Polydopamine Coating in Trabecular Bone-Like
Scaffolds With Strontium-Substituted
Nanohydroxyapatite for Bone Tissue Engineering

Adones Almeida Rocha, Vinicius Oliveira dos Santos, Alexandre Malta Rossi, Anna Carla Araujo, Rossana Mara da Silva Moreira Thiré

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Adones Almeida Rocha, Vinicius Oliveira dos Santos, Alexandre Malta Rossi, Anna Carla Araujo, Rossana Mara da Silva Moreira Thiré. Polydopamine Coating in Trabecular Bone-Like Scaffolds With Strontium-Substituted Nanohydroxyapatite for Bone Tissue Engineering. Journal of Applied Polymer Science, 2025, 143 (9), 10.1002/app.70116. hal-05489050

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RESEARCH ARTICLE

OPEN ACCESS

# Polydopamine Coating in Trabecular Bone-Like Scaffolds With Strontium-Substituted Nanohydroxyapatite for Bone Tissue Engineering

Adones Almeida Rocha<fnref n="1" />,² ID | Vinicius Oliveira dos Santos<fnref n="1" />,³ | Alexandre Malta Rossi⁴ | Anna Carla Araujo² | Rossana Mara da Silva Moreira Thiré<fnref n="1" />

**Correspondence:** Rossana Mara da Silva Moreira Thiré (rossana@metalmat.ufrj.br)

**Received:** 12 September 2025 | **Revised:** 15 November 2025 | **Accepted:** 19 November 2025

**Keywords:** biomaterials | biomimetic | biopolymers and renewable polymers | nanocrystals | nanoparticles | nanowires

## ABSTRACT

Bone fractures can be effectively treated through the utilization of supportive structures for tissue growth: the scaffolds. Strontium-substituted hydroxyapatite (HASr) is a promising ceramic improving bone regeneration. However, composite scaffolds made from PLA and hydroxyapatite lack optimal surface properties. Polydopamine (PDA) exhibits potential for functionalizing biomaterials surfaces by providing ideal physical-chemical properties for cell adhesion. This study proposes applying a PDA coating on PLA/HASr scaffolds to enhance their physico-chemical properties for use as biomaterials in bone engineering applications. Fused Filament Fabrication 3D printer was used to produce scaffolds with trabecular bone-inspired geometry using a digital model based on the cell bone dynamics. Wettability analysis demonstrated that PLA/HASr/PDA scaffolds exhibited improved hydrophilicity (31.2° ± 5.8°) compared to neat PLA (88.3° ± 0.4°). These scaffolds exhibit several characteristics suitable for bone tissue engineering applications, such as enhanced release of signaling ions (Ca²⁺ and Sr²⁺ ions from modified hydroxyapatite) and potential improved cell adhesion properties.

## 1 | Introduction

Currently, the increasing incidence of bone injuries is an international issue as the contemporary lifestyle habits such as poor nutrition and sedentarism, which cause the weakening of bone tissue [1], proliferate in the world. Also, the world population is getting old, as life expectancy increases with the advances in medicine [2, 3], so the incidence of bone injuries is likely to rise even further. These factors show the importance of the topic for research in bone injury treatment.

The gold standard for the healing of bone injuries is the use of material self-provided by the patient, also known as the

autologous graft [4]. This technique has some disadvantages such as the necessity of two surgical procedures and limitations in treating large defects. The substitution of autologous grafts by scaffolds in association with bioactive molecules and cells is the focus of Bone Tissue Engineering (BTE) [4, 5]. These scaffolds provide three-dimensional support for new tissue growth in an injury.

For optimal performance, scaffolds should mimic both the micro and macrostructural characteristics of native bone, which exhibits a hierarchical and anisotropic morphology [2]. The interior part of the bone is highly porous, and it is known as cancellous or trabecular bone. While regular lattice and

<footnote n="1">¹Program of Metallurgical and Materials Engineering, Alberto Luiz Coimbra Institute for Graduate Studies and Research in Engineering, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil | ²Institut Clément Ader (ICA), Univ Toulouse, IMT Albi, INSA Toulouse, ISAE-SUPAERO, CNRS, ICA, Toulouse, France | ³National Institute of Technology (INT), Rio de Janeiro, Rio de Janeiro, Brazil | ⁴Applied Physical Department/Centro Brasileiro de Pesquisas Fisicas (CBPF), Rio de Janeiro, Brazil</footnote>
implicit surface structures with repeating pore geometries can replicate some aspects of bone morphology, trabecular bone-like architectures have been shown to better emulate the natural bone environment, providing larger surface areas and concave pores that enhance cell adhesion and proliferation [6–8]. Recent studies have demonstrated that 3D-printed trabecular bone-like scaffolds implanted in femoral con- dyles outperform regular scaffolds in terms of biological response, promoting superior bone ingrowth and integration [9, 10]. Therefore, the adoption of trabecular bone-like geometries represents a promising strategy for improving scaffold biofunctionality.

