Vol.2, No.11, 1239-1248 (2010)
Copyright © 2010 SciRes. http://www.scirp.org/journal/HEALTH/
Openly accessible at
Simulation of perforated rectangular cantilever
immunosensor for estimation of bacterial pathogens
J. Sakthi Swarrup1, K. Govardhan2, V. Velmurugan1
1Center for Nanotechnology Research, School of Electronics Engineering, VIT University, Vellore-632 014, Tamil Nadu, India
Received 13 February 2010; revised 22 February 2010; accepted 1 March 2010.
ABSTRACT gens are essential for the potential treatment of the dis-
eases. However, it is difficult to treat for gram negative
and positive bacteria due to inherent and acquired resis-
tance to antimicrobial agents. Endospore-forming gram-
positive bacteria produce a unique resting cell called an
endospore. Bacillus Anthrax spore formers cause anthrax
in domestic animals, which may be transmitted to hu-
mans. Pseudomonas aeruginosa pathogen is generated
from gram-negative bacteria. It is the quintessential op-
portunistic pathogen of humans. It is a leading cause of
hospital-acquired infections (nosocomial infections). It is
difficult to eradicate due to its resistance to most antimi-
crobial agents. There is probably no tissue that cannot
become infected by Pseudomonas if the host defenses
are weakened. It is usually involved in soft tissue infec-
tions, urinary tract infections and pneumonia. The best
known and most widely studied species is Coryne Bac-
terium Diptheria, gram-positive bacteria, the causal agent
of Diptheria. The genus corynebacterium consists of a
diverse group of bacteria including animal and plant
pathogens, as well as saprophytes. Some coryne bacteria
are part of the normal flora of humans, finding a suitable
niche in virtually every anatomic site. One of the major
pathogens of humans from spirochetes a phylogenetically
distinct group of bacteria, is Treponema pallidum, the
agent of syphilis, a sexually transmitted disease.
Micro fabricated and multilayered perforated can-
tilever beam immunosensor was modeled using
CoventorWare for the estimation of bacterial an-
tigens of Bacillus Anthrax, Pseudomonas aeru-
ginosa, Coryne Bacterium Diptheria and Tre-
ponema pallidum. A rectangular cantilever beam
with perforations was simulated with dimensions
as length-200 µm, width-10 µm and thickness-0.5
µm. Each perforation is rectangular with length-
10 µm, width-5 µm and thickness-0.5 µm. The
theoretical and FEM simulations were carried out
with five immunoglobulin antibodies, IgA, IgD,
IgE, IgG and IgM for the estimation of bacterial
antigens. The effect of perforation in cantilever
beam and molecular size of antibody and antigen
on the performance of the sensor has been stu-
died. The cantilever beam without perforation
showed a deflection of 1.8 e + 02 µm whereas the
cantilever beam with perforation showed addi-
tional deflection of 1.9 e – 02 µm. With IgG, the
difference between analytical and simulation
values is positive and low especially with low
molecular weight antigens Pseudomonas aeru-
ginosa and Treponema pallidum. The low mo-
lecular weight IgG influences the antigen-anti-
body interaction more favourably. The simulated
perforated rectangular cantilever beam with IgG
antibody is a more promising model for the fab-
rication of a sensor for the estimation of highly
motile Pseudomonas aeruginosa and Treponema
Infectious diseases are generally detected with immu-
nohistochemistry (IHC), flow cytometry, immuno-electron
microscopy, ELISA, western blotting, polymerase chain
reaction (PCR) etc. These systems use an antibody-based
method to detect a specific protein. One of the main
drawbacks with IHC staining is difficulties in over-
coming specific or non-specific background. Optimisation
of fixation methods and times, pretreatment with block-
ing agents, incubating antibodies with high salt, and opti-
mising post-antibody wash buffers and wash times influ-
ence the quality immunostaining. Flow cytometry is less
effective for detecting extremely rare cell populations.
