Challenges and opportunities for small volumes delivery into the skin.

Each individual's skin has its own features, such as strength, elasticity, or permeability to drugs, which limits the effectiveness of one-size-fits-all approaches typically found in medical treatments. Therefore, understanding the transport mechanisms of substances across the skin is instrumental for the development of novel minimal invasive transdermal therapies. However, the large difference between transport timescales and length scales of disparate molecules needed for medical therapies makes it difficult to address fundamental questions. Thus, this lack of fundamental knowledge has limited the efficacy of bioengineering equipment and medical treatments. In this article, we provide an overview of the most important microfluidics-related transport phenomena through the skin and versatile tools to study them. Moreover, we provide a summary of challenges and opportunities faced by advanced transdermal delivery methods, such as needle-free jet injectors, microneedles, and tattooing, which could pave the way to the implementation of better therapies and new methods.

Spontaneous water adsorption-desorption oscillations in mesoporous thin films.

Understanding fluid transport and phase changes in nanopore structures is of great interest to many application fields, from energy conversion to water harvesting. This work discusses the spontaneous oscillations of the water saturation of mesoporous thin films, in the zone adjacent to a sessile water drop, at ambient conditions. The wetting-front dynamics onto the film is described by considering three coexisting phenomena: infiltration from the water drop, condensation from air vapor, and evaporation to the ambient. It was found that the oscillations follow spontaneous condensation-evaporation imbalances, which are governed by the hysteretic character of the adsorption-desorption behavior of the mesoporous material. The outcomes of this work provide insights on the complex interplay between water and nanopore structures, which has practical implications for the handling of humid microenvironments in lab-on-a-chip technology, as well as for many processes that take part of the cycle of water in nature.

An Open Pit Nanofluidic Tool: Localized Chemistry Assisted by Mesoporous Thin Film Infiltration.

Nanofluidics based on nanoscopic porous structures has emerged as the next evolutionary milestone in the construction of versatile nanodevices with unprecedented applications. However, the straightforward development of nanofluidically interconnected systems is crucial for the production of practical devices. Here, we demonstrate that spontaneous infiltration into supramolecularly templated mesoporous oxide films at the edge of a sessile drop in open air can be used to connect pairs of landmarks. The liquids from the drops can then join through the nanoporous network to guide a localized chemical reaction at the nanofluid-front interface. This method, here named "open-pit" nanofluidics, allows mixing reagents from nanofluidically connected droplet reservoirs that can be used as reactors to conduct reactions and precipitation processes. From the fundamental point of view, the work contributes to unveiling subtle phenomena during spontaneous infiltration of fluids in bodies with nanoscale dimensions such as the front broadening effect and the oscillatory behavior of the infiltration-evaporation front. The approach has distinctive advantages such as easy fabrication, low cost, and facility of scaling up for future development of ultrasensitive detection, controlled nanomaterial synthesis, and novel patterning methods.

Modeling the abnormally slow infiltration rate in mesoporous films.

Mesoporous films have been shown to exhibit striking behaviors in capillary-driven infiltration experiments. The process has been shown to follow classical Lucas-Washburn dynamics, but the effective pore radius has been calculated from hydrodynamic resistance considerations to be orders of magnitude lower than measured pore dimensions. In addition, the infiltration rate has been observed to decrease with increasing pore diameter, in contrast to the expected trend for capillary-like pores. Here, we present a simple model accounting for the mechanism behind these anomalous effects. We found the infiltration rate to be inversely proportional to the cubed ratio of pore to neck size. This physical scaling correctly modeled both the magnitude of the infiltration rate and its variation with pore diameters, for a wide range of experimental data. The model established a connection between capillary filling dynamics and nanoscale pore structure, which is of practical interest for the design and characterization of mesoporous films.

Complex Filling Dynamics in Mesoporous Thin Films.

The fluid-front dynamics resulting from the coexisting infiltration and evaporation phenomena in nanofluidic systems has been investigated. More precisely, water infiltration in both titania and silica mesoporous films was studied through a simple experiment: a sessile drop was deposited over the film and the advancement of the fluid front into the porous structure was optically followed and recorded in time. In the case of titania mesoporous films, capillary infiltration was arrested at a given distance, and a steady annular region of the wetted material was formed. A simple model that combines Lucas-Washburn infiltration and surface evaporation was derived, which appropriately describes the observed filling dynamics and the annulus width in dissimilar mesoporous morphologies. In the case of wormlike mesoporous morphologies, a remarkable phenomenon was found: instead of reaching a steady infiltration-evaporation balance, the fluid front exhibits an oscillating behavior. This complex filling dynamics opens interesting possibilities to study the unusual nanofluidic phenomena and to discover novel applications.