Structural optimization of porous CPC scaffolds and the effect of eliminating the outer wall on mechanical properties for bone regeneration

Additive manufacturing was utilized to fabricate rotationally symmetrical scaffolds from CPC, which exhibit sufficient mechanical stability to function as bone replacement and possess sufficient accessible surface area for subsequent release of active ingredients. An existing geometry was further developed for this purpose. The experimental protocol entailed an initial phase of solidification in an atmosphere saturated with water, followed by a post-solidification phase in Phosphate Buffered Saline (PBS). Furthermore, a pause was inserted after every five layers during three-dimensional plotting, and the green bodies were sprayed with water. The study also investigated the influence of water content on mechanical strength. A comprehensive examination of the test specimens was conducted under macroscopic, microscopic, and mechanical scrutiny. The scaffolds demonstrated an adequate capacity to withstand a load of 2,000 N (N). Subsequent to consolidation in Phosphate Buffered Saline (PBS), there was no observed increase in the maximum tolerated force. At this breaking load, the majority of test series exhibited an average deformation of 5%. The resultant stiffness was measured at 1,100 MPa. Consequently, the samples exhibited a strength level that was lower than that of spongy bone. The investigation revealed that the novel geometry, featuring an open outer ring, exhibited adequate mechanical stability while concomitantly augmenting the surface area accessible from the exterior for subsequent drug release. The advent of mass production with the new geometry is now a possibility.

1 Introduction

The treatment of substantial or infected bone defects remains a significant challenge in the field of regenerative medicine. The materials used for bone replacement must meet specific criteria to ensure their effectiveness and safety. These materials must be biocompatible and osteoconductive, meaning they must be able to integrate with the body’s natural biological processes without causing an immune response (Qin et al., 2024; Santoro et al., 2025). Additionally, they should exhibit sufficient mechanical stability to prevent degradation or structural failure within the body. If they are biodegradable, they should be sufficiently mechanically stable at the outset and degrade at the same rate as the new bone is formed (Tajvar et al., 2023; Liang et al., 2024). Ideally, these materials would also serve as carrier systems for local drug delivery, allowing for targeted treatment and reduced side effects (Skrinda-Melne et al., 2024). Calcium phosphate cements (CPCs) are promising due to their chemical similarity to the mineral bone matrix, robust bioactivity, and capacity to undergo in situ hardening (Vorndran et al., 2013; Vorndran et al., 2015; Bertrand et al., 2023; Blankenburg et al., 2022; Bohner et al., 2005). According to Van de Belt et al. (2000), approximately 90% of orthopedic surgeons in the United States utilize antibiotic-loaded bone cement. A study from 2025 also confirms the simplicity of mixing CPC pastes with antibiotics in the operating room (Zeiner et al., 2025). However, it should be noted that these systems are not without their limitations. A significant limitation shared by this and other contemporary systems is the suboptimal release kinetics of the incorporated antibiotics (van de Belt et al., 2000; Kanellakopoulou and Giamarellos-Bourboulis, 2000; Díez-Peña et al., 2002). Consequently, the minimum inhibitory concentration is frequently not attained (van de Belt et al., 2000). Such sub-therapeutic levels have been demonstrated to contribute to the development of antibiotic resistance (van de Belt et al., 2000). Incorporating antibiotics directly into the cement matrix has been shown to compromise its mechanical strength by up to 30% (Vorndran et al., 2013). Moreover, none of these systems are designed as pre-printed scaffolds, enabling the fabrication of patient-specific implants. CPC-scaffolds offer a distinct advantage in terms of their osteoinductive capacity which is optimal when the scaffold is printed within a diameter of 10 mm (Meininger et al., 2019). Contemporary additive manufacturing processes, including 3D bioplotting, facilitate the precise fabrication of cylindrical, porous CPC structures with controllable macrogeometry (Bertrand et al., 2023; Blankenburg et al., 2022). By meticulously designing the pore architecture (Muallah et al., 2021), the mechanical load-bearing capacity and the specific surface area of the scaffold can be tailored to specific requirements (Wu et al., 2020; Ahlfeld et al., 2017; Alkhasawnah et al., 2021). Earlier studies have demonstrated that substantial enhancements in compressive strength can be achieved through layer rotation and strand thickness modification. In certain instances, these modifications have been observed to yield values exceeding 10 MPa, which is analogous to the compressive strength of spongy bone (Bertrand et al., 2023). At the same time, scaffold architecture plays a crucial role in the release of active substances, such as antibiotics, which are frequently utilized for local infection control in bone defects (Abbasi et al., 2020; Schweiker et al., 2024; Polo-Corrales et al., 2014). A central problem of earlier scaffold designs is the formation of a closed outer wall, which contributes to structural stability but limits the effective surface area for the binding and release of active substances (Bertrand et al., 2023). Recent developments in bone scaffold design have focused more and more on geometric optimization to balance mechanical performance and biological functionality (Bertrand et al., 2023; Blankenburg et al., 2022; Schweiker et al., 2024). One such approach is to create partially or fully open outer walls to increase the specific surface area and enhance local drug release. However, preserving sufficient structural integrity after removing the outer shell remains a critical limitation of current designs. In this study, we present a novel calcium phosphate cement (CPC) scaffold with an intentionally open outer surface. It is additively manufactured to maximize the specific surface area for controlled drug delivery while maintaining the compressive strength of conventional closed-wall scaffolds. This concept resolves the longstanding trade-off between drug release efficiency and mechanical stability, offering a promising approach for infection-controlled bone replacement therapy. This study primarily aims to characterize the compressive properties of the open-structured CPC scaffold and evaluate its clinical applicability for load-bearing, infection-susceptible bone defects.

