2.1 Digital model development

A 3D scanner was employed to capture the anatomical features of the spinal body part. The scanning process ensured high precision and accuracy, producing a digital representation of the patient upper body. Once acquired, the data was processed using modeling software that employs dynamic mesh techniques such as Autodesk Meshmixer (Autodesk Inc., San Rafael, CA, USA) to generate a positive model, which served as the foundation for subsequent design stages. The positive model was then digitally transformed, partitioned, and topologically optimized according to the patient's needs using the computer aided design (CAD) software Autodesk Fusion 360 (Autodesk Inc., San Rafael, CA, USA). This process culminated in an assembly of well-defined parts, forming a negative mold suitable for brace manufacturing.

Two distinct software tools were used to design the embedded structures within the final model. The gyroid TPMS lattice was designed using the Functional Lattice Package (FLatt Pack) program (2021, University of Nottingham),²³ with a unit cell size that goes from 4 to 8 mm and a volume fraction of 0.25. The lattice structure was constructed as a linear repetition of cells along the custom-selected mesh body. The kirigami square-based auxetic structures were created through generative design using Rhinoceros® and its integrated parametric counterpart Grasshopper (Robert McNeel & Associates, Seattle, WA, USA), featuring a cell size of 8 mm and a thickness of 1.5 mm. Both structures were integrated into the assembly in accordance with the selected shape directly exported from the negative brace model.

2.2 Materials

Two thermoplastic filaments are employed for the components of the designed spinal brace. The first is medical grade Polypropylene (PP) with density of 0.9 g/cm³ and 1.75 mm diameter (P-lene 4, Treed Filaments, Italy), which was used as hard polymer filament. The second material is a soft medical grade thermoplastic polyurethane (TPU) with 80 Shore A, 1.22 g/cm³ material density, and 1.75 mm filament diameter (FlexMark8, Treed Filaments, Italy). All materials were used without further modifications and handled with gloved hands to minimize any possible contamination.

2.3 Printing technology and assembly of the brace

Each component of the brace was 3D-printed using the MEX technology through Original Prusa XL (Prusa Research, Prague, Czech Republic). The Cartesian printer has a maximum building volume of 360 × 360 × 360 mm³ and operates at a layer resolution of 0.1–0.3 mm, using a nozzle with a diameter of 0.4 mm. The optimized digital model was exported in stereolithography (.stl) format and sliced using PrusaSlicer software to be printed. To ensure the adhesion of the structures to the surface of the printing platform, a layer of commercial duct tape was applied on the heated print sheet. The 3D-printed components were manually assembled to form the complete orthopedic brace.

2.4 Breathability performance of 3D-printed brace

The breathability performance of the 3D-printed brace was evaluated in comparison to two traditional braces (perforated texture brace and no-perforated texture brace). The primary objective of this test was to quantify the breathability of the TPMS-integrated structure within the brace. Each brace sample was placed in direct contact with a beaker containing 100 mL of distilled water. The system was then positioned on a hot plate set to 37 °C to simulate physiological conditions. The test was conducted over a 24-h in a controlled environment with a relative humidity (RH) of 52 %. At the end of the test, the amount of evaporated water was determined by measuring the weight difference of the system between the initial time point (t0) and the final time point (t24). To ensure statistical significance, each experiment was performed in triplicate. The permeability of the TPMS-based porous scaffold was calculated using the following equation:permeability=WVP·thicknesswhere WVP represents the water vapor permeance measured at the end of the experiment, and thickness refers to the thickness of both the 3D-printed and traditional braces, which is 5 mm.

2.5 In vitro simulation of physiological diaphragm displacement

To simulate physiological diaphragm displacement in vitro – characterized by a 2.5 cm expansion and a corresponding air volume of 0.5 L – the 3D-printed TPU kirigami structure was affixed to a balloon and subjected to a constant pressure of 5 cm H2O. This pressure was maintained using an EO-150 pulmonary ventilator, which also enabled real-time monitoring to ensure system integrity and prevent leakage. Displacement of the kirigami structure during the ventilation test was quantitatively analyzed using Kinovea software. Each experiment was performed in triplicate to ensure reproducibility.