In this context, scaffolds are produced from biomaterials that have biological acceptance. Poly(lactic acid) is a widely used polymer in BTE for its biocompatibility and biodegradability properties. Also, PLA is FDA (Food and Drug Administration—USA) approved for biological application [4]. However, this polymer has low bioactivity, so the PLA composites with phosphates such as beta-tricalcium [5] and hydroxyapatite [11] are applied to improve the osteogenic potential of PLA scaffolds.

Hydroxyapatite is a bioactive ceramic material that possesses high similarity with the inorganic phase of bone extracellular matrix [11]. The modification of hydroxyapatite with ionic substitution shows good improvement in the increase of water solubility [12, 13] and bioactivity [12–16], desirable properties for BTE scaffolds. Among the various substitutions investigated, strontium (Sr²⁺) has attracted considerable attention due to its dual role in stimulating osteoblast activity and inhibiting osteoclast-mediated resorption [12, 17–18]. Several studies have incorporated strontium-substituted hydroxyapatite (HASr) into polymeric [19–21] or ceramic scaffolds [16, 22], reporting improved osteogenic responses both in vitro and in vivo. Specifically, Qi et al. [23] demonstrated that controlled Sr²⁺ release from 3D printed HASr scaffolds enhanced cell proliferation and differentiation, highlighting the potential role of Sr. ions in the regeneration process.

Despite the significant progress achieved with PLA-based composites containing hydroxyapatite, their surface properties often remain suboptimal for cell attachment and proliferation [24, 25]. Cells preferentially adhere to hydrophilic surfaces or those bearing functional chemical groups Tolabi et al. [26]. Since cell adhesion is the initial and crucial step in tissue regeneration, influencing proliferation, migration, and differentiation [27], strategies to modify surface chemistry have emerged as a critical strategy in the design of bioactive material.

Polydopamine (PDA) has emerged as an effective and versatile coating for improving scaffold surface properties. PDA has a lot of functional groups, such as catechol, amine, and imine that allow it to establish connections between materials and cells, enhancing their interaction and facilitating cell adhesion to material surfaces. PDA represents a relatively simple and efficient strategy for endowing scaffolds with bioactive surface properties, typically achieved through a straightforward immersion process in some ideal conditions. Also, this technique does not

compromise the internal structure and mechanical strength of the material, underscoring its significant potential for clinical applications across diverse fields [26].

The development of an ideal bone graft substitute remains a major challenge for researchers. To the best of our knowledge, the combination of biomimetic trabecular bone-like structures, strontium-substituted hydroxyapatite, and PDA surface coating has not yet been investigated. This integrated approach presents promising potential to enhance bioactivity and manufacturing feasibility. Therefore, this work aims to fabricate PLA/HASr/ PDA scaffolds with biomimetic trabecular bone-like geometries using fused filament fabrication (FFF) for bone tissue engineering applications. Additionally, this study also investigates the physicochemical modifications introduced by HASr incorporation and PDA coating, with particular focus on surface characteristics that can enhance the potential biological performance of the scaffolds.

## 2 | Materials and Methods

### 2.1 | Materials

PLA filaments and PLA/HASr composite filaments with a nominal proportion of 90%/10% (w/w) were acquired from the NanoIn company (PLA-NanoIn and Osteoprint10-NanoIn filaments). According to the manufacturing datasheet [28], the PLA and PLA/HASr filaments contain ~3% D-isomer PLA polymer and have a molecular weight of approximately Mn = 145,000 g/mol. Dopamine hydrochloride 98% (H8502) was purchased by Sigma Aldrich.