2 Materials and methods

The printer needle with an inner diameter of 0.3 mm (Article No.: 500883) was purchased from VIEWEG GmbH (Kranzberg, Germany). The CPC paste used for printing (20 mL, Article No.: 087-020-PL) was purchased from Innotere (Radebeul Germany). Dulbecco’s Phosphate-Buffered Saline (DPBS; Gibco, Life Technologies, part of Thermo Fisher Scientific, Waltham, MA, United States) was obtained from Thermo Fisher Scientific (Catalog No. 14190094).

2.1 3D printing process

3D printing was performed on a 3D Bioplotter (Envisiontec GmbH, Gladbeck, Germany) by using the low temperature printing head with a conical dispensing needle (Vieweg). The CPC paste used in this study was manufactured by Innotere. It comprised synthetic calcium and phosphate salts finely dispersed in a biocompatible oil phase made of short-chain triglycerides (caprylic/capric triglycerides), along with two additional emulsifiers: polyoxyl-35 castor oil and cetyl phosphate. Both the triglycerides and the polyoxyl-35 castor oil were derived from pure vegetable-based raw materials.

2.2 3D printing parameters

As we have seen in previous works (Bertrand et al., 2023; Blankenburg et al., 2022) printing parameters always depend on the geometry of the scaffold. Therefore, parameter optimization was done directly on the geometries to be printed, instead of using the printer software’s “Parameter Tuning” function, which prints only lines. The parameters for the new printing process must be set so that the high viscosity of the CPC does not prevent the printing of the low-viscosity alginate. In reverse, the alginate should not break through the outer CPC shell. To achieve the following printing parameters were varied:

• Pressure CPC [bar]: 1.1–1.3

• Printing speed [mm/s]: 6–8

• Pre-flow CPC [s]: 0.15

• Post-flow CPC [s]: 0

The needle offset [mm] did not require adjustment, because it is calculated from 80% of the needle inner diameter. Unlike in previous works (Blankenburg et al., 2022; Huber et al., 2022), it was not possible to identify the printing parameters using the “Parameter Tuning” of the “Visuals Machines” software. This program does not take into account the required difference in pressure of the different materials, which is why they were tested directly in a modeled geometry.