3.1 Digital design

After obtaining the positive model from the 3D scan of the spinal body part (Fig. 2A), a crucial phase that involves the complete personalization of the spinal brace shape and support function is the modeling of the negative mold. Truly, the core strength of digital design is that therapy adjustments or growth-related adjustments can be made directly on the virtual model in a limited amount of time.

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Fig. 2 (A) Positive surface generated from the 3D-scanned trunk. (B) Refined, closed mesh of the negative model extracted and elaborated from the positive surface. (C) Optimized spinal brace model with highlighted the support zones (in blue) and the containment zones (in green).

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Firstly, to achieve a high degree of control in modeling the brace's surface, a uniform, high-resolution mesh surface is generated in Autodesk Meshmixer. Actually, the digital model consists of a large number of small facets of the same size, typically represented by three- or four-sided polygons. A high resolution ensures a highly detailed surface structure, preserving fine details, contours, and shapes from the original data but, at the same time, a large quantity of polygons produces complex files which need a high amount of time to be processed. Truly, a balancing between detail and computational cost is performed to provide both precise and easy processable models.

Next, the main shape of the brace is extracted from the positive model, ensuring a smooth outline and providing a shape that protects the underarm and hip areas, which is essential for patient comfort during fitting, with final dimensions of 208 × 176 × 358 mm (Fig. 2B). Thanks to the prior surface optimization, the negative model surface can be successfully exported in “.stl” format and imported into Autodesk Fusion 360. Here, an optimized surface thickness is selected. By leveraging the CAD software capabilities, a uniform thickness of 4 mm is chosen, resulting in a less invasive and lighter design compared to traditional braces, which can reach 7–8 mm in thickness.

Once achieved the general shape of the brace, the model was further optimized through the integration of advanced structural elements to enhance material efficiency while preserving structural integrity. To accomplish this topological optimization, the brace was segmented into specific regions, each assigned distinct structural characteristics, namely postural support and containment, to balance support, flexibility, and patient comfort as depicted in Fig. 2C.

The remaining section of the model forms the structural backbone of the brace, thus providing the necessary framework for overall stability and support. To enhance both mechanical performance and weight distribution, these regions were also filled with advanced structural elements. The intricate lattice structure was strategically oriented to follow the natural curvature of the brace, ensuring seamless integration with the design while keeping structural efficiency. Indeed, the brace achieves an optimal balance between strength, flexibility, and material efficiency, further improving patient comfort and guaranteeing postural stability while sitting. Lastly, the backbone of the brace was designed with buttonholes and Velcro openings to accommodate the fastening of soft inserts and secure the brace to the patient.

3.1.1

3.1.1 Triply periodic minimal surface (TPMS) structures for postural balance

The side areas of the brace (namely support zone), responsible for hip stabilization, and the mid-lower back region of the spine emerged as critical zones for posture balance, providing strength and stability. These regions require both high support and rigidity to ensure the brace functions correctly, preventing undesired deformations of the orthosis caused by the patient's body forces.

To meet these demands while keeping low weight and breathability, these originally solid areas were replaced with a gyroid TPMS matrix directly embedded in the core structure of the spinal brace. By using the Flatt Pack program, it is possible to tune the geometrical parameters of these lattices, starting from the type of lattice, hence the shape of the cells, to the cell size, allowing control over the dimension of the porosities. Additionally, the volume fraction can be adjusted to modify the wall thickness of the lattice, ensuring either greater resistance or enhanced breathability, tailored directly to the patient's needs for optimal performance. Moreover, the software enables the filling of a customized “.stl” file with lattices and allows the orientation of the selected structure to follow the natural curvature of the orthosis, thereby improving the overall performances of the brace.

