## 2 Materials and Methods
### Characterization of the Filaments and the 3D-Printed Objects  
Thermal characterization was performed using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). For the DSC measurements, about 3 mg of samples were placed in aluminum crucibles, next were sealed hermetically, and were tested in a Q200 (TA Instruments, New Castle, DE, USA) apparatus from 25 to 300 \({}^{\circ}\)C at a heating rate of 10 \({}^{\circ}\)C/min and under nitrogen atmosphere at a flow rate of 50 mL/min. TGA measurements were conducted with a TG Q500 instrument (TA Instruments, New Castle, DE, USA). In the case of filaments testing, approximately 45 mg mass obtained from five different sites of each filament was placed in a platinum pan and heated over a temperature range from 25 to 900 \({}^{\circ}\)C at a heating rate of 10 \({}^{\circ}\)C/min, under synthetic airflow of 50 mL/min.  
The morphological characteristics of the fillers, filaments, and the printed products were examined with Scanning Electron Microscope (SEM) using a Zeiss SUPRA 35 VP instrument (Zeiss, Oberkochen, Germany) at an acceleration voltage of 10 kV equipped with an energy dispersive X-ray (EDX) analysis system. Samples were mounted on aluminum stubs using a conductive silver paste (RS 186-3600) and gold-coated before the examination. Furthermore, HAp crystals were examined using Transmission Electron Microscopy (TEM) (JEM 2100, JEOL, Akishima-Shi, Japan). The size of the crystals was measured from the corresponding SEM or TEM images using the image analysis software ImageJ.  
The purity of raw materials and the crystallinity of filaments and printed objects were assessed by Powder X-ray Diffractometry (XRD) by using a D8 Advance instrument (Bruker AXS, Karlsruhe, Germany). Data were acquired with Ni-filtered CuKa1 radiation, operating at 40 kV and 40 mA at a scanning speed of 0.6 s/step and a step size of 0.02\({}^{\circ}\). Phase identification was done by the Bruker software Eva.  
Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopy applied to assess possible interactions between polymers or polymers and filler materials. The spectra were recorded using an IRTracer-100 (Schimadzu, (Shimadzu, Tokyo, Japan) FTIR spectrometer equipped with the ATR accessory MIRacle(tm) Single Reflection (Madison, WI, USA). The spectra were acquired after 50 scans at a resolution of 4 cm\({}^{-1}\) and in the spectral range between 550 and 4000 cm\({}^{-1}\).  
Tensile strength tests were carried out at room temperature on pure PLA and PLA/PCL filaments using an Instron universal testing machine (Instron 5582, Norwood, MA, USA) at a crosshead displacement rate of 5 mm/min. Tensile mechanical properties were calculated as a mean value of five specimens for each composition. Piezoelectric measurements were performed by a PM300 piezometer (Piezo Test LTD, London, UK). The longitudinal piezoelectric coefficient (d\({}_{33}\)) was measured by applying a force of 0.25 N at a frequency of 110 Hz.  
Electrical response of nanocomposites was examined through Broadband Dielectric Spectroscopy (BDS) in the frequency range from 0.1 Hz to 10 MHz, using an Alpha- N Frequency Response Analyzer and a BDS 1200 dielectric cell both supplied by NovothermSystem (Novocontrol Technologies, Hundsagen, Germany). The temperature set at 37 \({}^{\circ}\)C and was controlled by a Novotherm System. The number of measuring points during each frequency scan were 70. The calibration procedure was open terminal auto calibration as suggested by the Frequency Response Analyzer's manufacturer.