2.3 New geometry

The aim of this research was to extend the outer boundary in order to achieve a larger surface area for subsequent drug delivery applications. The new geometry was based on the approach of Bertrand et al. (Vorndran et al., 2013). To this end, the outer ring of the geometry was opened, and the middle ring was omitted. The rotation of the geometry around the center point by 1° per layer was retained. Autodesk Inventor, Version 2025 (Autodesk Inc., San Francisco, CA, United States), was used to create the new geometry. Figure 1 shows the old and new geometries as well as the resulting 3D prints. For comparison purposes, always 20 layers were printed, regardless of the geometry or post-treatment.

Figure 1

Figure 1. Old versus new geometry, including corresponding 3D prints, Sample size 1 cm.

2.4 Post-treatment

The aim was to investigate how various post-treatments affect mechanical strength. All 3D-printed scaffolds (old and new geometry) were incubated in a chamber with double-distilled water at 37 °C for 5 days, creating a water-saturated atmosphere. Additionally, half of the samples were incubated in PBS at room temperature for 7 days. In the second approach, we examined the effect of spraying water during 3D printing to stabilize the layer and prevent it from collapsing under its own weight Bertrand et al. (2023). Additionally, the effect of water quantity during 3D printing on the mechanical properties of the scaffolds was investigated. For this purpose, two groups were examined:

Group 1: five scaffolds were printed simultaneously, ensuring that all scaffolds received the same amount of water.

Group 2: five scaffolds were printed sequentially, ensuring that the first scaffold received the most water, and the last scaffold received the least. This printing was repeated three times, so that at least three samples received the same amount of water.

2.5 Sample characterization

The dimensions of the scaffolds were measured after the post-treatment using a digital caliper gauge (Precise PS 7215, Burgwächter, Wetter-Volmarstein, Germany). After the respective post-treatment, the samples were characterized using a stereomicroscope (Olympus, SZ-61, Shinjuku, Japan) at ×20 magnification to assess strand thickness. The strand widths were measured using ImageJ (FIJI) version 1.53t to determine whether the samples swelled or shrank as a result of the post-treatment. All treated samples were compared with the untreated reference sample. In addition, surface roughness was determined using a 3D laser scanning microscope (VK-X 210, Keyence, Osaka, Japan) at 10 different positions on each scaffold. The samples were also examined under the scanning electron microscope SEM (FEI Quanta 250 FEG, FEI, Hilsboro; OR, United States) with an EDX unit (Oxford Instruments, Abingdon, United Kingdom). To analyze the surface properties a Large Field Detector (LFD) and a Backscattered Electron Detector (vCD) were used. The aim of the ESEM investigations was to determine the influence of post-treatment, including the spraying of water during the 3D printing to stabilize the layers and prevent them from collapsing under scaffold own weight. Phase composition was examined by means of XRD (Bruker D8 Advance, Bruker Corp., Billerica, MA, United States). Measurement conditions were Bragg-Brentano geometry, equipped with Cu anode and secondary graphite monochromator, scintillation counter, 40 kV/40 mA, 1°2theta/min, step size 0.02°2theta. Rietveld Refinement analysis was performed by using Profex 5.2.2.

Following microscopy, the mechanical properties of the printed and post-treated geometries were assessed using a compression test. A preload of 1 N was applied using a universal testing machine (Zwick Z005, Zwick/Roell, Ulm, Germany), and the samples were then compressed at a speed of 1 mm/s until mechanical failure or a maximum deformation of 50%, with the test load limited to 2000 N.

2.6 FEM simulation with ANSYS

Finite Element Analysis (FEA) was performed using ANSYS Workbench 2023 R1 to investigate the mechanical response under compressive loading of two calcium phosphate cement (CPC) scaffold geometries, each consisting of 20 layers with a wall thickness of 0.25 mm. The 3D models were created in SolidWorks 2025 and subsequently imported into ANSYS Workbench for simulation.

2.6.1 Model setup and boundary conditions

The material was assumed to be homogeneous and isotropic, with a yield strength of 954 MPa and a Poisson’s ratio of 0.25. Both scaffold models were subjected to the same boundary conditions, the bottom surface was constrained using a fixed support, while the top surface was subjected to displacement-controlled loading of up to 0.25 mm, applied in five incremental steps of 0.05 mm. Large-deflection and nonlinear material options were activated to account for geometric and material nonlinearity.

