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Iversen, et al.
A B C D D

G F E
G F E

H

H

Figure 1. Schematic depicting the fabrication process of the on-skin patch using BIOX printer: (A) printed polydimethylsiloxane
substrate, (B) attachment of copper sheet wires, (C) printing of Ag electrodes, (D) printing Single-walled carbon nanotubes for pH sensing,
(E) printing of encapsulation layer, (F) connecting female pins and (G) connecting the socket to the copper sheet wires. (H) Exploded view
of the patch with printed components.
were kept in the buffer solution until the measured Ω
resistance obtained a steady state, after which they Sensitivity = pH
were transferred to another buffer solution with a
different pH. The buffer cycle followed the change Sensitivity = mV
from pH 9.18 – 6.86 – 4 – 6.86 – 9.18. To measure the pH
sensitivity of the fabricated sensor, similar method was
employed except that the pH of buffer solutions used 2.8. Offset drift
were 5.8, 6.2, 6.6, 7, 7.7 and 7.8.
The patch was submerged in a beaker filled up to ¾ of
2.4. Hydration sensor buffer solution with pH 6.82 for 8 days. The beaker was

To measure the change in the resistance, patch was placed sealed off with parafilm to prevent evaporation of buffer
on a hotplate at 37°C to simulate normal skin temperature. and measurement was taken once in a day.
A piece of fabric was placed onto the patch to avoid pre-
load. A total of 300 µL (3 × 100 µL) of a buffer solution 3. Results and discussion
with a pH of 6.86 was dropped on the fabric at different 3.1. FEA results
intervals in a span of 15 min.
FEA simulations were performed to investigate the
2.5. Repeatability stress around a wound when a load was applied to the
The patch was continuously shifted 12 times between skin. The central idea was to observe the effect of patch
buffer of pH 4 and 6.86 over 90 min to obtain resistance shape on the stress around the wound site. Reduced
versus time plot. stress in a wound is crucial to lower the potential of
fracturing a wound, thereby reducing the healing time.
2.6. Response time Wounds are reported to heal about 3 times faster in
Data of resistance detection in buffer solution with absence of any mechanical stress [56,57] . Further, it reduces
different pH were used to find out the response time by the chances of reinfection [58] . Shape of the type II patch
calculating the mean time taken to stabilize the patch on was found to relieve the maximum stress applied on
changing the pH. the skin with a reduction of 66% (Figure S1). Oval
shape of type II patch distributed the stress over a large
2.7. Sensitivity area as indicated by lighter blue color in Figure S3 and

Sensitivity was calculated using two equations given reduced the maximum stress at the narrowest points of
below: the wound.

International Journal of Bioprinting (2022)–Volume 8, Issue 1 43