  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.