2.6.2 Output and evaluation

The following quantities were analysed from the simulations:

• Minimum principal stress, evaluated to represent the compressive stress.

• Reaction force at the top surface, analysed as the output result.

These parameters were used to assess the mechanical response of the CPC scaffold geometries under compressive loading.

2.6.3 Assumptions and limitations

The models assume linear-elastic, brittle CPC behavior and do not include diffusion - degradation coupling. Crack propagation is driven purely by mechanical stress without chemical weakening effects. Nonetheless, this approach allows quantitative comparison of the mechanical impact of scaffold geometry on fracture resistance.

2.7 Micro-CT

The micro-CT data of all specimen was acquired on a SkyScan 1,276 (Hamamatsu L10321-67 source; XIMEA MH110XC-KK-TP camera) at 100 kV and 200 µA with Al + Cu filtration. Projections were recorded with 2 × 2 binning (object-space pixel size 13.6 µm), 962 ms exposure, frame-averaging 3, flat-field correction enabled, and random vertical movement (amplitude 10) to mitigate rings. A 360° scan with 0.4° steps yielded ∼900 projections in a round trajectory (step-and-shoot). Reconstruction was performed in NRecon with beam-hardening correction set to 65%, no smoothing, and no ring filtering.

2.8 Statistics

In accordance with DIN standards (DIN EN 843-2; DIN EN ISO 527; DIN EN ISO 6892-1) for mechanical testing, at least five samples were mechanically tested. All results are expressed as means ± standard deviations. The test for normal distribution was performed using the Shapiro-Wilk test. The measured values were analyzed using ANOVA with Tukey post hoc test at a significance level of p < 0.05. Origin 2023 Professional SR1 (OriginLab, Northampton, MA, United States) was used for all statistical analyses.

3 Results

3.1 Sample dimensions and surface roughness

A comparison of the geometry from a previous project (Bertrand et al., 2023) (referred to as “old” for simplicity) with the newly created geometry, without an outer ring to increase the accessible surface area, is shown in Figure 1. The dimensions of both geometries were identical. The scaffolds of both geometries had a diameter of 10 ± 0.5 mm. The strand width ranged between 0.37 ± 0.04 and 0.53 ± 0.03 mm. There was no significant difference in strand width between incubation in PBS and non-incubation. This applied to both geometries. Figure 2 shows the strand widths of the individual groups: old/new and no PBS/PBS.

Figure 2

Figure 2. Comparison of strand widths for the different groups, significant difference with p < 0.05 (*).

The surface roughness values exhibited a decline in correlation with the prolongation of exposure duration to spray water during the additive manufacturing process. The surface roughness value for the lowest exposure to water (Group 1/S1) demonstrates no significant difference from the surface roughness values of the scaffolds produced concurrently (Group 2/S1). Furthermore, an increase in roughness was observed during the incubation process in PBS. A comparison between the old and new geometries reveals no significant differences in surface roughness (see Table 1).

Table 1

Table 1. Surface Roughness of the different Samples (n = 10).

3.2 Determining the active surface area

The active surface area was determined using SolidWorks 2025 for the CAD designs and 3D slicer 5.8.1 (www.slicer.org, Freeware) based on µCT data for the 3D-printed scaffolds. The resulting values are summarized in Table 2 below. With the new geometry, the outer surface opens up, expanding the accessible area by a factor of 4. The values determined using CAD design correspond almost exactly to the actual measured values, which speaks for the precision of 3D printing.

Table 2

Table 2. Comparison of the active surface area for both geometries.

3.3 FEM simulation with ANSYS

The simulation results show the principal stress and reaction force for the old and new scaffold geometries under a 0.25 mm compressive displacement.

• The old geometry exhibited a maximum compressive stress of −93.62 MPa, with the high-stress regions (blue contour) concentrated on the outer walls.