The TPMS gyroid structure was selected due to its isotropic properties,²⁴ distinctive of this lattice type. Lattices with different cell sizes were chosen to accommodate the anatomical variations of different regions (Fig. 3A). A cell size of 5 mm was used in the lateral hip-placement sections, where stability and resistance are of primary importance, while a cell size of 8 mm was employed in the back region to enhance breathability (Fig. 3B). Both configurations are characterized by a volume fraction of 0.25 to preserve the stretching-dominated properties of the lattice, which exhibit a linear scaling behavior of strength and stiffness with relative density.²⁴ Moreover, the final meshes were selected, exported, and digitally assembled with the core body of the brace without skin to ensure air permeability.

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Fig. 3 (A) Gyroid single cell and accordingly generated hard section for the lower back postural support of the spinal brace. (B) Magnified view of the gyroid lattice structures inserted in the spinal brace, featuring an 8 mm cell size for the lower back zone and a 5 mm cell size for the lateral hip-placement sections. (C) Permeability performance of different spinal brace designs. The 3D-printed braces with TPMS gyroid structures are shown with an 8 mm cell size (back region, in red) and a 5 mm cell size (lateral hip placement, in blue). These are compared to a traditional brace with perforated texture (4 mm pores, in black) and a non-perforated traditional brace (in green).

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Additionally, gyroid TPMS structures were integrated also into the skeletal frame of the spinal brace to enhance mechanical performance by distributing forces evenly while minimizing material usage thus weight. In this case, the cell size corresponds to the thickness of the brace (4 mm), and the volume fraction remains 0.25 to maintain structural continuity with other regions. Here, the mesh was selected with skin to provide a smoother surface for improved comfort during fitting.

Next, it has been evaluated the breathability of the gyroid TPMS matrix integrated directly into the core structure of the spinal brace, comparing their performance to two traditional designs. All samples were tested over a 24-h period on a hot plate maintained at 37 °C and 52 % RH to simulate human skin conditions.

Fig. 3C illustrates the breathability results for the various designs: the 3D-printed brace with a 8 mm cell size (back region, shown in red), the 3D-printed brace with an 5 mm cell size (lateral hip placement, shown in blue), a traditional perforated texture brace with a 4 mm of dimensions of breathable pores (shown in black), and a traditional brace with no perforated texture (shown in green). Notably, the TPMS gyroid matrix exhibited a significant increase in breathability (0.0008 g/Pa·s·m and 0.0013 g/Pa·s·m for 8 mm and 5 mm cell size, respectively) compared to the traditional design with perforated texture (0.00001 g/Pa s m); as expected the traditional brace with no perforated texture it turned out to be non-breathable. This improvement is attributed to the greater distribution of engineered interconnected porosities of the gyroid matrix, which facilitates greater mass transport across the brace. Overall, these findings highlight the critical role of design parameters – particularly the gyroid configuration – in enhancing brace breathability, thereby potentially reducing discomfort during extended use.

3.1.2

3.1.2 Auxetic kirigami structures for supporting breathing and digestion

The upper back region (namely containment zone) of the spine and the abdominal zone primarily serve a containment function rather than requiring rigid support. In these areas, excessive stiffness could hinder natural physiological movements such as breathing and digestion. To address this issue was taken inspiration from architecture and civil engineering facades, the rigid structure was replaced with bendable, soft kirigami square-based auxetic pattern inserts. These structures exhibit a negative Poisson's ratio behavior, allowing them to expand and contract dynamically in response to body movements. This improves patient comfort and adaptability while further reducing the overall weight of the brace. Moreover, the kirigami auxetic pattern was selected because it can efficiently accommodate the deformation of the skin following volume variations caused by physiological motions.^25,26

