A Multi-disciplinary Senior Design Topic: Biofilms, Electrical Engineering and Sensor
Technology: J.N. Zemel, H. Nedwill Ramsey Professor (Emeritus) of Sensor
Technologies, zemel@ee.upenn.edu
1. What are Biofilms?
Biofilms are ubiquitous and complex biological structures generated by an assemblage of
bacterial and/or fungal and/or algae species at a wide variety of interfaces, as illustrated in
Figure 1. In some respects, these films resemble a multicellular system. What is intriguing
about the biofilm environment is that these different species can promote, protect and
propagate the biofilm environment for the benefit of all the different unicellular participants.
The substantial variety of single cell
species involved in generating complex
biofilm structures and the broad
conditions under which they prosper are
themselves
interesting
scientifically.
Examples are the dental plaque in our
mouth and the slime on filter beds in
water treatment plants. The latter impacts
the power industry since the buildup of
biofilms in power plant can seriously
impact the efficiency of heat exchangers
leading to significant economic costs.
However, not all biofilm systems are
problematic. One example taken from
Figure 1: Illustration of the biofilm community
reference 2 is quoted here: “An often
cited example of a bacterially produced biopolymer is xanthan gum, produced by
Xanthamonas campestris. This biopolymer is produced for use as a thickening agent in a
variety of food and consumer products.”
2. Why detecting the earliest stages of biofilm formation is important:?
Reference 4 at the end of this document contains the following relevant comment:
Sampling strategy The most common way to address biofouling problems is sampling of the
water phase. This can usually be performed relatively easily but will provide no information
about the location and the extent of deposits on surfaces. It is very important to keep in mind
that microbial analysis of the water phase is not suitable to accurately locate or quantify
biofilms as the contamination of the water phase occurs not continuously but randomly and
does not reflect the site or extent of biofilms in a system. This emphasis on sampling of
surfaces may sound trivial, however, in practice, surface sampling is often difficult.
“Surface sampling is often difficult.” That statement indicates that a measurement system
that can follow the earliest stages of biofilm growth could be a valuable tool for better
understanding the attachment, growth and development of the biofilm. What is known is that
in the earliest stages of biofilm formation, biopolymers that exist in most conventional water
systems deposits on a surface. As this precursor layer forms, planktonic (free floating) cells
material that generates the biofilm.. The structure and early characteristics of the biofilms
can be studied using a wide variety of microscopic methods involving staining and
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fluorescence. This works well in the laboratory but a more desirable approach would involve
a detection and measurement methodology that can be used on site within systems such as
heat exchangers. The aim of this project is to make early surface sampling of biofilm
formation simple and routine.
3. What property might be used to detect the early formation of the biofilms?
The problem created by the growth and accretion of biofilm is that they impede heat transfer
from a surface. The reduction in heat transfer is because their thermal conductivity is
significantly lower than that of H2O as evidenced in high temperature heat exchange piping.
While this is certainly the end result of biofilm build-up, the reduced thermal conductivity
might also be a route for detecting the early stages of biofim accumulation. The major
question is whether a sensor method can be devised that is sufficiently sensitive when
operated near or at ambient temperatures.
4. A bit of background.
Some years ago, there was a graduate program in ESE exploring the use of the pyroelectric
effect for monitoring fluid flow (mostly gases), referred to as pyroelectric anemometry or PA
(References 4-7). It was shown that: (a) the physical process involved was convective heat
flow from the solid to the gas; (b) the thermal conductivity of the gas was the major variable
controlling the signal level; (c) the sensitivity of the measurement was exquisite and covered
almost 5 orders of magnitude in flow; the measurements were extraordinarily reproducible
from device to device. Measurements of liquid flow have not been carried out so that would
be the first order of business. That is, to build a system/device to measure the operational
characteristics of a water-based PA. The fundamental properties of the PA won’t change in
going from a gas like argon to liquid water since the operation of the PA depends on
convective heat transfer at the surface.
It should be kept in mind that heat transfer is the central issue in this proposed project. If the
surface is gunked up with a biofilm, the heat transfer will be seriously impacted. Because
biofilms on solid surfaces impede thermal transport from the solid, this property provides a
possible means for detecting their early growth, provided the sensors employed have
adequate thermal sensitivity. As can be seen in references 1-3, the heat transfer from a sold
surface to water drops when the biofilm forms and not infrequently the drop is profound. To
measure the changes in the PA signal generated as a biofim grows, sensitive analog and
digital electronics are needed. The design and construction of a suitable flow system is likely
to include microfabricated microfluidic structures (see reference 7) into which currently
available PA chips would be incorporated.
In summary, what is proposed here is an extension of a known measurement technology
based on well-established scientific and engineering principles to address a known problem
created by biological life forms. It’s an interdisciplinary effort that offers insight into how
engineering and biology can cooperate to address a real industrial issue.
5. A brief aside on EE education.
While computers, communication and control are core aspects of any EE education, there is
growing awareness that the rapid growth and expansion of pure and applied biology is
having a profound impact in engineering. The tools and methods of the physical sciences
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(chemistry, physics and engineering) play central roles in contemporary biological areas of
industry and academia. The proposed project presented below illustrates how the
application
of
material
physics,
thermal
engineering,
electronics
and
microfluidics/microfabrication technology could lead to a device that provides useful
information on the early growth and development of this extraordinary life form, the biofilm.
Some Reference Material (I found that Firefox is best for viewing the first 3 documents):
1. If you are interested in this project, I urge you to scan through the material presented in
http://www.biofilm.montana.edu/. A variety of overviews of both the behavior and
industrial significance of biofilms is available on Google’s “Biofilm Thermal Conductivity”
search site. I haven’t found too much on the Penn Library website
2. A good overview can also be found at http://www.wet-usa.com/data/bullets/46.pdf . Note
that most of the discussion related to significant deposits of biofilm material rather than
early stage detection, what I hope will be possible with PA based system.
3.
Another interesting document from a European consortium is found at:
http://www.efcweb.org/efcweb_media/Downloads/EFC+WP10/BR99_02b.pdf
Methods for Investigation of Biofilms, 1999
Steps in biofilm sampling and characterization in biofouling cases
Gabriela Schaule1, Thomas Griebe2 and Hans-Curt Flemming2
1
IWW Center for Water Research, Moritzstr. 26, D- 45476 Mülheim
2
University of Duisburg, Dep. Aquatic Microbiology, Geibelstr. 41, D-47057
4. H. Y. Hsieh, J. N. Zemel and Haim H. Bau "Pyroelectric Anemometry: Theory of
Operation",Sensors and Actuators, A49, 125-132, (1995).
6. H. Y. Hsieh, and J. N. Zemel, "Pyroelectric Anemometry: Frequency, Geometry And Gas
Dependence",Sensors and Actuators, A49, 133-140, (1995)
7. H. Y. Hsieh, A. Spetz and J. N. Zemel, "Pyroelectric Anemometry: Vector and Swirl
Measurements", Sensors and Actuators, A49, 141-147, (1995)
8. Yu Dun, H. Y. Hsieh, and J N. Zemel, "Microchannel Pyroelectric Anemometers for Gas
Flow Measurements", 39, 29-35, 1993
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