging them with a microscope.

This degree of accuracy could open the door to studying the basic molecular interactions that make life possible, says Edward Boyden, senior author on the study. "If you could see individual molecules and identify what kind they are, with single-digit-nanometer accuracy, then you might be able to actually look at the structure of life."

Expansion microscopy was introduced by Boyden's lab in 2015. In a 2017 paper, the team demonstrated resolution of around 20 nanometers, using a process in which samples were expanded twice before imaging. This approach, as well as the earlier versions of expansion microscopy, relies on an absorbent polymer made from sodium polyacrylate. However, these gels are not completely uniform in structure or density, which leads to small distortions in the shape of the sample when it's expanded, limiting the accuracy that can be achieved.

To overcome this, the researchers developed a new gel called tetra-gel, which forms a more predictable structure. By combining tetrahedral PEG molecules with tetrahedral sodium polyacrylates, the researchers were able to create a lattice-like structure that is much more uniform than the free-radical synthesized sodium polyacrylate hydrogels they previously used.

The researchers demonstrated the accuracy of this approach by using it to expand particles of herpes simplex virus type 1 (HSV-1), which have a distinctive spherical shape. After expanding the virus particles, the researchers compared the shapes to the shapes obtained by electron microscopy and found that the distortion was lower than that seen with previous versions of expansion microscopy, allowing them to achieve an accuracy of about 10 nanometers.

The researchers also used their new hydrogel to expand cells, including human kidney cells and mouse brain cells. They are now working on ways to improve the accuracy to the point where they can image individual molecules within such cells. One limitation on this degree of accuracy is the size of the antibodies used to label molecules in the cell, which are about 10 to 20 nanometers long. To image individual molecules, the researchers would likely need to create smaller labels or to add the labels after expansion was complete.