When the capsules were dried, they adopted the bowl-like shape which made an exact measurement of wall thickness difficult. Apparently, the thickness of the capsule wall increased to 55 -85 µm, which corresponds to approx. two times the wall thickness, which would be expected when the shell bulges. The permeability of the microcapsule shell was studied using fluorescent dyes of different sizes (Figure and Supporting Information Videos). A small molecule like fluorescein (340 Da) could diffuse into the capsules, as shown in the supporting information video V1 and Figure . However, FITC-labelled dextran (40 kDa) did not diffuse into the cavity (Error! Reference source not found.). Similarly, when FITC-labelled dextran was encapsulated in the inner cavity during capsule formation, the macromolecular cargo did not diffuse out of the capsules (Error! Reference source not found.). Thus, small molecules such as fluorescein permeated through the APCN capsule wall, while the APCN was impermeable to the macromolecular dextran. As 40 kDa dextran has a radius of 4.5 nm, the microcapsules should likewise retain similarly-sized colloids. The influence of dehydration-rehydration cycles on the permeability of the microcapsule shell was studied with encapsulated fluorescein as a model compound. NR-labelled empty capsules were incubated for 30 min in a fluorescein solution to load them with the fluorescent dye, then centrifuged, quickly washed, and resuspended in water. Encapsulated fluorescein was then allowed to diffuse out of the capsules for 10 min. The capsules were sedimented by mild centrifugation, the supernatant was collected, and its fluorescence was measured. To normalize the fluorescence of released fluorescein to the concentration of capsules in the sample, the microcapsules were resuspended in water and collected to measure their NR fluorescence (Figure , Entry: 0 Rehydration cycles; Figure ). The same fluorescein uptake and release procedure was carried out with APCN microcapsules that had been dried and rehydrated for one or more cycles (Figure ! Reference source not found.). The experimental results show that dried and rehydrated microcapsules released more fluorescein than the never dried ones. However, the release did not increase with increasing number of drying-rehydration cycles. As it can be assumed that the amount of fluorescein released after a fixed amount of time is proportional to the permeability of the shell, the results suggest that the permeability increased once upon drying and rehydration and remained constant in the subsequent cycles. Two possibilities could account for these results. Either the first drying induced small defects in the capsule shell that do not enlarge or increase in number in further drying events. However, such defects are not observed in the SEM images of the capsules, and it is also unlikely that additional drying steps would not damage the shell further. Alternatively, the arrangement of the hydrophilic PHEA and the hydrophobic PDMS domains in the APCN might change upon the first drying and rehydration step, for example by increasing the co-continuity of the two polymer phases in the APCN, thereby making it easier for the water-dissolved fluorescein to diffuse through the water-swollen microcapsule shell. One possible application of APCN microcapsules is as microcompartments for chemical reactions. To explore this potential application, the capsules were loaded with platinum nanoparticles (PtNPs) that are catalysts, for example, for the decomposition of hydrogen peroxide into oxygen and water. PtNPs were added to the inner aqueous phase during capsule formation by microfluidic double emulsification.