## 3 Results and Discussion  
As it will be shown later in this manuscript, using LIPEs as templates gave access to a high degree of versatility in terms of formulations and functionalization. As with any other emulsion, LIPE is made of two immiscible phases that can be possibly loaded by any compatible fillers. Therefore, functionalities can be imparted to the polymer matrix and the porosity in a specific and selective way. (The high volume and mass fraction of the oil phase makes possible a high relative loading of the polymer matrix generated by conversion of the LIPE into porous HCPEG via photopolymerization. This level of loading would not be achievable in the case of a high internal phase emulsion due to the reduced volume of the oil phase. As anticipated previously, the material processability was investigated and tested towards several runs of microneedles arrays fabrication by replica molding, the main aspects of which will be later discussed more in detail. In order to fabricate practical microneedles arrays, the formulation of the low internal phase emulsion was optimized to reach the best compromise between mechanical strength and porosity once photo-crosslinked into a monolithic porous polymer material. Ideally, the Young's Modulus of a microneedle should be 1.0 GPa or more to enable skin piercing [47]. On the other hand, the presence of open and dense porosity was also desirable from many applicative standpoints, e.g., microneedles patch for drug delivery, body fluids suction, and bio-sensing devices. Eventually, we optimized a material featuring dense meso-porosity without compromising the mechanical performance in respect to the bulk plastic material. SEM morphological analysis (Figure 1b) of our porous HCPEG showed an inner porosity composed of several sub-micrometric pores randomly distributed throughout the material. Nevertheless, a closer look at the material by means of TEM morphological analysis (Figure 1c,d) clearly revealed nano-pores throughout the inner structure. It was evident from the morphological analysis that this material featured micro and nano-porosities. Nanoindentation mechanical characterizations of both porous and non-porous HCPEG showed a decrease in Young's Modulus from 2.16 GPa down to 1.63 GPa (Figure 2) because of the porous inner structure. Despite this remarkable decrease, the Young's Modulus was still well above the 1.0 GPa threshold. Notably, the Young's Modulus did not scatter throughout all the indentation depth, indicating that the porosity was homogenous throughout the inner structure of porous-PEG on the nanoindenter length scale. So far, we have achieved a material showing mechanical strength, micro, and nano-porosity. A closer inspection of the porosity by SEM (Figure 1b) shows mesopores sitting on the walls of micropores (indicated by the red arrows in the insert). Seemingly, the morphological analysis indicated an open porous structure. Following this conclusion, we further tested it by carrying out molecular permeability studies. Specimens of the monolithic material (porous and non-porous) were immersed in sulforhodamine B solutions and their inner structures were afterward observed with confocal fluorescence microscopy (Figure 3a-c). Notably, the fluorescence signal coming from the probe molecules was found to be evenly distributed in the porosity throughout the material. At the same time, the bulk non-porous plastic material did not allow probe fluorescent molecules to penetrate the inner structure (Figure 3a). It is obvious that in presence of a closed porous structure the fluorescent probe molecules must be found just in proximity of the specimen surface, as it happens for the bulk non-porous material. Nevertheless, the fluorescent probe can be found in the internal pores of the specimen only if the pores are somehow interconnected and molecules can freely move throughout the material. On this basis, we deduced that molecules may permeate throughout the polymer mesh with the pores acting as micro/nano-flasks.