Immuno-electron microscopy can be technically challen-
ging, expensive and require rigorous optimisation. ELISA
test does not identify the amount of antigen present in
Keywords: Modeling; Perforated Rectangular
Cantilever Beam; Immunosensor; Immunoglobulin
Antibodies; Estimation of Bacterial Pathogen Antigen
Identification and quantification of bacterial patho-
J. S. Swarrup et al. / Health 2 (2010) 1239-1248
Copyright © 2010 SciRes. http://www.scirp.org/journal/HEALTH/
the sample and is expensive and time consuming. West-
ern blot test is technically demanding, expensive, subject
to interpretation, presence or absence of bands, intensity
of those bands. Major disadvantages of the PCR protocol
include length of time needed (2-3 days). PCR tests also
require more sophisticated equipment and greater exper-
tise and hence expensive. Urine and blood tests takes a
long time to analyze. The test should be carried out in
the lab only. Errors may occur due to the carelessness of
the technician.
Openly accessible at
Immunosensors are a type of biosensor, which uses
antibodies or antigen as the biospecific sensing element,
and are based on the ability of an antibody to form com-
plexes with the corresponding antigen. The reaction be-
tween the antibody-antigen reactions is highly selective,
and is analogous to a lock and key fit. The piezoelectric
sensing method is thought to be one of the most sensitive
analytical instruments developed to date, being capable
of detecting antigens in the picogram range. This trans-
duction method is relatively easy to use, cost effective,
and offers direct label-free analysis and overcomes all
the dis-advantages of the existing system. In addition, it
is able to provide the option of several immunoassay
formats for increased sensitivity and specificity.
Development of a simple and multifunctional trans-
ducer to detect two or more species by one cantilever at
one time is a challenge. The cantilever deflection and
output voltage can be influenced by various forces acting
on the probe molecule such as molecular size of the
probe and target molecules, DNA hybridization of probe
molecule [1], nature of grafting of probe molecules on
the surface whether it is ordered or is disordered [2],
grafting density [3] and design and thickness of the can-
tilever beam. Therefore it is relevant to develop simula-
tion methods and design rules that will enable the design
of microcantilever-based biosensing systems. In this pa-
per the effect of molecular size of both antibody and
antigen on the deflection of microcantilever upon ad-
sorption of probe molecules and binding of target mole-
cules has been studied by modeling perforated rectangu-
lar cantilever with immunoglobulin for the detection of
bacterial pathogens of Bacillus Anthrax, Pseudomonas
aeruginosa, Coryne Bacterium Diptheria and Treponema
2.1. Modeling of Perforated Rectangular
Cantilever Beam
The design of the perforated rectangular cantilever beam
was simulated using commercial CoventorWare-DESIGN-
ER software. The first step involved in the modeling of
the perforated rectangular cantilever beam starts with the
Process Editor, where the properties of the materials for
cantilever beam are applied. The antibodies IgG, IgM,
IgD, IgA and IgE are added to the process editor with
their properties like Young’s modulus, density, weight
Crystalline silicon (Young modulus 155.8 Gpa, Pois-
son coefficient 0.21 and density 2330 Kg/m3) was used
as the substrate with thickness, 5 µm in the present simu-
lation. Boron phosphor silicate glass (BPSG) material
was deposited for a thickness, 35 µm (selected from the
Materials Database) as a temporary support for the can-
tilever beam using Stack Material option. For construc-
tion of the cantilever beam, certain region in the layer
has been etched in the BPSG using the options Delete
and Straight Cut. Deposition and selective etching of the
silicon material was then carried out using Planar Fill
option and Straight Cut options respectively. A layer of
gold was deposited to give good attachment of the anti-
bodies. Deposition and selective etching of the gold on
the perforated cantilever was then carried out using Pla-
nar Fill option and Straight Cut options respectively. The
sequence of modeling of the cantilever beam using the
Process Editor is given in Figure 1. Once the cantilever
beam was constructed the antibodies were coated in the
beam surface.
To improve the sensitivity of the immunosensor to de-
tect nanogram material, a simple way was to reduce the
size of the beam. The final simulated perforated rec-
tangular cantilever beam sensor has dimensions length-
300 µm, width-100 µm and total thickness-40.5 µm. The
thickness of the cantilever beam is 0.5 µm (Figure 2).
