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Biomimetic Hydrogel-CNT Network Induced Enhancement of Fluid-
Structure Interaction for Ultrasensitive NanosensorsMeghali Bora1†, Ajay Giri Prakash Kottapalli1*†, Miao Jianmin2, Michael S. Triantafyllou3
1Center for Environmental Sensing and Modeling (CENSAM) IRG
Singapore-MIT Alliance for Research and Technology (SMART),
1 Create Way, Create Tower, Singapore 1386022School of Mechanical & Aerospace Engineering, Nanyang Technological University,
50 Nanyang Avenue, Singapore 6397983Department of Mechanical Engineering, Massachusetts Institute of Technology,
77 Massachusetts Avenue, Cambridge, MA 02139, USA*Corresponding author (Email): [email protected], (Tel): +65 65165702
Supporting Information
Biomimetic NEMS sensor fabrication
Figure S1 shows a schematic of all the unit processing steps involved in NEMS flow sensor
fabrication. Aligned PVDF nanofibers were formed by placing the aluminum collection foil (as
shown in Figure S1a) on the rotating electrode of a far field electrospinning setup. Spinning for
duration of 30 min produced a PVDF membrane of thickness ~20 µm. The PVDF nanofiber film
was then transferred on to an AB-1170 optically clear adhesive (OCA) film (ThermoFisher
Scientific, Singapore), which is a pressure sensitive transparent film of 100 µm thickness. This
OCA film was punched with an array of through holes of 2 mm diameter as shown in Figure S1b.
The aluminum foil with the nanofibers was placed on the OCA film with the side containing
nanofibers facing the film.
As shown in Figure S1c, upon application of pressure, the nanofibers were transferred to the
OCA film due to its pressure sensitive adhesiveness. Following the transfer of fibers, another OCA
film prepared with a similar pattern of cavities as the first film was positioned on top of the fibers.
The circular cavities of 2 mm diameter on the top and bottom OCA films were aligned with each
other as shown in Figure S1d. Application of pressure sealed the top and bottom OCA films
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sandwiching the PVDF nanofiber membrane. A series of conducting copper tapes mounted on both
sides of each circular membrane formed the contact pads for each sensor.
Vertically standing high aspect ratio CNTs with high degree of alignment were grown on a
silicon substrate through atmospheric pressure chemical vapor deposition (APCVD) process. The
APCVD growth of the VACNT bundles was conducted at CVD Equipment Corp, New York. A 2
nm Fe / 20 nm Al2O3 (iron terminated) metal precursor was deposited by electron beam evaporation
and patterned into circular patterns of diameter 350 µm. The distance between consecutive
precursor patterns was kept the same as the center-to-center distance between the PVDF membranes
to allow a direct transfer of the CNTs after growth. The VACNTs were grown at a temperature of
750 °C for a total duration of 3 hours.
In order to transfer the VACNTs, a micro-drop of EPO-TEK-H20E non-conductive epoxy
was drop-cast at the center of each PVDF membrane. The silicon substrate with VACNTs was
carefully aligned and positioned on the PVDF membranes using a precise X-Y-Z position
controller. The distal tips of VACNTs were brought in contact to the epoxy and the epoxy was
cured for 12 hours at 55 °C. Removal of silicon wafer after curing transferred the VACNTs onto the
membranes while breaking them off at the root where they were connected to the silicon substrate
(Figures S1e and f). Individual devices were then diced out.
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Figure S1: Schematic of the fabrication process flow. (a) Aligned PVDF nanofibers formed on
an aluminum substrate through electrospinning. (b) OCA film with an array of cavities of 2 mm and
copper contacts. (c) Transfer process involving pressure application and peeling of the aluminum
foil. (d) Alignment of the second OCAS film on top of the first and pressure application to seal. (e)
Transfer of the array of VACNT bundles on to the PVDF membranes using a micro-drop of non-
conductive epoxy followed by curing. (f) Dicing to separate the individual sensors
Hydrogel network structure parameters
Table S1 summarizes the approximate values of the various network structure parameters
calculated from swelling study of HA-Tyr hydrogels.
