Bio-inspired microneedle design for efficient drug/vaccine coating

Biomimetics is the interdisciplinary scientific field focused on the study and imitation of biological systems, with the aim of solving complex technological problems. In this paper, we present a new bio-inspired design for microneedles (MNs) and MN arrays, intended for rapidly coating the MNs with drug/vaccine. The biomimetic approach consists in ornamenting the lateral sides of pyramidal MNs with structures inspired by the external scent efferent systems of some European true bugs, which facilitate a directional liquid transport. To realize these MNs, two-photon polymerization (TPP) technique was used. Liquid coating capabilities of structured and non-structured MNs were compared. Moreover, both in-vivo and ex-vivo skin tests were performed to prove that MNs pierce the skin. We show that the arrays of MNs can be accurately replicated using a micro-moulding technique. We believe this design will be beneficial for the process of drug/vaccine loading onto the needles' surfaces, by making it more efficient and by reducing the drug/vaccine wastage during MN coating process.

The external scent efferent system of selected European true bugs (Heteroptera): a biomimetic inspiration for passive, unidirectional fluid transport

In this work, we present structured capillaries that were inspired by the microstructures of the external scent efferent system as found in different European true bug species (Pentatomidae and Cydnidae). These make use of small, orientated structures in order to facilitate fluid movement towards desired areas where defensive substances are evaporated. Gland channels and microstructures were investigated by means of scanning electron microscopy and abstracted into three-dimensional models. We used these models to create scent channel replicas from different technical substrates (steel and polymers) by means of laser ablation, laser structuring and casting. Video analysis of conducted fluid-flow experiments showed that bug-inspired, artificial scent fluid channels can indeed transport different fluids (water solutions and oils/lubricants) passively in one direction (velocities of up to 1 mm s-1), while halting the fluid movement in the opposite direction. At the end of this contribution, we present a physical theory that explains the observed fluid transport and sets the rules for performance optimization in future work.

Adaptive camouflage: what can be learned from the wetting behaviour of the tropical flat bugs Dysodius lunatus and Dysodiusmagnus

The neotropical flat bug species Dysodius lunatus and Dysodius magnus show a fascinating camouflage principle, as their appearance renders the animal hardly visible on the bark of trees. However, when getting wet due to rain, bark changes its colour and gets darker. In order to keep the camouflage effect, it seems that some Dysodius species benefit from their ability to hold a water film on their cuticle and therefore change their optical properties when also wetted by water. This camouflage behaviour requires the insect to have a hydrophilic surface and passive surface structures which facilitate the liquid spreading. Here we show morphological and chemical characterisations of the surface, especially the cuticular waxes of D. magnus Scanning electron microscopy revealed that the animal is covered with pillar-like microstructures which, in combination with a surprising chemical hydrophilicity of the cuticle waxes, render the bug almost superhydrophilic: water spreads immediately across the surface. We could theoretically model this behaviour assuming the effect of hemi-wicking (a state in which a droplet sits on a rough surface, partwise imbibing the structure around).  Additionally the principle was abstracted and a laser-patterned polymer surface, mimicking the structure and contact angle of Dysodius wax, shows exactly the behaviour of the natural role model - immediate spreading of water and the formation of a thin continuous water film changing optical properties of the surface.

Bioinspired polymer microstructures for directional transport of oily liquids

Nature has always served as an inspiration for scientists, helping them to solve a large diversity of technical problems. In our case, we are interested in the directional transport of oily liquids and as a model for this application we used the flat bug Dysodius lunatus. In this report, we present arrays of drops looking like polymer microstructures produced by the two-photon polymerization technique that mimic the micro-ornamentation from the bug's cuticle. A good directionality of oil transport was achieved, directly controlled by the direction of the pointed microstructures at the surface. If the tips of the drop-like microstructures are pointing towards the left side, the liquid front moves to the right and vice versa. Similar effects could be expected for the transport of oily lubricants. These results could, therefore, be interesting for applications in friction and wear reduction.