Once the whole structure was built in the 2D the struc-
ture was exported to the MEMMECH analysis, where
the boundaries were fixed and meshing was done. Finite
Element Method was used for the analysis. The model of
the mesh of cantilever beam is given in Figure 2. After
meshing, the pressure was applied at the top surface of
the cantilever equivalent 1 mole (molecular weight in
gram) of antigen for the sensing. The pressure/stress
equivalent 1 mole (molecular weight in gram) of antigen
was calculated using the equation,
-04 2
(Weight ofantigen1.66090210e)
Pressure (kg/cm)
((LB)(e) )
where L is length of the cantilever, and B is width of the
cantilever. The pressure in Kg/cm2 was converted into
pressure in Mpa.
2.2. Detection of Antigens
Four different diseases viz, anthrax, nosocomial infec-
tions, diphtheria and syphilis and the corresponding
bacteria were considered for the present study. In the
present modeling static deflection mode was considered
J. S. Swarrup et al. / Health 2 (2010) 1239-1248
Copyright © 2010 SciRes. http://www.scirp.org/journal/HEALTH/
Figure 1. Sequence of modeling of cantilever using process editor of the coventorware.
for the detection. For static deflection, one can apply
Openly accessible at
Stoney’s formula,
where R is the can- The antibody and antigen reaction is an important pro-
tective mechanism in human being against invading
foreign substances. The antibody and antigen reaction,
together with phagocytosis, constitute the immune res-
ponse (humoral immune response). Invading foreign
substances are antigens while the antibodies (immuno-
tilever’s radius of curvature, ν is Poisson’s ratio, E is the
substrate’s Young modulus, t is the thickness of the canti-
lever, and Δσ is the differential surface stress. In the pre-
sent modeling, uniaxial and one dimensional approxima-
tion was followed in which Poisson coefficient was not
taken in to account. However, in order to get accurate
results, simulation was carried out with a non zero value
of Poisson coefficient. When analytes bind to only one
side of the cantilever’s surface, the cantilever bends up or
down depending on the side to which the analytes bind.
The deformation of the cantilever, which arises from va-
riations in surface stress (Δσ), is measured from changes
in the resistance of the piezoresistive material. The equi-
valent pressure corresponding to 1 mole of antigen was
applied to the beam and the displacement, voltage and
stress was noted. The deformation induces the current in
the quartz piezo-patch. The output voltage at the upper
surface of the piezo-patch is measured. The deformation
of the beam depends on the applied pressure as well as on
the geometry of the device, molecular size of the anti-
body and antigen-antibody interaction. According to the
deflection of the beam and out put voltage, the intensity
of the disease could be judged.
globulins), are specific proteins generated (or previously
and present in blood, lymph or mucosal secretions) to
react with a specific antigen. Gamma globulins are pro-
duced by B lymphocytes when antigens enter the body.
Immunoglobulins (Ig) have been selected as substrate
antibodies for the construction of cantilever. Immuno-
globulins (Ig) have a basic four-chain monomeric struc-
ture consisting of two identical heavy chains and two
identical light chains with interchain disulfide bonds.
There are five heavy chain classes (M, D, G, E and A),
four G subclasses (G1-4), and two A subclasses (A1,2).
There are two light chain isotypes, K (kappa) and L
The biological characteristics of human immunoglobulins
are given in Table 1. IgA molecules have an average mo-
lecular weight of 250,000 daltons. IgA is found in body
fluids such as tears, saliva, mucosa, and other bodily
secretions. It provides a first line of defense against
invading pathogens and allergens. IgD appears to act in
J. S. Swarrup et al. / Health 2 (2010) 1239-1248
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Figure 2. Displacement profile of simulated plain and perforated rectangular cantilever beams under applied pressure.
J. S. Swarrup et al. / Health 2 (2010) 1239-1248
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conjunction with B and T cells to help them in location
of antigens. IgG is the most common type of antibody. It
is the most common immunoglobulin against microbes.
It acts by coating the microbe to hasten its removal by
other immune system cells. It gives lifetime or long-
munoglobulins to recognize an antigen. An antigen has on
its surface a combining site that the immunoglobulin
recognizes from the combining sites on the arms of its
Y-shaped structure. In response to the antigen that has
called it forth, the immunoglobulin wraps its two com-
bining sites like a “lock” around the “key” of the antigen
combining sites. The mode of action of the present im-
munoglobulin varies with different types of antigens. With
its two-armed Y-shaped structure, the immunoglobulin
can interact two antigens at the same time with each arm.