• The new geometry exhibited a maximum compressive stress of −165.37 MPa under the same displacement, also concentrated on the outer walls.

The reaction force at the top surface was −1473 N for the old geometry and −902 N for the new geometry. Both geometries exceed the typical compressive strength of CPC scaffolds 41.6 MPa (Bertrand et al., 2023), indicating that fracture is likely to initiate at the outer surface, particularly in regions of stress concentration. Based on the simulation, the old geometry exhibits lower compressive stress and higher reaction force under the same displacement, indicating that it can handle higher compressive loads than the new geometry.

3.4 Mechanical tests

The Young´s Modulus of the scaffolds was calculated by determining their base area using Solidworks 2025. The stress and Youngs modulus of the new geometry were observed to be 1/3 higher than those of the old geometry. This phenomenon can be attributed to the one-third reduction in the base area of the new geometry. The subsequent figure offers a synopsis of the values ascertained for stress and E-moduli, offering a comparative analysis between the old and new geometries.

If post-treatment is included, additional incubation in PBS (or not) results in additional values. This doubles the maximum failure load for the old geometry. With the new geometry, there is no significant difference in the maximum failure load for samples with/without PBS. The Young´s Modulus (YM) of the old geometries increases by 25% due to incubation in PBS. The YM of the new geometry without incubation in PBS is in the same range as the PBS-incubated samples of the old geometry. The difference is significant. However, there is no significant difference in YM between the samples with/without PBS of the new geometry. Figure 3 summarizes the maximum failure load and YM for incubation in PBS (or not), as well as for old and new geometries (Figure 4). Figure 5 shows the stress-strain curves for both geometries with and without PBS post-treatment.

Figure 3

Figure 3. Maximum failure load and Stiffness depending on post-treatment with/without PBS for both geometries (0.2 mm nozzle, n = 5, *…p < 0.05).

Figure 4

Figure 4. Comparison of compressive Strength and Young´s Modulus of both geometries, *…p < 0.05.

Figure 5

Figure 5. Stress-strain curves for both geometries.

The amount of water had no significant influence. While the values in Figure 6 for the simultaneously printed scaffolds (group 2) tend to vary less than those for the individually printed scaffolds (group 1), no significant differences were found in maximum failure load, Young´s Modulus, or compressive strength.

Figure 6

Figure 6. The influence of water quantity on failure load, stiffness and compressive strength.

The fracture behavior of the new and old geometries is analogous: the outer rings of the old geometry and the open wave structure of the new geometry demonstrate fracturing, whereas the core remains unharmed in both geometries. The following Figure 7 provides a visual representation of this comparison. As shown in Figure 8, the areas with the highest stresses are the areas that yield and break during mechanical loading.

Figure 7

Figure 7. Fracture behavior of the different geometries, before and after mechanical testing, left microscopic images, right µCT reconstructions by 3D slicer 5.8.1; diameter of the scaffold: 10 mm, diameter of the remaining core: 6 mm.

Figure 8

Figure 8. Finite element analysis (ANSYS 2023 R1) of two CPC scaffold designs under compressive loading. (A) Scaffold with an open outer wall exhibiting localized stress concentrations at the junctions between the outer rim and the internal struts (minimum principal stress: −165.37 MPa). (B) Scaffold with a closed outer wall showing a more homogeneous stress distribution and a lower principal stress of −93.61 MPa. The comparison highlights the trade-off between mechanical stability and open-surface geometry for enhanced drug release.

3.5 Light and electron microscopy

In a manner akin to that observed in microscopic images obtained using a stereo microscope, cracks manifested on the outer surface of samples imaged using a scanning electron microscope. It was observed that the presence of cracks on the surface was directly proportional to the amount of water that was sprayed during the 3D printing process. The samples exposed to water for the longest time have the most surface cracks, while the last printed sample, exposed to water the least, has no surface cracks. These correlations are illustrated in Figure 9.

Figure 9

Figure 9. SEM images of CPC scaffolds exposed to different amounts of water, (A) Group 2 all scaffolds printed together, minimal water exposure; (B–D) Group 1; printed one after the other; increase in water exposure from B to D; (A,B) comparable water exposure.

