It
is well-known that polymer brushes can degraft in aqueous liquids.
Here we show that brushes can deteriorate in humid air too. We observe
that the detachment rate of the brushes increases with increasing
relative humidity and hydrophilicity of the brushes. We relate this
to the increase in water absorption as these parameters are increased.
Our results imply that protective measures that are at present being
developed for applications of brushes in liquids will also be key
in enabling the long-term storage and utilization of hydrophilic brushes
in air.
Sensor
platforms can benefit from the incorporation of polymer
brushes since brushes can concentrate the analyte near the sensor
surface. Brushes that absorb acetone vapor are of particular interest
since acetone is an important marker for biological processes. We
present a simple procedure to synthesize acetone-responsive poly(methyl
acrylate) brushes. Using spectroscopic ellipsometry, we show that
these brushes respond within seconds and swell by more than 30% when
exposed to acetone vapor. Moreover, quartz crystal microbalance measurements
demonstrate that the brushes can be exploited to increase the acetone
detection sensitivity of sensors by more than a factor 6. Surprisingly,
we find that the swelling ratio of the brushes in acetone vapor is
independent of the grafting density and the degree of polymerization
of the polymers in the brush. This is qualitatively different from
swelling of the same brushes in liquid environments, where the swelling
ratio decreases for increasing grafting densities. Yet, it indicates
that the brushes are robust and reproducible candidates for implementation
in vapor sensor systems.
High speed imaging was used to capture the fast dynamics of two injection methods. The first one and perhaps the oldest known, is based on solid needles and used for dermal pigmentation, or tattooing. The second, is a novel needle-free micro-jet injector based on thermocavitation. We performed injections in agarose gel skin surrogates, and studied both methods using ink formulations with different fluidic properties to understand better the end-point injection. Both methods were used to inject water and a glycerin-water mixture. Commercial inks were used with the tattoo machine and compared with the other liquids injected. The agarose gel was kept stationary or in motion at a constant speed, along a plane perpendicular to the needle. The agarose deformation process due to the solid needle injection was also studied. The advantages and limitations of both methods are discussed, and we conclude that micro-jet injection has better performance than solid injection when comparing several quantities for three different liquids, such as the energy and volumetric delivery efficiencies per injection, depth and width of penetrations. A newly defined dimensionless quantity, the penetration strength, is used to indicate potential excessive damage to skin surrogates. Needle-free methods, such as the micro-jet injector here presented, could reduce the environmental impact of used needles, and benefit the health of millions of people that use needles on a daily basis for medical and cosmetic use. arXiv:1811.00510v1 [physics.app-ph] 1 Nov 2018
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