Proteins in focus: Dynamically tunable protein microdevices were built up by a simple “top-down”, maskless, femtosecond laser direct writing approach with bovine serum albumin. This technique was used to produce biocompatible microlenses that swell and shrink reversibly in response to changes in the pH of the surrounding solution. These responses to environmental stimuli can be used to focus the microlenses.

Whether
it’s right under our nose or far away, when we observe an object we see
it—provided we have healthy eyes and normal vision or suitable
glasses—in focus. For this to work, muscles deform the lenses of our
eyes and adjust them to a suitable focal distance. For miniaturized
technical devices, microscale lenses with a similar adaptable focus
could be an advantage. In the journal Angewandte Chemie,
Hong-Bo Sun and a team from Jilin University, China, have described a
new approach to the production of adjustable microlenses made from
protein gels.

Proteins
are potentially useful as “building materials” for microcomponents
because they are readily available, inexpensive, and biocompatible. They
can also change their properties in response to external stimuli, which
makes them an interesting material for use in adjustable microlenses.
However, lenses must be extremely precise in order to meet optical
requirements—something difficult to achieve with proteins. In addition,
they must be fast, simple, and inexpensive to produce.            

The
Chinese researchers have now met this challenge: They used a laser to
“write” the desired micrometer-sized lens shape out of a solution of
bovine serum albumin, a protein. Methylene blue acts as a
photosensitizer, which captures the light energy like an antenna and
triggers a crosslinking reaction of the protein molecules. Driven by a
computer, the laser cuts out the desired three-dimensional form voxel by
voxel. A voxel is a 3D pixel, a tiny segment of volume. The irradiation
used is in femtosecond pulses, which lasts on the order of 10-13
seconds. The crosslinking reaction only takes place in the locations
that are irradiated. After the reaction, the protein molecules that have
not reacted can simply be rinsed away. What stays behind is a
cross-linked, aqueous protein gel in the shapes of micrometer-sized
lenses.

Direct
writing with lasers usually results in structures that have too rough a
surface for optical applications. By optimizing the duration of the
laser pulse, the pulse intensity, and the protein concentration, Sun and
his team obtained lenses with outstanding optical properties.

The
special trick in this case is that the amount of liquid absorbed by the
protein gel depends on the pH value of the solution. Increasing the pH
causes the lens to swell. If the increase in thickness is limited by a
glass surface, the lens primarily grows in width and becomes flatter. If
the pH value is reduced, the gel shrinks and the lens is more curved.
Because the radius of curvature determines the focal length of the lens,
this method can be used to focus the microlenses.

Because
the protein lenses are biocompatible, they may be used in optical
analytical systems for medical diagnostics or lab-on-a-chip technology.
           

Dynamically Tunable Protein Microlenses

 

SOURCE: Wiley