3.6 XRD

The XRD spectra (Figure 10) were analyzed using a Rietveld refinement analysis (Profex 5.2.2). There was no significant difference between the samples with or without incubation in PBS. Mainly CDHA, HA, and traces of alpha TCP were detected.

Figure 10

Figure 10. Xrd pattern of the CPC.

4 Discussion

4.1 Sample characterization

The strand width of the old/new printed scaffolds was between 0.37 and 0.53 mm when a 0.2-mm nozzle was used. In the study by Bertrand et al. (2023), the strand widths were between 0.27 and 0.37 mm. Therefore, they were significantly thinner despite similar 3D-printer parameters. Blankenburg et al. (2022) achieved strand widths between 0.25 and 0.55 mm at 1.5 bar with a 0.25-mm nozzle. Liu et al. (2006) reported the thixotropic behavior of CPC pastes. Lin et al. (2008) came to the same conclusion, which may also explain the difference in strand width. Looking at the active surface area between the old and new geometries reveals a fourfold increase simply because the outer ring was opened. The minimal difference between the CAD design and the actual design suggests that the printing was precise.

4.2 Mechanical properties

Previous studies have shown that spraying the scaffolds with water after printing five or eight layers results in more stable structures that do not collapse under their own weight. Additionally, the scaffolds were found to withstand forces well above 3,000 N at a diameter of 1 cm. Furthermore, the sample could be compressed 4%–5% without breaking (Bertrand et al., 2023). The new geometry is similarly flexible and can be compressed up to 6%. In contrast, comparable sintered ceramics, such as those used by Bertrand et al. (2023) or Bo et al. (2003) or Seidenstuecker et al. (2021), break at 0.05%–0.1% compression. However, the new geometry could only withstand maximum forces of up to 2,000 N due to its greater porosity compared to the geometry of Bertrand et al. (2023). The Young´s Modulus was higher for the samples that were not post-treated in PBS.

According to Bertrand et al. (2023), the compressive strength of 3D-printed β-TCP ceramic scaffolds with different numbers of layers is in the range of 17.5–37 MPa. The scaffolds can withstand only 1000 N. Sintered ceramics are in the range of 25 MPa. Despite the smaller surface area resulting in lower failure loads, the compressive strength of the new geometries is in the range of 50–60 MPa. Blankenburg et al. (2022) found that the compressive strength is 5–10 MPa due to the low height and thin walls of 3D-printed scaffolds. Lode et al. (2014) CPC scaffolds only exhibited a compressive strength of 6.1 MPa. However, their base area was larger than in the present study. Ahlfeld et al. (2017) determined by their small samples (designed for filling a 5 mm defect in the rat femur) a compressive strength of 0.2–0.7 MPa. Tian et al. (2021) described a compressive strength similar to that in the previous study, which was in the range of 25 MPa.

Considering compressive strength alone, the values obtained in this study fall within the range of full ceramics (Reilly and Burstein, 1975; Patel et al., 2008). Depending on the location, 70 MPa is very close to cortical bone (100–200 MPa) and significantly above cancellous bone. Therefore, its use in treating bone defects or infections (if loading with antibiotics is anticipated) is conceivable. This involves replacing parts of the bone, which are 6–10 mm in diameter (Vajgel et al., 2014) and of various heights, especially for critical-size bone defects, which are of the same order of magnitude. This means that the additively manufactured replacement does not have to bear the full load, especially if patients are unable to bear full weight (Sparks et al., 2023).