Adsorption of biomolecules on a surface of a microcanti-
lever generates surface stresses that cause the cantilever to
deflect [4-6]. In the present studies, adsorption of various
antigens on the perforated rectangular cantilever influence
displacement, voltage and stress.
standing immunity against infectious diseases. It is highly
mobile, passing out of the blood stream and between
cells, going from organs to the skin where it neutralizes
surface bacteria and other invading microorganisms. IgE
is responsible for allergic reactions. IgE acts by attach-
ing to cells in the skin called mast cells and basophil
cells. In the presence of environmental antigens, IgE
releases histamines from the mast cells. The histamines
cause the nasal inflammation. IgM is the largest among
the immunoglobulins. IgM usually form clusters that are
in the shape of a star. It is effective against larger micro-
organisms. Because of its large size (it combines 5 Y-
shaped units), it remains in the bloodstream where it
provides an early and diffuse protection against invading
antigens. IgG is produced by the plasma cells. IgD and
IgE are susceptible to denaturation by the treatment with
heat and reducing agents under conditions that have little
effect on other immunoglobulins.
To improve the sensor performance and to enhance
the signal transduction to detect nanogram level antigen,
a novel sensor platform with perforated rectangular can-
tilever beam was modeled. By modeling and simulation,
the high surface stress region on the cantilever was iden-
tified. Optimization of cantilever width, thickness and
perforation was carried out with the software packages.
Specific perforated cantilever designs were targeted to
optimize stress localization at the base of the cantilever.
The cantilever beam without perforation showed a de-
flection of 1.8e + 02µm whereas the cantilever beam with
perforation showed additional deflection of 1.9e – 02µm
(Figure 2). Therefore the cantilever beam with perfora-
tion was modeled. The layout of perforated can- tilever
beam is given in Figure 3.
The present selected bacteria, Bacillus Anthrax (1),
Pseudomonas aeruginosa (2), Coryne Bacterium Dipthe-
ria (3), and Treponema pallidum (4), have different mo-
lecular weight and physical characteristics (Table 2). All
immunoglobulins are in a Y-shape with differences in the
upper branch of the Y. These structural differences of in
each of the immunoglobulins enable the individual im-
Openly accessible at
Table 1. Biological characteristic of human immunoglobulins.
Antigen species Shape Gram staining Motility Intra/Extracellular Molecular
Weight (Dalton)
Bacillus Anthrax
Toxin (1) Rods Gram-positive Nonmotile Extracellular 86,000
Aeruginosa (2) Rods Gram-negative Motile Extracellular 33,000
Coryne Bacterium
Diptheria (3)
Small, slender,
pleomorphic rods
(unevenly) Nonmotile Extracellular 60,000
Pallidum (4)
Long, slender, flexible,
spiral- or corkscrew-
shaped rods
but stains poorly Highly motileExtracellular 34,000
Table 2. Characteristics of antigen species.
Characteristics IgA IgD IgE IgG IgM
Molecular weight (Dalton) 250000
(150000-350000) 180000 190000 150000 900000
Carbohydrate (Approx %) 7 12 12 3 12
Sedimentation coefficient (S20,w) 7 (9-15) 7 8 7 19
Biological survival (plasma T ½ days) 6 3 2 3 5
Serum concentration (mg per 100ml) 250 3 0.01 1100 100
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Layout of perforated cantilever beam
(Base: Length-300 µm, width-100 µm and thickness-40.5 µm , Beam : Length-200 µm, width-10µm and thickness-0.5 µm,
Antibody-loaded mesh pattern : Length-20 µm, width-100 µm and thickness-0.5 µm)
Base Perforated
Cantilever beam
Antibody-loaded mesh pattern
Quartz peizo patch
Figure 3. Layout of perforated cantilever beam.