### 2.2 | Scaffolds Designs Production

The design of the scaffolds was based in the biomimetic geometry obtained by solving a mathematical model that simulates the bone cell remodeling, as detailed in the authors' previous work [29]: the Komarova's Model for Bone remodeling. Briefly, the mathematical model is based on the partial differential equations (Equation 1–4), that describes the bone cell remodeling in terms of the osteoblasts (B), osteoclasts (C) and bone mass density (ρ) in function of spatial (u=x, y, and z dimensions) and temporal parameters (t).

$$ \frac{\partial C(u, t)}{\partial t} = D_C \nabla^2 C(u, t) + \alpha_1 C(u, t)^{g11} B(u, t)^{g21} - \beta_1 C(u, t), \quad (1) $$

$$ \frac{\partial B(u, t)}{\partial t} = D_B \nabla^2 B(u, t) + \alpha_2 C(u, t)^{g12} B(u, t)^{g22} - \beta_2 B(u, t), \quad (2) $$

$$ \frac{\partial \rho(u, t)}{\partial t} = -k_1(u, t) \max(0, C(u, t) - \underline{C}) + k_2 \max((u, t) - \underline{B}). \quad (3) $$

The parameters in Equations (1–3) are related to signaling molecules and biological properties for each kind of cell, as explained in Table 1. The boundary conditions are detailed in Rocha et al. [29].

The computational models were obtained in Matlab 2022a software. The numerical method of Laplace's discretization
using a finite-difference scheme was used for the spatial part (Equations 4 and 5, below) and the Adaptative Runge-Kutta method for the temporal part.

<table><caption>TABLE 1 | Summary of parameter values for the Equations (1-3).</caption><thead><tr><th>Parameters</th><th>Description</th><th>Value</th></tr></thead><tbody><tr><td>α<sub>1</sub></td><td>Production rate of osteoclasts</td><td>3 [day<sup>-1</sup>]</td></tr><tr><td>α<sub>2</sub></td><td>Production rate of osteoblasts</td><td>4 [day<sup>-1</sup>]</td></tr><tr><td>β<sub>1</sub></td><td>Apoptosis rate of osteoclasts</td><td>0.2 [day<sup>-1</sup>]</td></tr><tr><td>β<sub>2</sub></td><td>Apoptosis rate of osteoblasts</td><td>0.02 [day<sup>-1</sup>]</td></tr><tr><td>k<sub>C</sub></td><td>Rate of bone's resorption</td><td>0.45% [day<sup>-1</sup>]</td></tr><tr><td>k<sub>B</sub></td><td>Rate of bone's formation</td><td>0.0448% [day<sup>-1</sup>]</td></tr><tr><td>D<sub>C</sub></td><td>Diffusion coefficient of osteoclasts</td><td>1×10<sup>-6</sup> [bone mass mm<sup>-1</sup>]</td></tr><tr><td>D<sub>B</sub></td><td>Diffusion coefficient of osteoblasts</td><td>1×10<sup>-6</sup> [bone mass mm<sup>-1</sup>]</td></tr><tr><td>C</td><td>Cell population of osteoclasts in steady-state</td><td>1.06 [cell mm<sup>-1</sup>]</td></tr><tr><td>B</td><td>Cell population of osteoblasts in steady-state</td><td>212.13 [cell mm<sup>-1</sup>]</td></tr><tr><td>g11</td><td>Autocrine signaling factor of osteoclasts</td><td>0.5 [single]<br>1.1 [osc.]<br>[dimensionless]</td></tr><tr><td>g21</td><td>Paracrine signaling factor of osteoclasts</td><td>-0.5<br>[dimensionless]</td></tr><tr><td>g22</td><td>Autocrine signaling factor of osteoblasts</td><td>0.0<br>[dimensionless]</td></tr><tr><td>g12</td><td>Paracrine signaling factor of osteoblasts</td><td>1.0<br>[dimensionless]</td></tr></tbody></table>

$$ \frac{C_{i,j,k}^{(n+1)} - C_{i,j,k}^{(n)}}{\Delta t} = \frac{k_{i,j,k} C_{i+1,j,k}^{(n+1)} - 2C_{i,j,k}^{(n)} + C_{i-1,j,k}^{(n)}}{\Delta x^2} + \frac{k_{i,j,k} C_{i,j+1,k}^{(n+1)} - 2C_{i,j,k}^{(n)} + C_{i,j-1,k}^{(n)}}{\Delta x^2} + \frac{k_{i,j,k} C_{i,j,k+1}^{(n+1)} - 2C_{i,j,k}^{(n)} + C_{i,j,k-1}^{(n)}}{\Delta x^2} + \alpha_1 C(u, t)^{g_{11}} B(u, t)^{g_{21}} - \beta_1 C(u, t) \Delta y^2, \quad (4) $$

$$ \frac{B_{i,j,k}^{(n+1)} - B_{i,j,k}^{(n)}}{\Delta t} = \frac{k_{i,j,k} B_{i+1,j,k}^{(n+1)} - 2B_{i,j,k}^{(n)} + B_{i-1,j,k}^{(n)}}{\Delta x^2} + \frac{k_{i,j,k} B_{i,j+1,k}^{(n+1)} - 2B_{i,j,k}^{(n)} + B_{i,j-1,k}^{(n)}}{\Delta x^2} + \frac{k_{i,j,k} B_{i,j,k+1}^{(n+1)} - 2B_{i,j,k}^{(n)} + B_{i,j,k-1}^{(n)}}{\Delta x^2} + \alpha_2 B(u, t)^{g_{12}} B(u, t)^{g_{22}} - \beta_2 B(u, t). \quad (5) $$

