- Printing biological molecules
Thanks to advances in ink-jet printing
technology, cells and fluorescently labeled proteins can now be printed at a maximum
density while maintaining their biological function. That capability could pay dividends
for researchers. For example, cells have long been grown in a dish, but that has
not been of much benefit to the field of tissue engineering and tissue replacement.
Cells in a dish don’t function exactly as
they do in vivo. Being able to print growth factors in a geometric design at varying
concentrations would allow researchers to detect what is happening, using fluorescence
as readout. The resulting data ultimately could be used to build a scaffold that
is seeded with cells, and the assembly could function in a biologically and therapeutically
Biosensors — such as those designed
to detect pathogens in the event of man-made or natural pandemics — also would
benefit from printing. Mass production of biosensors is difficult because biological
molecules must be reproducibly delivered in a space-constrained and well-controlled
fashion. Current sensors are constructed in cleanrooms under ideal conditions and
often are produced only once — not the type of setup required for widespread
Associate chemistry professor David
Wright and his research group at Vanderbilt University in Nashville, Tenn., are
printing biological molecules using a system from Dimatix Inc. of Santa Clara, Calif.,
which recently announced it was being acquired by Tokyo-based Fuji Photo Film Co.
Printing biological materials can be helpful for various applications. A printed multiwalled
carbon nanotube with bound DNA (left) was treated with a DNA-sensitive DAPI fluorescent
probe, showing the presence of DNA on the nanotube (right).
The printer was built with techniques
used to construct microelectromechanical systems and has an ink-jet array of 16
nozzles that each deposit 10-pl droplets of fluid. The resulting features measure
as small as 40 μm across after spreading out on a surface.
Unlike thermal ink-jet printers, the
system uses piezoelectric materials to form the droplets. The piezoelectric substance
creates an acoustic vibration that forces ink out of the nozzle and draws ink in
from a reservoir to replace what was expelled. The formation method is important
for biological applications because biomolecules can be denatured or otherwise altered
Jan Sumerel, manager of biomedical
sciences at the company, noted that, in adapting an ink-jet printer for biological
molecules, some changes had to be made. Biological fluids generally are not like
typical inks in terms of viscosity and surface tension.
“It’s in a different territory,
and so we have to tweak our biological materials so that they can be printed,”
Traditionally, jetting conditions require
inks that have a surface tension of about 30 mN/cm, which is not changed by the
printing process, and the inks need a viscosity of 0.010 to 0.012 Pa·s, which
can be different during printing because it is coupled to temperature. Most biology
takes place in an aqueous environment, where the surface tension is much higher,
about 70 mN/cm. At the same time, the viscosity is much lower, often <0.003 pa·s.
Water itself has a viscosity of <0.001 pa·s at room temperature.
In addition to changes in the biological
materials, the investigators adjusted the operating parameters of the printer. In
particular, they altered the waveform, frequency and magnitude of the voltage that
drove the piezoelectric material. These adjustments overcame the viscosity and surface
tension differences and allowed drops to be generated. A stroboscopic camera mounted
on the printer monitored drop formation and enabled feedback so that further adjustments
could be made.
This flexibility was important in a
research situation, noted Wright. Being able to quickly tune the printer for various
molecules and fluids meant that there was not a need to rebuild it every time a
new formulation was tried.
Besides the lack of heat, the piezoelectric
approach also enabled very high density, which was determined by the linear encoder
resolution of 5 μm. The printer produced a little more than 5000 dpi, with
a fluid volume in each drop measured in the picoliters instead of the nanoliters
commonly dispensed by other ink jets. A downside, though, was that the ink-jetting
required low frequencies and, therefore, was quite a bit slower.
Wright said that the technique does
preserve biological function, as proved by printing and the subsequent amplification
of genomic DNA. As for the relatively low speed, he said that this was being worked
on through the development of suitable inks.
He noted that the technique could possibly
be used for applying quantum dots in a controlled manner or for manipulating membrane
proteins or membrane-bound receptors.
Contact: Jan Sumerel, Dimatix Inc.,
Santa Clara, Calif.; e-mail: firstname.lastname@example.org. David Wright, Vanderbilt University,
Nashville, Tenn.; e-mail: email@example.com.
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