The data on the interaction of antigen with IgA, IgD,
IgE, IgG and IgM are given in Table 3. The analytical
and simulation data reveal the variation of displace-
men t, voltage and stress with the interaction of 1 mole
of antigen with the antibody. With all toxins, the dis-
placement, voltage and stress increases with the increase
of molecular weight of the antigen. The variation of can-
tilever response with molecular weight of antigen is
given in Figure 4. The difference between analytical
and simulation values is attributed to the uniaxial and
one dimensional approximation. Moreover the point of
interaction of antigen and antibody in the length of the
cantilever is also an influential factor. Only the part
from the clamping edge to the force applying point pro-
duces a stress. Ramos et al. [7,8] analyzed the effect of
bacterial adsorption onto cantilevers and claimed that
the cantilevers’ response depends on the stiffness of
the sample as well as on its mass. The authors also re-
ported that a stiffer cantilever is relatively insensitive
to the mechanical properties of bioparticles but that its
mass sensitivity is limited. A pliant cantilever usually
has better mass sensitivity, but its response may be
complex [9].
The force applying point is again influenced by the
molecular weight of immobilised-antibody and antigen
and concentration of binding sites. The variation of can-
tilever response with molecular weight of the all immo-
bilised-antibodies is given in Figure 5. With a particular
antigen the output voltage during simulation decreases
with increase of molecular weight of the all immobi-
lised-antibodies except with IgM (Table 4, Figure 5).
However, stress decreases only up to the molecular
weight of 180000. Afterwards the stress increases with
increase of molecular weight of the immobilised-anti-
body (Figure 5). In the case of displacement, the dis-
placement decreases only up to the molecular weight of
190000. Afterwards the displacement increases with
increase of molecular weight of the immobilised-anti-
body (Figure 5). IgM having high molecular weight due
to polymeric nature, the relationship does not hold good.
With all other antibodies, the low molecular size influ-
ences the antigen-antibody interaction more effectively.
Antibody, IgG having low molecular weight yields
higher output voltage with Bacillus Anthrax Toxin which
has higher molecular weight in comparison with all other
antigens. Antibody, IgG yields lower output voltage with
Pseudomonas aeruginosa which has lower molecular
weight. While comparing the output voltage of all the
antibodies, IgG yields higher out put voltage. IgG having
low molecular weight may allow lesser strain on the can-
tilever beam in comparison with the high molecular
weight immunoglobulin. Immunoglobulin having low
molecular weight may favour molecular level changes in
terms of physical and morphological nature of adsorbed
It has been reported that the physical and morpho-
logical nature of the probe molecules influence the de-
flection. Hagan and Chakraborty have investigated the
effect of the structure of the adsorbed layer of probe
molecules on the cantilever on the transport of targets on
to the layer and subsequent organization of complement-
tary regions leading to nucleation and completion of
hybridization [1]. The nature of grafting of probe mole-
cules on the surface whether it is ordered or is disordered
influence the deflection [2]. Disordered distributions,
which are most likely in practice, lead to much larger
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cantilever deflections. Molecular level self-assembly con-
trols the response of the microdevice. The grafting den-
sity also influences the deflection [3]. At larger grafting
density hybridization of probe molecule does not occur,
but rather, the target chain can bridge across more than
one probe molecule.
Table 3. Response of immunoglobulins to 1 mole of antigen.
