Fiber running through STED

DOUGLAS FARMER, SENIOR EDITOR DOUG.FARMER@PHOTONICS.COM

Thanks to stimulation emission depletion (STED) microscopy, scientists can capture subcellular dynamics, such as the plasticity of dendritic spines that are responsible for transferring electrical signals. These specialized structures on neurons are thought to be the sites where memory consolidation occurs through synaptic activity.

Due to the complexity of a STED microscope, this equipment is generally only accessible in a research laboratory. This may change soon if a commercial partner is found for a grant-funded project at the University of Colorado (CU) for the development of a fiber-based STED system.

STED is a superresolution technique in which a focused laser beam excites fluorescence in a small diffraction-limited region of a sample, and then a second laser beam is used to return the surrounding fluorophores to their ground state. The result is the so-called “doughnut hole” of high resolution. Typically, STED systems include pulsed lasers, a single-photon detector, dichroic mirrors, a phase plate, an objective, a data acquisition unit, and a scanner and are often used on cell cultures or tissue slices, which I delve into in my cover story here.

A team led by Emily Gibson and Diego Restrepo at CU Denver, along with Juliet Gopinath at CU Boulder, is developing a miniaturized two-photon, fiber-coupled microscope to enable STED resolution in previously inaccessible areas. In their design, a Ti:sapphire laser is fired through a polarization-maintaining fiber, and along with other components — such as a beamsplitter, a spatial light modulator, and a photomultiplier tube — can deliver both the excitation and depletion beams at 915 nm and 592 nm, respectively.

The project has garnered grant support from the National Institutes of Health and the National Science Foundation. The miniature fiber optic microscope could reveal the spatial and temporal detail of brain activity and complete this while attached to the heads of laboratory animals that are accomplishing various tasks, since it is not affected by bending or changes in temperature. Ultimately, according to researchers, neural activity could be monitored in real time.

As I describe in my feature article, researchers at other institutions are busy enhancing details that existing STED systems have already uncovered. The team led by Stefan Hell, who was awarded the Nobel Prize in Chemistry in 2014 for developing STED microscopy, is using a technique called MINSTED, in which the co-aligned beams are adjusted in a circular motion around a fluorescent molecule, which has the potential to track cellular processes. And companies have developed STED modules that can be integrated into traditional confocal microscopes and created data analysis tools to process the structural detail that STED provides.

As with other superresolution methods, the smaller the detail, the bigger the lesson it can provide about the foundation and evolution of life.

Enjoy the issue!
Douglas J. Farmer