The matrices produced after binarization were processed using Matlab 2022a tools: *isosurface*, which generates a three-dimensional surface from matrix data, and *stlwrite*, which creates an STL digital file through triangulation (Figure 1).

## 2.3 | Scaffold Fabrication

The STL files were prepared using the slicing software, IdeaMaker 1.1, to generate G-codes. The N2 Raise3D printer was used for printing the scaffolds. The printing conditions include a bed temperature of 40°C–60°C, printing temperature of 210°C, layer height of 0.15 mm, and a printing speed of 40 mm/s. These parameters were optimized to achieve the best dimensional accuracy of the printed pieces. Cylindrical pieces were printed with a diameter of 1 mm. Specifically printed pieces, square samples of 1 cm², were used for FTIR, XRD, and wettability analysis, produced using the same 3D printer apparatus as the scaffolds.

After additive manufacturing, the scaffolds were soaked in a 1 M NaOH (Sigma Aldrich) solution at 45°C for 15 min and washed in distilled water. Then, they were dried at room temperature for 24 h. The alkaline treatment is a process that increases chemical groups on the scaffold surface, such as hydroxyl and carboxylic

<figure><img src="image_4.png" /><figcaption>FIGURE 1 | Digital images of the geometry applied in the study: (a) isosurface trabecular bone-like geometry obtained in our preview work Rocha et al. [29], and (b) Region of interest selected with 8 mm of diameter and height of 2 mm. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>
groups, also increasing the surface roughness [4, 30]. This treatment usually provides more hydrophilicity for scaffolds and impacts cellular adhesion, improving the bioactivity of polyester polymers Richbourg et al. [31].

For Polydopamine coating, the scaffolds were immersed in a 3 mg/mL dopamine solution Tris–HCl buffer at pH 8.5 for 24 h, protected from light, with constant stirring. Both types of printed scaffolds (PLA and PLA with hydroxyapatite) were undergo surface treatment with polydopamine. Therefore, four types of scaffolds were produced in this study, coded here as PLA, PLA/HASr, PLA-PDA, and PLA/HASr-PDA.

## 2.4 | Characterization

### 2.4.1 | Scanning Electron Microscopy (SEM)

The microscopic morphology of the scaffolds was observed using a scanning electron microscope (SEM, Tescan-Vega3) operated at 10 kV acceleration in secondary electron mode. The SEM equipment can be coupled with the Energy Dispersive Spectroscopy detector (EDS, Bruker) to evaluate elemental composition. To prevent electrical charging during SEM imaging, a thin gold coating was applied to the scaffolds (Denton Vacuum—Desk V).

### 2.4.2 | Chemical and Microstructural Characterization

The functional groups of the samples were studied using a Fourier transform infrared spectrometer (FTIR, Perkin Elmer, model Spectrum 100) operating in attenuated total reflectance mode (ATR mode) over the range of 4000–400 cm⁻¹, with 64 scans and a resolution of 4 cm⁻¹. Printed pieces and HASr were characterized for crystallinity using X-ray diffraction (XRD, Shimadzu XDR-6000).

### 2.4.3 | Wettability

The hydrophilic capacity of the scaffolds was assessed using a contact angle meter (Ramé-Hart NRL A 100-00 goniometer). For contact angle measurements, 2 µL of distilled water was deposited on the material surface at room temperature; this analysis was conducted in triplicate, and the average value will be obtained from 100 measurements per sample.

