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3D Printed Electronic Patch for pH and Hydration Sensing
There has been some work done in fabricating of stress, and the shape was used to fabricate the wound
pH-sensing wound patches based on fibre optics , patch in this work.
[29]
ion-sensitive field-effect transistor , nuclear magnetic
[30]
resonance , Ultraviolet-visible spectroscopy [32,33] , near- 2.2. Fabrication of patch
[31]
infrared spectroscopy , Raman spectroscopy [35,36] , Firstly, the substrate was fabricated according to the
[34]
potentiometric sensor , pH-responsive dyes and stimulated design. For this purpose, polydimethylsiloxane
[37]
quantum dots [38-48] . Despite the efforts, there are limitations (PDMS) (Sylgard 184, DOW, Diatom, Denmark) was
of reliability, limited pH range, low sensitivity, complex used and mixed with a curing agent (10:1 ratio) to give a
design, autofluorescence, use of toxic dyes, and high solution with viscosity ~3.5 Pa.s. To adjust the viscosity
production cost. Therefore, there exists a need to develop of the solution, silicon dioxide nanoparticles (SiO NPs)
an easy-to-apply, non-toxic and low-cost system, which (5 – 20 nm, Merck Life Science, Denmark) were added
2
can quantify both wound exudate and pH levels in a in different ratios (Figure S2). The mixture was heated
wound. at 80°C for 10 min to get a homogenous solution. To
This work demonstrates the potential of flexible and remove trapped air and bubbles, solution was centrifuged
printed electronics for integrated wound monitoring by at 6000 rpm for 10 min followed by degassing at 50°C.
engineering an on-skin platform. The approach adopted The patch was printed using Celllink’s BIOX bioprinter
in this work was based on the development of printed with a mixture containing 20 wt% NPs. Substrate and
on-skin sensor patch, which can provide a simple result top layer were printed using nozzle size and pressure of
for the pH of the wound fluid. Single-walled carbon 22 G, 110 kPa and 25 G, 190 kPa, respectively, and a
nanotubes (SWCNT) were used for pH sensing owing speed of 8 mm/s. Ag ink (Smart Fabric Ink tc-c4007, TC)
to their excellent sensitivity. In earlier reported work, was used for printing electrodes and hydration sensor.
a composite of carbon nanotubes (CNT) conductive Printing was performed using nozzle size of 25 G with
polymers were used to increase the sensitivity but speed and pressure of 5 mm/s and 200 kPa, respectively.
required the use of reference electrode, which is known to The temperature of printbed was kept at 60°C. The pH
suffer from leakage of electrolyte [49,50] . Here, the change sensor was fabricated by printing SWCNT aqueous ink,
in CNT resistance is measured as a function of H ion (0.2 mg/mL, Merck Life Science, Denmark) using an
+
concentration without the requirement for a reference inkjet printhead. A total of 30 layers were printed for
electrode [51,52] . The patches are also capable of stimulating homogeneity and interconnection between SWCNTs with
the wound electrically for direct wound management a speed and pressure of 8 mm/s and 13 kPa, respectively.
using hydration sensing mechanism. Fabricated Valve open and cycle time was 1 ms and 100 ms,
multifunctional patches show satisfactory repeatability, respectively. The temperature of the printbed was kept as
reusability, and response performance. To the authors 60°C with a curing time of 60 s. The complete fabrication
knowledge, this is the first report on combining the two process is depicted in Figure 1.
functionalities of pH and hydration sensing in a single The complete patches were designed with four
patch for wound management. We have used the printing printed layers of PDMS modified with SiO NPs as
route to fabricate the wound patch due to its attractive substrate material and commercial Ag and SWCNT
2
features of low cost, material saving, and fast processing. inks (Figure 1H). Components 1-4 and 6 were printed
Printed technologies are a fast-growing area with many and collectively called as patch while components five,
techniques under its umbrella namely, material jetting, seven, and eight were used as attachments to carry out
vat polymerization, and material extrusion [53-55] . We the measurements without interfering with the printed
have chosen material extrusion as the preferred mode of
printing due to the range of the viscosity of the materials parts. Top insulated layer (1 of Figure 1H) was designed
used in this work. Material extrusion is also promising to to support the structure. The presence of three pockets
create planar platforms with multi-material printing. allows easy access of fluid for pH and hydration sensors
(2 and 4 of Figure 1H).
2. Materials and methods 2.3. Resistance detection in buffer solution with
2.1. Finite element analysis (FEA) simulation different pH

Solidworks 2019 – 2020 student edition was used for Patches were tested in a buffer solution (McIlvaine
the simulations of all models (Figure S1). Same forces buffer, Merck Life Science) and resistance change
and fixtures were applied to all models. Patches were was measured with a digital Keithley model 2110. The
subjected to forces in two directions: horizontal and sensor output was recorded using Kickstart (version 2.4)
vertical, as marked by yellow and green colored bars. It software. The sampling rate was 1 kHz, and the data was
was found that the patch type II has the largest reduction reviewed with the software KickStart 2. The patches

42 International Journal of Bioprinting (2022)–Volume 8, Issue 1