The toxicity of silver nanoparticles to zebrafish embryos increases through sewage treatment processes

Silver nanoparticles (AgNPs) are widely believed to be retained in the sewage sludge during sewage treatment. The AgNPs and their derivatives, however, re-enter the environment with the sludge and via the effluent. AgNP were shown to occur in surface water, while evidence of a potential toxicity of AgNPs in aquatic organisms is growing. This study aims to examine the toxicity of AgNPs to the embryos of the aquatic vertebrate model zebrafish (Danio rerio) before and after sewage treatment plants (STPs) processes. Embryos were treated with AgNP (particle size: >90 % <20 nm) and AgNO3 in ISO water for 48 h and consequently displayed effects such as delayed development, tail malformations and edema. For AgNP, the embryos were smaller than the controls with conspicuously smaller yolk sacs. The corresponding EC50 values of 48 hours post fertilization (hpf) were determined as 73 μg/l for AgNO3 and 1.1 mg/l for AgNP. Whole-mount immunostainings of primary and secondary motor neurons also revealed secondary neurotoxic effects. A TEM analysis confirmed uptake of the AgNPs, and the distribution within the embryo suggested absorption across the skin. Embryos were also exposed (for 48 h) to effluents of AgNP-spiked model STP with AgNP influent concentrations of 4 and 16 mg/l. These embryos exhibited the same malformations than for AgNO3 and AgNPs, but the embryo toxicity of the sewage treatment effluent was higher (EC50 = 142 μg/l; 48 hpf). On the other hand, control STP effluent spiked with AgNPs afterwards was less toxic (EC50 = 2.9 mg/l; 48 hpf) than AgNPs in ISO water. This observation of an increased fish embryo toxicity of STP effluents with increasing AgNP influent concentrations identifies the accumulation of AgNP in the STP as a potential source of effluent toxicity.

Moisture harvesting and water transport through specialized micro-structures on the integument of lizards

Several lizard species that live in arid areas have developed special abilities to collect water with their bodies' surfaces and to ingest the so collected moisture. This is called rain- or moisture-harvesting. The water can originate from air humidity, fog, dew, rain or even from humid soil. The integument (i.e., the skin plus skin derivatives such as scales) has developed features so that the water spreads and is soaked into a capillary system in between the reptiles' scales. Within this capillary system the water is transported to the mouth where it is ingested. We have investigated three different lizard species which have developed the ability for moisture harvesting independently, viz. the Australian thorny devil (Moloch horridus), the Arabian toadhead agama (Phrynocephalus arabicus) and the Texas horned lizard (Phrynosoma cornutum). All three lizards have a honeycomb like micro ornamentation on the outer surface of the scales and a complex capillary system in between the scales. By investigation of individual scales and by producing and characterising polymer replicas of the reptiles' integuments, we found that the honeycomb like structures render the surface superhydrophilic, most likely by holding a water film physically stable. Furthermore, the condensation of air humidity is improved on this surface by about 100% in comparison to unstructured surfaces. This allows the animals to collect moisture with their entire body surface. The collected water is transported into the capillary system. For Phrynosoma cornutum we found the interesting effect that, in contrast to the other two investigated species, the water flow in the capillary system is not uniform but directed to the mouth. Taken together we found that the micro ornamentation yields a superhydrophilic surface, and the semi-tubular capillaries allow for an efficient passive - and for Phrynosoma directed - transport of water.

Slippery surfaces of pitcher plants: Nepenthes wax crystals minimize insect attachment via microscopic surface roughness

Pitcher plants of the genus Nepenthes efficiently trap and retain insect prey in highly specialized leaves. Besides a slippery peristome which inhibits adhesion of insects they employ epicuticular wax crystals on the inner walls of the conductive zone of the pitchers to hamper insect attachment by adhesive devices. It has been proposed that the detachment of individual crystals and the resulting contamination of adhesive organs is responsible for capturing insects. However, our results provide evidence in favour of a different mechanism, mainly based on the stability and the roughness of the waxy surface. First, we were unable to detect a large quantity of crystal fragments on the pads of insects detached from mature pitcher surfaces of Nepenthes alata. Second, investigation of the pitcher surface by focused ion beam treatment showed that the wax crystals form a compact 3D structure. Third, atomic force microscopy of the platelet-shaped crystals revealed that the crystals are mechanically stable, rendering crystal detachment by insect pads unlikely. Fourth, the surface profile parameters of the wax layer showed striking similarities to those of polishing paper with low grain size. By measuring friction forces of insects on this artificial surface we demonstrate that microscopic roughness alone is sufficient to minimize insect attachment. A theoretical model shows that surface roughness within a certain length scale will prevent adhesion by being too rough for adhesive pads but not rough enough for claws.