Robustness was initially not the only drawback to the technology—equally important was instrument cost. The original prototype
instruments used lasers as an excitation source and were consequently financially unsuitable for practical applications. The
use of high-intensity LEDs for excitation, which have in the last years dropped in price to a couple of dollars, together
with efficient optical and electronic designs, addresses that problem: fluorescence-based sensors are potentially cheaper
than conventional probes. Much has been done on the design of a new generation of sensing patches that can be demonstrated
to not leach into the culture media or otherwise affect the growing cells.
Even with all of these advances, the technology still has a potential disadvantage in that the sensing patches are subject
to photobleaching over time. Because of improved formulations which greatly increase the lifetime of the patches, and the
use of phase shift or ratiometric measurements, the sensor's measurements are immune to changes in the patch response caused
by photobleaching, except after long-term use. In such a case, the signal to noise ratio drops with time until the measured
signals become noisy and less accurate after an excessive number of measurements. Because the effective lifetime of the sensing
patch is measured in tens of thousands of readings, this is a problem only with very long processes monitored at frequent
intervals (seconds versus minutes or hours). Even in this case, an easy solution exists: a vessel can be equipped with two
or more patches in place of a single patch and the sensing head can be moved to a fresh patch whenever the measurements start
to become noisy. In this way, processes of arbitrary duration can be monitored with no loss of accuracy and no drift over
time.
CONCLUSIONS
As the need for more cost-effective and rapid development of drugs and other bioprocess-related products increases, so does
the tolerance for new technologies and approaches in what has been a rather conservative industry. Fluorescence-based sensing
technologies, which can greatly decrease overall development time, labor, and costs, become an increasingly useful tool, particularly
because their use permits a degree of miniaturization, scalability, and multiplexing previously unavailable. Although there
are limitations, particularly in patch development, continued development and maturation in this area holds great promise
for the development of a widespread paradigm-shifting technology in the near future.
Joe Qualitz is the president and CEO of Fluorometrix Corporation, Stow, MA, 978.461.2468, support@fluorometrix.com
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