Paper
RSC Advances
technique lies on the simplicity of its fabrication method, as it
requires only a PDMS stamp, with the designed features, along
with the paper and ink. In contrast to the view that printing
techniques using standard inks are not suitable for the
fabrication of paper-based microfluidic devices,16 we demon-
strate that, by taking advantage of the absorbing capability of
filter paper, it is possible to create paper-based microfluidics
by simple contact stamping. While the use of indelible ink to
create microfluidic structures has been reported pre-
viously,19,20 to the best of our knowledge this is the first time
that direct contact stamping of paper-based microfluidics has
been performed using commercially available ink.
version 2012 and converted to the final toolpaths using
AutoDeskE HSMxpress.
The inking process was performed by gently pushing the
PDMS stamp three times against the ink saturated stone pad.
This step was followed by exposing the inked PDMS to air for 5
s, in order to remove any air bubbles formed on top of the
PDMS stamp. The presence of air bubbles on the surface of the
inked PDMS occurs as a consequence of the air that is expelled
from the pores of the stone during the inking process. Finally
the PDMS stamp was placed in contact with the laboratory
filter paper for three seconds, without the application of any
force.
The glucose paper based assay was performed by spotting 2
mL of a fresh phosphate buffered saline (PBS) solution (pH =
7.4) containing 67 U mL21 of glucose oxidase (GOx), 100 U
mL21 of Horseradish peroxidase (HRP), 0.3 mM Trehalose, 6
Materials and methods
The microfluidic platform was fabricated using standard
laboratory filter paper (WhatmanE grades 1 and 595), which
adsorbs the ink through its full thickness and defines the
borders and so the flow channels of the microfluidic structure.
Three different inks were examined: Black fountain pen ink
from Noodler’s InkTM (Product Code: 19001), Black 214 Stamp
Ink from HITT Marking Devices Inc. and LumocolorE
Permanent Universal Black Ink (kindly provided by Staedtler
Mars GmbH & Co. KG). LumocolorE ink viscosity was reduced
by using a solvent mixture made of a 1 : 1 v/v ethanol-n-
propanol, 10 : 1 ink-solvent mixture. The main attractive
characteristics of the listed inks are their hydrophobic nature
when dry, and short drying times.
mM
4-Aminoantipyrine
(4AAP)
and
12
mM
is
4-Hydroxybenzoic acid (HBA). All the chemical were purchased
form Sigma-Aldrich and used as received. Functionalised
paper-based microfluidic devices were dried at RT and used
the same day. Photographs of the paper-based microfluidic
devices were taken using a Canon PowerShot G7 camera in a
controlled light intensity area. After transferring the images to
the computer, the analysis of the sensing area colour change
was performed using a script and functions on Matlab
R2007bE (The MathWorks, Inc., MA, USA).
Results and discussion
The contact stamping is performed using PDMS stamps
(10 : 3 w/w monomer-curing agent), cured at 60 uC for 8 h. In
order to control the volume of ink transferred from the PDMS
stamp to the filter paper, the inking of PDMS is performed
using a stone ink-pad (299 6 499 rectangle from HITT Marking
DevicesE). Based on the company specifications, the ink-pad is
capable of providing a constant ink flow from the porous stone
when in contact with the stamp. Moreover, the ink-pad can be
used with solvent/acid based industrial inks, which cannot be
employed with standard pads. The PDMS stamp was incorpo-
rated onto a custom made rectangular prism made of
aluminum and a layer of a black ceramic material (see Fig.
Ink selection
During the evaluation of the most suitable ink for the
stamping process, the three selected inks (see above) were
tested by simply drop casting 1 mL of each onto WhatmanE
filter paper grade 1. The Noodler’s InkTM sample was found to
be unsuitable for stamping the filter paper for two reasons.
Firstly, a yellowish ring (most probably due to separation of
components in the ink) formed around the outer rim of the
ink spot (see Fig. S2, ESI ) and secondly, despite its
hydrophobic nature, this ink did not coherent hydrophobic
3
barriers in the paper (see Video 1 ESI ). For these two reasons
3
it was decided not to use Noodler’sTM Black Ink for further
experiments. In contrast, neither the Black 214 nor the Black
LumocolorE showed this yellowish ring after being drop cast
on paper.
S1, ESI ). The dimensions of the prism are 4.35 6 5.1 6 7.6
3
cm (H 6 W 6 L) with a total weight of 440 grams. The
stamping device (PDMS stamp/rectangular prism) will be
referred to as PDMS stamp from now on.
After the ink stains were dried at room temperature for 5
min (enough time to assume that the solvent of the inks is
completely evaporated), the hydrophobicity of the two inked
papers was then characterised by placing a 2 mL drop of DI
water on top. Black 214 is described by the manufacturer as a
‘‘waterproof ink with excellent adhesion’’, while Black
LumocolorE ink is similarly claimed to be a water- and
weather-proof ink with drying time of seconds. However,
Black 214 did not make the paper hydrophobic enough,
displaying similar behaviour to Noodler’sTM Black Ink.
Furthermore, a yellowish substance leached out from the
inked area just after the DI water droplet was absorbed by the
The PDMS negative molds were fabricated through a
micromiller (CAT3D, Datron, UK) using 4 mm thick PMMA
(poly methyl methacrylate) master as substrate, although other
materials could be employed. First, a 3 mm flat endmill was
used to shave off 300 mm of material in order to flatten the
PMMA surface. Following this, a 600 mm deep pocket of
suitable x and y dimensions was milled out. Using flat
endmills of appropriate dimensions, the required negative
mold was milled out starting from the bottom of the pocket.
All designs were performed using Solid WorksE Student
18812 | RSC Adv., 2013, 3, 18811–18816
This journal is ß The Royal Society of Chemistry 2013