## 2 Experiments and Methods  
### Materials and Sample Preparation  
The chemical reagents used in this experiment include chitosan (deacetylated \(\geq\) 75%) and glacial acetic acid (CH\({}_{3}\)COOH), both sourced from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China, primarily for preparing the chitosan solution. Calcium chloride (CaCl\({}_{2}\), purity \(\geq\) 96%) and sodium bicarbonate (NaHCO\({}_{3}\), ACS grade) were also obtained from the same supplier and used as reactants in the mineralization process. Sodium hydroxide (NaOH, ACS grade, 97%, flakes) was procured from Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China and utilized in the preparation of chitosan hydrogels. All reagents were used as received, without further purification, and deionized water was employed throughout the experiment. The preparation of chitosan hydrogels involved dissolving chitosan powder in the CH\({}_{3}\)COOH solution to create a homogeneous solution, followed by film formation using 3D printing technology. The chitosan molecules were then coagulated in a NaOH solution, resulting in the formation of a three-dimensional network structure [28; 29; 30]. The detailed procedure was as follows: First, an appropriate amount of chitosan powder was weighed and gradually added to a 0.5 M CH\({}_{3}\)COOH solution to prepare an 8% chitosan solution by weight. The mixture was ultrasonically stirred at room temperature to ensure the complete dissolution of the chitosan, resulting in a uniform solution. To prevent bubbles from affecting the hydrogel's structure during molding, the solution was placed in a degassing mixer until all bubbles were eliminated. The degassed chitosan solution was then used to create chitosan films through a 3D printing technique, utilizing a self-assembled extrusion 3D printer in the laboratory. Given the viscosity of the chitosan solution, a balance between printing speed and needle aperture size was critical. If the printing speed was too fast or the needle aperture too small, the extrusion of the solution became discontinuous or failed altogether, preventing film formation. Conversely, if the printing speed was too slow or the needle aperture too large, aggregation occurred, affecting the uniformity and size of the film. After optimization, the printing speed was set to 3 mm/s, and a 400 \(\upmu\)m needle was chosen. Due to the mobility of the slurry, when extruded from the needle of the 3D printer, it spreads on the glass sheet, making the chitosan filament size larger than the 400 \(\upmu\)m pinhole. If the printing pitch is too small, it causes the excessive aggregation of the chitosan solution, while too large pitch prevents the extruded chitosan filament from spreading to form a film. After testing, it was found that the optimal printing pitch for forming the film is 800 \(\upmu\)m. The size of the films, as shown in Figure 2a,b, allows for subsequent mechanical testing and characterization (Figure S5). The dried chitosan film samples were neutralized using a 1M NaOH solution at room temperature for 6 h. After gelation, the chitosan hydrogel samples were thoroughly rinsed with deionized water to remove any residual NaOH, and then stored in deionized water for future use. The in situ growth of CaCO\({}_{3}\) nanocrystals in the previously prepared chitosan hydrogel was conducted using an ion diffusion technique as follows: First, the chitosan hydrogel sample was immersed in a 0.1M CaCl\({}_{2}\) solution for 24 h. Ca\({}^{2+}\) was introduced into the hydrogel network by diffusion, forming a Ca\({}^{2+}\)-loaded chitosan hydrogel. This was followed by a biomimetic mineralization reaction, where the Ca\({}^{2+}\)-loaded chitosan hydrogel samples were immersed in a 0.1 M NaHCO\({}_{3}\) solution for reaction times ranging from 5 min to 12 h, resulting in CaCO\({}_{3}\) nanocrystals of varying sizes and morphologies produced at different time intervals. The mineralization reactions were conducted at room temperature.