4.3 Surface analysis using SEM

During incubation in aqueous environments, such as PBS or water baths, calcium phosphate cement (CPC) samples often develop microcracks on their surfaces due to swelling and drying stresses. This phenomenon is particularly evident in layered CPC structures, where wetting decreases adhesion between layers and encourages microcrack propagation across the surface (Bagnol et al., 2021; Kucko et al., 2020). This study found the same: the longer or more frequently the samples were exposed to water, the more cracks appeared on the surface. However, additional incubation in PBS after solidification in a water-saturated atmosphere did not result in additional cracks. Bohner et al. (2005) demonstrate in their study that the temperature and relative humidity during the curing process significantly impact the reactivity and mechanical stability of CPC. Therefore, controlling the humidity is recommended. Additionally, Richter et al. (2023) emphasize in their review that an initial maturation phase at a high relative humidity (e.g., 95%–100%, 37 °C) before water or buffer incubation results in more uniform hydration and significantly reduces surface cracking. Akkineni et al. (2015) also suggest incubation in a water-saturated atmosphere.

4.4 Relationship between scaffold geometry and drug release behavior

Introducing an open outer surface in rotationally symmetrical molded bodies fundamentally alters the diffusion and release mechanisms within the CPC scaffold. In conventional closed-wall geometries, drug transport is primarily governed by Fickian diffusion through interconnected micropores and the gradual dissolution of the outer layer. This limits the accessible surface area for release (Fu and Kao, 2010; Siepmann and Peppas, 2001). In contrast, the open-wall design of the present study exposes the internal pore channels directly to the surrounding medium. This substantially increases the accessible surface area, thereby accelerating the initial release phase (Guo et al., 2023). Owing to the intrinsic surface degradation behavior of calcium phosphate cements, the release process is expected to follow a non-Fickian or anomalous diffusion mechanism, in which both surface dissolution and matrix relaxation contribute to the transport kinetics (Fu and Kao, 2010). In contrast, the open-wall design of the present study exposes the internal pore channels directly to the surrounding medium. This substantially increases the accessible surface area, thereby accelerating the initial release phase (Seidenstuecker et al., 2017). Subsequently, as degradation proceeds toward the inner core, drug diffusion becomes increasingly controlled by the dissolution front and pore connectivity rather than by simple concentration gradients. Importantly, the temporal scale of drug release (2–4 weeks) is substantially shorter than the temporal scale of bulk scaffold degradation (3–6 months). This decoupling implies that the open geometry primarily modulates early-stage therapeutic availability without compromising the long-term mechanical stability required for bone regeneration. Thus, the controlled exposure of the internal structure enables high initial drug accessibility while maintaining sufficient mechanical integrity for the intended implantation period. In summary, the open outer geometry promotes a surface-driven, anomalous release mechanism that enhances early drug delivery while preserving the integrity of the scaffold. In comparison, fully open shapes with a rotation of 90° per layer (Ahlfeld et al., 2017) show a completely different release pattern. Here, the entire surface is accessible, which can significantly accelerate possible release kinetics (Akkineni et al., 2015; Fosca et al., 2022) or lead to a strong burst release. This must then be counteracted, for example, through the use of hydrogels (Lukina et al., 2023). These findings demonstrate how the geometry of a scaffold can be tailored to control diffusion kinetics independently of bulk degradation. This provides a rational design principle for bone-replacement scaffolds in infection-controlled therapy (Fu and Kao, 2010; Guo et al., 2023).

5 Conclusion

In this study, we present a novel, rotationally symmetric geometry developed for extrusion-based 3D printing of calcium phosphate cement (CPC). This geometry is intended for use in potential drug delivery applications and was designed to maximize surface area. The open outer ring structure increases the accessible surface area by a factor of 4 compared to the old geometry while ensuring sufficient mechanical stability. This design concept offers a promising basis for future functionalization studies.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

CP: Visualization, Validation, Writing – original draft, Investigation, Formal Analysis, Writing – review and editing. JL: Writing – original draft, Writing – review and editing, Data curation. SS: Writing – original draft, Methodology, Validation, Writing – review and editing. MP: Formal Analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review and editing. HS: Methodology, Conceptualization, Software, Writing – review and editing, Resources, Writing – original draft. MS: Funding acquisition, Data curation, Methodology, Writing – original draft, Software, Resources, Supervision, Conceptualization, Validation, Project administration, Writing – review and editing.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was used in the creation of this manuscript. Generative AI was used to improve the English.

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