The kirigami auxetic pattern was modeled using Rhino and its integrated parametric counterpart, Grasshopper, serving of the Linketix plug-in. A rectangular geometry was selected, with both length and width set to 16 mm, corresponding to the cell size, leading to each individual square in the pattern measuring 8 mm per side. An offset of 1/24 of the cell size was applied to each square element along all perimeters, except for the edges to guarantee durability at multiple deformations. The number of repetitions was determined based on the shape and extension of the soft insert that must be added to the brace. The final pattern was then extruded by 1.5 mm to preserve flexibility and provide a minimally invasive solution for the patient. Finally, the pattern was exported and inserted into an opportunely modeled portion, which will later be attached to the core of the brace using a button-and-loop fastening system. Fig. 4A illustrates the kirigami square-based auxetic cell and the tailored inserts.

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Fig. 4 (A) Auxetic kirigami cell and accordingly modeled soft inserts for the abdominal and upper back containment area of the spinal brace. (B) Expansion of kirigami auxetic structure under simulated physiological diaphragm displacement; the kirigami cells exhibited displacements ranging from 1.89 ± 0.21 mm to 4.18 ± 0.1 mm, demonstrating their ability to support diaphragm movement and adapt to volumetric changes during breathing.

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To evaluate the auxetic behavior of the 3D-printed TPU-based kirigami structure under dynamic conditions mimicking physiological diaphragm displacement, an in vitro tests using a controlled pulmonary ventilator was conducted (for further details see Materials and Methods). Within the diaphragmatic expansion range of 0.5–2.5 cm, the kirigami cells exhibited displacements ranging from 1.89 ± 0.21 mm to 4.18 ± 0.1 mm (Fig. 4B). This dynamic response supports diaphragm movement and effectively accommodates the skin deformation induced by volume changes during breathing. These findings highlight the innovative potential of integrating auxetic kirigami structures into 3D-printed brace, offering enhanced conformability and improved comfort.

3.2 Material selection

In view of the fact that the spinal brace is designed with two specific functional zones (i.e., support and containment zones), to ensure accurate performances, two different types of materials were selected. Initially, polylactic acid (PLA) and polyethylene terephthalate glycol (PETG) filaments were considered for the production of the rigid components (data not shown). PLA was accounted since it is the most commonly used FFF filament material, allowing for the rapid and cost-effective production of a wide variety of objects and devices. On the other hand, PETG was deemed due to its high strength and durability, as it has already been used in the production of 3D-printed orthopedic braces and orthoses.⁹ However, both materials proved unsuitable for this application because, despite their toughness, they were brittle and prone to breaking under the minimal deflection. For this reason, PP was chosen instead. Although slightly more expensive, it offers superior properties such as high toughness and flexibility,²⁷ allowing the brace to bend without breaking, which is a required characteristic to properly fit the brace, while recovering its original shape after removing the stimulus. For the soft sections, TPU 80 Shore A was chosen due to its high elasticity, characterized by good resistance to hysteresis cycles and a deformation exceeding 250 %.¹³ Additionally, once printed, it features a smooth finish, which is particularly useful for ensuring comfort.