Table S1: Network structure properties of HA-Tyr hydrogels
Parameter Value
Mass swelling ratio 145
Equilibrium water content 99 %
Molecular weight between crosslinks 47.5 X 105 g/mol
Crosslinking density 0.26 X 10-6 mol/cm3
Pore size 7 µm
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Additional sensor testing results
Repeatability of Sensitivity enhancement due to HA-Tyr hydrogel cupula in air and water
The data below presents additional repeats of the experiment in Figures 4d and e, which describe
the sensitivity enhancement due to the presence of the HA-Tyr hydrogel cupula. Five sensors,
which feature a naked VACNT hair cell and another five sensors with HA-Tyr hydrogel dressed
VACNT structures were used for these experiments. In each experiment, a pair of sensors
consisting of one sensor of each type is located equidistance from the dipole in both air and water
ambiences. The average experimental sensitivity enhancement with the presence of the hydrogel
dressing is 2.5 times in air and 8.1 times in water. The sensitivity enhancement over three repeats of
experiments on three pairs of sensors (one of each kind) was highly repeatable with a percantage
error of only 4% and 1.8% in air and water respectively.
Figure S2: Enhancement in sensor output of HA-Tyr cupula-dressed VACNT sensor in
comparison to naked VACNT sensor in air and water flow at different amplitudes and a fixed
Sensitivity enhancement = 8.14
Sensitivity enhancement = 8.16
Sensitivity enhancement = 2.4Sensitivity enhancement = 2.6
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frequency of 35 Hz (a) 41 mVpp in air (b) 56.6 mVpp in air (c) 707.2 mVpp in water (d) 848.7
mVpp in water.
Original sensor output recording for Figures 4b and c Figure S3 below shows the original
sensor output (at 35 Hz stimulus signal) for the lowest and highest velocity points in the air and
water flow calibration plots in Figures 4b and c.
Figure S3: As-recorded sensor output at (a) 5 mm/s air flow (lowest velocity) (b) 72 mm/s air
flow (highest velocity) (c) 5 mm/s water flow (lowest velocity) (d) 95 mm/s water flow (highest
velocity).
Additional data of repeats of experiments presented in Figure 5
The experimental results of two repeats of the air pulse experiment (Figure 5a of the manuscript) on
two sensors are presented below in Figure S4.
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Figure S4: Air pulse detection. Response of the sensor to air pulses of velocity (a) 5 mm/s and (b)
10 mm/s.
The experimental results of two repeats of the hand pass experiment (Figure 5b of the manuscript)
are presented below in Figure S5.
Figure S5: Hand pass detection. Response of the sensor to a hand past at a distance of (a) 30 cm
and (b) 10 cm away from the sensor.
The experimental results of two repeats of the human subject walking towards the sensor (Figure 5c
of the manuscript) are presented below in Figure S6.
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Figure S6: Human subject walking toward the sensor. Sensor output as a human subject walked
towards the sensor to a distance of 100 cm from the sensor at (a) lower speed (b) faster speed.
The experimental results of five passes of the human subject walking parallel to the sensor (Figure
5c of the manuscript) at a distance of 100 cm from the sensor is shown below in Figure S7.
Figure S7: Human subject walking parallel to the sensor five times at a distance of 100 cm
from the sensor.
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Flow velocity field generated by a dipole
In the experiments conducted in this manuscript, a vibrating sphere (dipole) was used as a stimulus
to generate the fluid flow velocity. The flow velocity generated is directly proportional to both, the
frequency of vibration of the dipole and the amplitude of vibration of the dipole. This can be seen
through the equations below, which describe the flow velocity generated by a vibrating dipole at
any location of the sensor in the vicinity of the dipole. Therefore, the velocity generated by the
vibrating dipole can be varied either by sweeping the frequency, or the amplitude of vibration.
The parallel and perpendicular components of the flow velocity generated by a vibrating dipole at
any position of the x-y plane (where the sensor is located) are given by [1].
V ∥ , x ( x )=ωa A3[(2x2−P2)
{x2+ P2 }52 ]
and
V⊥ , x (x )= 3 ωa A3 Px
{x2+P2 }52
where A is diameter of dipole, f is frequency of oscillation of the dipole, a is displacement
amplitude of the dipole, P is observation distance (distance between dipole source and sensor), and
ω is the angular frequency (ω = 2πf).
Supplementary Video captions:
Video 1 showing the displacement of naked VACNT bundle
Video 2 showing the displacement of hydrogel cupula dressed VACNT bundle
References
[1] Asadnia, M. Kottapalli, A. G. P. Miao, J. M. Warkiani, M. E. & Triantafyllou, M. S. Artificial
fish skin of self-powered micro-electromechanical systems hair cells for sensing hydrodynamic
flow phenomena. J. R. Soc. Interface 12, 1-14 (2015). DOI: 10.1098/rsif.2015.0322.
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