## 2 Materials and Methods
### Micro-Mechanical Compression Testing  
Historically, techniques for testing the mechanical properties of microcapsules, as reviewed by [PERSON] et al., while comprehensive, often relied on intricate compression geometries to discern the microcapsules' behaviors, from elasticity upon contact to failure behaviors. Moreover, finite element modeling (FEM) has been instrumental in extrapolating stress-strain relationships for core-shell structures [13; 14]. Notable works, such as [PERSON] et al., have provided insights into microcapsules' mechanical characterization and failure stress estimation [23]. However, while these methods have advanced our understanding, they often lack the precision and sensitivity required for nuanced analysis. Figure 2: Schematic representation of wall formation via in situ polymerization. Transitioning to our approach, we recognized the need for a more refined and precise methodology. Nanoindentation measurements were performed using a nanoindenter iNano (KLA Corp., Milpitas, CA, USA) equipped with an inForce50 actuator (KLA Corp., Milpitas, CA, USA) to measure the compressive strength of the hollow polymeric microspheres. The experimental setup utilizes the nanoindenter actuator with a flat punch diamond tip with a diameter of 100 \(\upmu\)m, which applies controlled compression to the microspheres until rupture occurs. The microcapsule slurry was diluted in 5 mL of deionized water with a dilution ratio of 1:1000. Small droplets of the diluted solution were deposited on laboratory slides previously glued on top of nanoindentation stubs with hot mounting compounds. The solvent evaporated before testing. The compression process consists of two distinct phases. In the initial phase, the displacement of the punch is controlled, ensuring precise measurement and control of the applied force. The surface identification procedure begins with the initial determination of the substrate position in an area adjacent to the microsphere of interest. Once the substrate position is identified, the system is positioned above the microsphere, and the indenter tip oscillates at the resonant frequency specific to the nanoindentation system being used. These oscillations are performed under force control, with an amplitude of approximately \(\pm 10\) nm around zero position. The compression phase is conducted without imposed oscillation, and rupture is identified through the derivative of the load signal. Rupture manifests as a sudden change in the load-depth curve. In load-controlled instruments, this sudden variation corresponds to a rapid change in displacement, while in displacement-controlled instruments, it corresponds to an immediate change in the applied load. The employed instrument implements a pseudo-displacement-controlled protocol between the two characteristics above. Figure 3 illustrates the relationship between load and depth as the compression of a single microcapsule progresses. The different point labels are linked to the time-lapse of the compression, which is presented in Figure 4.  
Figure 3: Load vs. Depth compression test of a single microcapsule. Stages comprise: (a) microcapsule approach by nanoindenter tip; (b, c), the tip comes into contact with the microcapsule, leading to a subsequent increase in load as the microcapsule is compressed; (d) the microcapsule ruptures; Subsequently, at point (e), the tip compresses the substrate, resulting in an exponential rise in Load. To determine the diameter of the microsphere, a mechanical approach is utilized. The displacement position at which the surface is identified is compared to the displacement position at which the substrate is initially detected. Compression tests are conducted at a constant velocity of 2 \(\mu\)m/s. This uniform velocity ensures that any potential effects related to strain rate on the behavior of the polymeric membranes, which are sensitive to such factors, are decoupled and minimized.