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): ajay@smart.mit.edu, (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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