Analytical Simulation
Difference between simulation
and analytical
ment (nm)
ment (nm)
Displacement Output
voltage Stress
IgA immunoglobulin
I 0.318 13.30 5.7 0.063 5.06 5.8 -80.2 -61.5 1.8
II 0.150 5.49 2.3 0.026 2.09 2.5 -82.7 -61..9 8.7
III 0.265 9.97 4.2 0.047 3.80 4.5 -82.3 -61..9 7.1
IV 0.153 5.65 2.4 0.027 2.15 2.7 -82.4 -61..9 12.5
IgD immunoglobulin
I 0.308 12.61 5.7 0.170 15.88 4.6 -44.8 25.9 -19.3
II 0.142 5.20 2.3 0.071 6.55 1.9 -50.0 26.0 -17.4
III 0.234 9.46 4.2 0.130 11.91 3.4 -44.4 25.9 -19.1
IV 0.145 5.36 2.4 0.073 6.75 2.0 -49.7 25.9 -16.7
IgE immunoglobulin
I 0.310 12.94 5.7 0.051 5.83 5.8 -83.5 -55.2 1.8
II 0.146 5.34 2.3 0.021 2.40 2.4 -85.6 -55.1 4.3
III 0.240 9.71 4.2 0.038 4.37 4.4 -84.2 -55.0 4.8
IV 0.149 5.50 2.4 0.022 2.46 2.7 -85.2 -51.3 12.5
IgG immunoglobulin
I 0.295 14.01 5.7 0.220 22.1 6.0 -25.4 57.7 17.1
II 0.092 5.64 2.3 0.093 9.11 2.5 1.1 61.5 8.7
III 0.221 10.25 4.2 0.170 16.56 4.5 -23.1 61.6 7.1
IV 0.093 5.81 2.4 0.096 9.39 2.7 3.2 61.6 12.5
IgM immunoglobulin
I 0.029 12.30 5.7 0.077 6.93 5.8 165.5 -43.7 1.8
II 0.021 5.07 2.3 0.032 2.86 2.5 52.3 -43.6 8.7
III 0.030 9.22 4.2 0.057 5.20 4.7 90.0 -43.6 11.9
IV 0.021 5.23 2.4 0.033 2.95 2.8 57.1 -43.6 16.7
I: Bacillus Anthrax Toxin, II: Pseudomonas Aeruginosa, III: Coryne Bacterium Diptheria, IV: Treponema Pallidum Difference between simulation and ana-
lytical (%) = 100(zsim-zanal)/-zanal
Table 4. Variation of output voltage with molecular weight of antibody.
Output voltage with simulation (V) (x1011)
Bacillus Anthrax Toxin (I) 5.06 15.88 5.83 22.1 6.93
Pseudomonas Aeruginosa (II) 2.09 6.55 2.40 9.11 2.86
Coryne Bacterium Diptheria (III) 3.80 11.91 4.37 16.56 5.20
Treponema Pallidum (IV) 2.15 6.75 2.46 9.39 2.95
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Figure 4. Variation of cantilever response with molecular weight of antigens.
Openly accessible at
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Figure 5. Variation of cantilever response with molecular weight of immobilized-antibody.
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J. S. Swarrup et al. / Health 2 (2010) 1239-1248
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With IgG, the difference between analytical and simu-
lation values is positive especially with low molecular
weight and motile antigens Pseudomonas aeruginosa
and Treponema pallidum. The signal transduction is in-
fluenced by the motility of the target molecules. It has
been reported by Schnell and Turner [10] that if signal
transduction occurs on time scales that are slow com-
pared to the motility of the molecules and organelles that
constitute the crowding elements, the effects of crowd-
ing are qualitatively the same as in a homogeneous
3-dimensional medium. In contrast, if signal transduc-
tion occurs on a time scale that is much faster than the
time over which the crowding elements move, the ef-
fects of varying the extent of crowding are very different
when reactions occur both in 2 and 3-dimensional space.
For fast signaling, crowding agents attenuate signaling
and never enhances signaling. In contrast, slow signaling
cascades can be both enhanced and attenuated by
crowding agents [11]. Therefore for the signal transduc-
tion in the case of low molecular weight and motile an-
tigens Pseudomonas aeruginosa and Tr ep on ema pal-
lidum, IgG is an appropriate probe molecule.
The authors acknowledges the support provided by Prof. K. K. Ray,
Prof. Zachriah Alex and Dr. Gargi Raina School of Electrical Sciences,
IT Vellore.
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The theoretical and FEM simulations were carried out
for plain and perforated rectangular cantilever with dif-
ferent immunoglobulin antibodies for the detection of
bacterial antigens. The theoretical and computational ap-
proaches have identified important variables that affect
design of the cantilever beam which are essential for the
design of the prototype device. The cantilever beam with
perforation showed an improved response. With IgG, the
difference between analytical and simulation values is
positive and low especially with low molecular weight
antigens Pseudomonas aeruginosa and Treponema pal-
lidum. The studies reveal that simulated perforated rec-
tangular cantilever beam sensor (length-300 µm, width-
100 µm and thickness-40.5 µm) with IgG antibody is a
more promising system for the fabrication for the detec-
tion of highly motile Pseudomonas aeruginosa and Tre-
ponema pallidum. Monoclonal IgG antibody could be a
more suitable probe for the detection of Pseudomonas
aeruginosa and Treponema pallidum antigens.
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Openly accessible at