There’s nothing silly about this puttylike optical element
Hank Hogan, firstname.lastname@example.org
In a new twist on the idea of a flexible microscope, researchers
at California Institute of Technology in Pasadena have constructed a functioning
microscope out of rubber. Not only does the instrument work, but it also is small
enough to be helpful in such applications as handheld diagnostic equipment. Using
the rubbery elastomer polydimethylsiloxane (PDMS), the group fabricated a microscope
that is only 10 cm long, complete with solid immersion lenses, and used it to image
fluorescence in cells.
A solid immersion lens is one example of an aberration-free
spherical lens. A more familiar version is formed by cutting through the middle
of a sphere, creating a hemispherical lens that enhances the numerical aperture
of an optical system by the index of refraction of the lens material.
Researchers have developed a way to easily produce solid immersion
lenses using a rubbery elastomer for both the mold and the lens (left). The lenses
can be integrated as imaging elements above a microfluidic channel (right). Images
courtesy of Stephen R. Quake and California Institute of Technology.
In contrast, a solid immersion lens
is formed by slicing a sphere, with the cut made past the midpoint by a distance
equal to the sphere’s radius divided by the material’s index of refraction.
The result is a sphere missing a chunk at the bottom. Unlike a hemispherical lens,
a solid immersion lens enhances an optical system’s numerical aperture by
the square of its index of refraction.
“The solid immersion lens provides
extremely good light-collection ability in a compact optic, and integration with
microfluidics allows one to flow objects past the [lens] to perform cytometry,”
said Stephen R. Quake, a member of the institute’s research team who is now
a professor of bioengineering at Stanford University in California.
The adoption of solid immersion lenses
has been held back, however, because they are difficult to make — particularly
the undercut features found in the overhang at the bottom of each lens — which
require labor-intensive hand grinding and polishing to achieve high-performance
As described in the April 24 issue
of Applied Physics Letters, in making their rubber lenses, the researchers
exploited the physical and optical characteristics of PDMS. Optically, the material
has a refractive index of 1.41 and is transparent, and therefore it can function
as an optical element. Physically, PDMS is not rigid, and the researchers took advantage
of this as well. “Our insight here was that the flexibility of PDMS would
allow us to make the highly undercut structures that are needed for solid immersion
lenses,” Quake said.
The scientists developed a rubber microscope consisting of compound
solid immersion lenses manufactured using 395-μm-radius microspheres (A) and
other optical elements integrated with a microfluidic chip on one end and a CMOS
sensor on the other (B). Only 10 cm long, the microscope imaged two E. coli cells
expressing GFP (C). Art and image courtesy of the American Institute of Physics.
The ability of elastomers to stretch
and return to a previous shape without damage has been exploited to make large solid
immersion lenses by embedding stainless steel ball bearings in PDMS, with the top
of the ball extending out of the elastomer. Because PDMS stretches, the ball can
be removed without damaging the material. Left behind is a form that can be used
as a mold for the lenses.
Instead of steel bearings, the scientists
used ruby microspheres from Edmund Optics of Barrington, N.J., to create the lenses.
However, their attempts to extend the molding technique to microscopic dimensions
ran into a problem: The surface tension of the PDMS caused it to creep up the side
of the spheres. They overcame this by partially curing the PDMS — causing
it to shrink back a controlled amount — before extricating the spheres. This
process fixed the shape in a highly reproducible manner without trapping the spheres
permanently in the elastomer.
The researchers then freed each microsphere
and finished the curing. In the next step, they poured another layer of PDMS into
the mold, cured it and popped free the finished lens.
The ruby spheres used as the initial
positive mold do not have to be perfect; however, they cannot deviate too much from
ideal because they are the basis for the lens quality. The wavelength of the source
illumination to be used with the lens plays a part in determining the amount of
acceptable imperfection of the sphere. “[The material] needs to be optically
smooth, and PDMS can reproduce features much smaller than the wavelength of light,”
The investigators characterized the
resulting lenses by measuring the amount of fluorescence from a latex bead as imaged
with and without the solid immersion lenses. They used a microscope that had a numerical
aperture of 0.63 without the lens and found that the fluorescence intensity ratio
— with and without the lens — was 15.35 to 1, which implied that the
numerical aperture with the lens would equal 1.25, close to the theoretical prediction
of 1.26. Similar close agreement with predicted performance was found with lenses
made with spheres ranging in size from 395 μm to 2 mm.
Not content to make a rubber lens,
they also constructed a rubber microscope, fabricating six lenses and employing
150-μm ruby microspheres to create two solid immersion lenses. According to
Quake, the optics were all fabricated separately and then put into place.
One of the lenses was placed atop a
microfluidic chip, which also was constructed of PDMS. The other lens formed
part of an optical arrangement that began with the first solid immersion lens, continued
through the other molded lenses and ended in front of a CMOS sensor. The result
was a 10-cm-long structure comprising a microscope and an integrated microfluidic
The only nonrubber parts of the system
were the mounting frame, the blue LED light source, a filter to separate excitation
from emission light, and the CMOS sensor. Because the device was a monolithic, integrated
structure, it did not exhibit the light-robbing problems of more heterogeneous instruments.
Using the rubber microscope, the researchers
imaged E. coli cells expressing GFP. Without the solid immersion lenses,
the microscope could not detect the 1 x 2-μm cells. With the lenses, which
boosted the magnification by about a factor of two, the scientists successfully
imaged the cells.
The elastomer-molding technique could
be used in various systems as a way to integrate optics for readout or other uses.
PDMS is already used to make a number of microfluidic mechanisms, including valves,
pumps, cell sorters and DNA-amplification devices. Adding integrated optics could
expand their capabilities.