## Materials and Methods
### Experimental Techniques  
Microstructural observations of the cryo-fractured surfaces of the 3D-printed specimens were made to investigate the quality of the printing process and the interfacial interaction between microcapsules and the TPU matrix. A Zeiss Supra 40 (Zeiss, Oberkochen, Germany) high-resolution field-emission scanning electron microscope (FESEM), operated at an accelerating voltage of 2.5 kV was used. Samples were observed after the deposition of a thin platinum/palladium conductive coating on their surfaces. Differential scanning calorimetry (DSC) measurements of the 3D-printed specimens were made by using a Mettler Toledo DSC30 calorimeter (Mettler, OH, USA). Specimens with a mass of about 10 mg were analyzed under a nitrogen flow of 150 mL/min, applying a first heating run from \(-\)100 to 220 \({}^{\circ}\)C, followed by a cooling step from 220 \({}^{\circ}\)C to \(-\)100\({}^{\circ}\)C and a second heating run until 220 \({}^{\circ}\)C. All these scans were performed at 10 \({}^{\circ}\)C/min. The glass transition temperature (Tg) of TPU was evaluated as the inflection point in DSC thermogram, while the melting (Tm) and the crystallization (Tc) temperatures of M6D capsules within the nanocomposites were determined in accordance with the endothermic and exothermic peaks, respectively. The specific melting enthalpy (\(\Delta\)Hm) and crystal  
Figure 1: FFF machine equipped with a lab-made air-cooling system for the filament to avoid its failure via buckling. lization enthalpy (\(\Delta\)Hc) values were determined as the integral of their corresponding peaks, while the relative melting and crystallization enthalpy values of M6D within the nanocomposites were obtained by dividing \(\Delta\)Hm by \(\Delta\)Hc for the melting/crystallization enthalpy of the neat M6D capsules (equal to 158.4 J/g; see ref. [34]), taking into account the effective microcapsule concentration. Only one specimen was tested for each sample. Thermo-gravimetric analysis (TGA) was carried out with a TA Instruments TGA Q5000 thermostbalance (TA Instruments, New Castle, DE, USA) under a nitrogen flow of 100 mL/min and in a temperature range from 30 \({}^{\circ}\)C to 700 \({}^{\circ}\)C at a heating rate of 10 \({}^{\circ}\)C/min. Only one specimen was tested for each composition. Dynamical mechanical analysis (DMA) was carried out using a TA Instruments DMA Q800 (TA Instruments, New Castle, DE, USA) analyzer in tensile configuration, testing rectangular 3D-printed specimens 30 mm long, 5 mm wide, and 1 mm thick. The testing temperature range was set between \(-\)60 \({}^{\circ}\)C and 100 \({}^{\circ}\)C, the heating rate was equal to 3 \({}^{\circ}\)C/min, the testing frequency was 1 Hz, and the strain amplitude was 0.05 mm/mm. Using this approach, the trends of the dynamic moduli (E\({}^{\prime}\),E\({}^{\prime\prime}\)) and the loss tangent (tan\(\delta\)) as a function of temperature were highlighted. Moreover, the glass transition temperature of the TPU (Tg) was determined to be the inflection point of E\({}^{\prime}\) curves. Only one specimen was tested for each composition. The mechanical behavior of the 3D-printed materials was investigated through quasi-static tensile tests conducted at room temperature, carried out according to ASTM D638 standard [40] using an Instron 5969 universal tensile testing machine (Instron, Norwood, MA, USA) equipped with a load cell of 50 kN. All samples were tested with a crosshead speed of 100 mm/min. A long-travel extensometer Instron 2603-080, with a gauge length of 25 mm, was used to monitor deformation during the tests. At least five specimens were tested for each composition in order to determine the chord modulus at a deformation of 10% and the stress and the strain at break. The specific fracture energy, representing the energy required for material failure divided by the cross-sectional area, was also determined.