3.3 3D printing and assembly of the customized spinal brace

Two distinct 3D printing processes were utilized to produce both hard and soft components, ensuring the desired printing quality and functionality. For the hard section of the brace, the.stl file was imported into the slicer, where the printing parameters for PP were fine-tuned to accommodate the specific size and shape of the piece. First, the model was optimally oriented and tilted within the build area to minimize the number of organic supports along the z-direction, thereby improving material consumption, surface finishing, and printing time. Due to the chemical nature of PP, which bonds only with itself, a 5 mm brim was added as a printing parameter, and a thin layer of adhesive PP tape was placed on the printing plate. A build platform temperature of 68 °C was selected to enhance the model adhesion. The extrusion multiplier was set to 1.2, while an extrusion temperature of 255 °C and a fan speed of 80 % were chosen to obtain the highest possible printing definition. A coasting distance of 1 mm and a retraction of 4 mm were applied to prevent the formation of material drops and accumulations during layer deposition. The layer height was set to 0.3 mm due to the brace's dimensions, balancing printing resolution, especially for TPMS-filled sections, with limited printing time. A layer width of 0.44 mm was selected to ensure homogeneous infill and high resolution for each layer. The infill density, using a gyroid pattern, was fixed at 13 % to reduce the overall weight of the brace and improve comfort. An outline overlap of 30 % was applied to prevent under-extrusion and avoid overheating issues. Given the rigid nature of PP, a printing speed of 2400 mm/min was chosen to optimize manufacturing time without compromising detail quality. Additionally, the fan speed was set to 80 % from the second layer onward to enhance uniformity and overall printing quality. For sake of clarity, the PP printing parameters are summarized in Table 1. After 35 h, the hard section of the spine brace was successfully printed. The time required to remove the supports and refine the edges of the model did not exceed 30 min. A preliminary visual inspection of the finished part confirmed that the fine-tuned parameters resulted in a well-printed brace with a stable structure, precise texture, and highly detailed features.

Conversely, TPU printing parameters were established based on previously reported research.²⁸ The soft, deformable regions were sliced in a planar configuration on x-y axis to avoid the need for support structures. Moreover, the material adhered to the build plate without requiring any external agent. Table 1 outlines the printing parameters for TPU soft components. The printing speed was reduced to 800 mm/min specifically for small sections, such as button-like features, to ensure the necessary accuracy and definition. The manufactured parts were printed within a few hours, resulting in a smooth surface free of defects or lumps. This provided a perfect interlock with the hard matrix of the brace while maintaining the required flexibility to fulfill their restraining function.

Lastly, the printed parts of the brace were assembled aligning the hard matrix and soft components, therefore ensuring seamless integration of all parts to provide a customized fit for the patient. After a qualitative assessment, the final prototype (Fig. 5A) resulted in a spine brace that conforms to the patient's anatomy while maintaining structural integrity, thanks to the gyroid TPMS structures, which provide adequate support. Furthermore, the 3D-printed spinal brace demonstrated a deformation capacity of up to 580 % in its rigid PP component (Fig. 5B). This exceptional flexibility significantly enhances the brace's wearability, particularly in pathological conditions where patients are bedridden or exhibit limited residual mobility, ensuring improved comfort and adaptability to the body's movements. Lastly, the incorporation of kirigami auxetic structures enhances comfort by effectively reducing pressure on sensitive areas.

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Fig. 5 (A) Final 3D-printed brace with hard and soft sections assembled, and a wearing test on the corresponding orthopedic manikin. (B) Deformation images of the rigid polypropylene (PP) component of the 3D-printed spinal brace, showing a maximum strain capacity of up to 580 % obtain by Kinovea analysis.

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Overall, the AM process offers significant advantages over traditional production methods, particularly in terms of precision, efficiency, and biomechanical performance. The manual thermoforming process requires highly skilled technicians to ensure a perfect fit to the patient's anatomy while maintaining a consistent thickness, a limitation that can be effectively addressed through digital modeling and 3D printing, ensuring greater reproducibility and repeatability. The FFF process enables cost-effective production of complex parts, allowing for the creation of lighter and more comfortable braces by optimizing material distribution and adapting both the inner and outer geometries to the patient's pathological needs. The incorporation of locally tuned advanced structures, such as TPMS for structural support and auxetic designs for flexibility, enhances patient comfort by improving breathability, heat dissipation, and localized adaptability to movement, reducing pressure points while maintaining postural stability. From a biomechanical perspective, these structures enable a more anatomically responsive fit, improving load distribution and ensuring better force absorption and dissipation, which reduces the risk of discomfort, pressure sores, and long-term musculoskeletal complications. Lastly, 3D-printed spinal brace is environmentally friendly, as AM minimizes material waste compared to traditional subtractive methods, while the ability to fabricate braces on demand supports a more sustainable and patient-specific approach to orthopedic care.