### 2.4.4 | Thermal Characterization

The thermal behavior of the scaffolds was determined using simultaneous thermogravimetric and differential thermal analysis (TGA/DTA, DTG60, and Shimadzu). Approximately 25 mg of each sample will be heated from room temperature to 500°C in an Argon atmosphere. Differentials scanning calorimetry (DSC, DSC-250, TA instruments) was applied to access the crystallinity index using the Equation (6), using 93 J/g enthalpy of fusion ΔHf to utterly crystalline PLA measured at the equilibrium melting point [32], and ΔHf obtained from one heating cycle with a heating rate of 10°C/min at a nitrogen atmosphere and flow rate of 50 mL/min.

$$X_c(\%) = \frac{\Delta H_f}{\Delta H_f^\circ} \qquad (6)$$

### 2.4.5 | In Vitro Ion Release

The scaffolds were initially weighed (m₀). Subsequently, the scaffolds were soaked in an adapted Simulated Body Fluid (SBF) at 37°C under constant agitation (150 rpm). The scaffolds will be removed from the SBF solution at 1, 7, 14, 21, and 28 days intervals, dried at room temperature for 24 h, and reweighed (mᵣ). The solution was replaced in each day's interval to ensure no mineral deposition in the scaffolds because of chemical equilibrium. The supernatant solutions were stored for analysis (refrigerator) to determine Ca²⁺ and Sr²⁺ ionic concentrations using inductively coupled plasma-optical emission spectroscopy (ICP-OES, Perkin Elmer, Model Optima 7000DV).

### 2.4.6 | Bioactivity Essay by Mineralization In Vitro

Mineralization in vitro was performed in order to evaluate the bioactivity of the scaffolds. The scaffolds were soaked in 20 mL of Simulated Body Fluid (SBF, pH = 7.4) at 37°C under constant agitation (150 rpm). After the incubation time of 24 h, the scaffolds were dried and evaluated by SEM (SEM, Tescan-Vega3).

### 2.4.7 | Compressive Testing

The compression test was carried out to evaluate the mechanical behavior of the scaffolds using a universal mechanical testing machine (3R—Recherches et Réalisations Remy, model 8801). The length of the specimens was a cylinder with a radius of 7 mm. The compression rate was 0.5 mm/min until 30% deformation with a load cell of 10 kN.

### 2.4.8 | Protein Adsorption Testing

Scaffolds were immersed in ethanol to remove surface impurities, then incubated in bovine serum albumin (BSA, 100 µg/mL in SBF) for 4 h at 37°C with agitation (70 rpm). After incubation, scaffolds were rinsed with SBF, and the total unabsorbed protein was quantified using a MicroBCA kit (ThermoFisher) and UV-vis spectrophotometry (PerkinElmer Lambda 25, 625 nm). The adsorbed protein (Cads) was calculated by Equation (7):

$$C_{ads} = C_i - (C_{n-ads} + C_{free}) \qquad (7)$$

where Cads is the amount of adsorbed protein, Ci is the initial amount, Cn-ads is the amount not adsorbed (washing solution) and free is the amount of free protein (BSA solution after 4 h incubation). The use of this protein is usual in to provide an overview of the scaffold capacity to adsorbed proteins, once albumin has hierarchical and structural resemblance with integrins [20].
2.4.9 | Statistical Analysis

The data will be shown as means ± for the standard deviation. Data with * indicates significant differences

between samples that were compared through the analysis
of variance and Tukey's test (p < 0.05). All statistical anal-
yses were performed using GraphPad Prism 7.0 Software
(GraphPad Inc.).

<figure><img src="image_5.png" /><figcaption>**FIGURE 2** | Scanning Electron Microscopy images of the printed scaffolds before coating: (a) EDS data spectra of the PLA and PLA/HASr scaffolds and (b) EDS mapping of PLA/HASr scaffold. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>
# 3 | Results and Discussion

## 3.1 | Morphology Aspects

The SEM images were analyzed to reveal the details in the morphology of the printed pieces and the presence and morphology of PDA deposition. Furthermore, EDS was employed to ascertain the presence and distribution of the HASr in the scaffolds.

### 3.1.1 | SEM/EDS

Figure 2 presents the SEM images of PLA and PLA/HASr. The presence of HASr in PLA/HASr scaffolds introduces noticeable defects, as observed in microtomography images. Figure 2a illustrates the chemical composition of the scaffolds using EDS. In neat PLA, strong signals from C and O atoms dominate, consistent with the polymer's composition. In contrast, Figure 2b

<figure><img src="image_6.png" /><figcaption>FIGURE 3 | Scaffolds PLA and PLA/HASr with PDA coating onto the surface in different magnifications.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**FIGURE 4** | Wettability results for the scaffolds: (a) photographs of water droplets on the surface of each scaffold, (b) Contact angle values for each scaffold (n = 4, p < 0.05). [Color figure can be viewed at wiley-onlinelibrary.com]</figcaption></figure>

reveals additional signals from Ca, P, and Sr atoms in PLA/HASr scaffolds, indicating the presence of hydroxyapatite (HA) on the surface of the scaffolds. The theoretical HA formula in this study, Ca<sub>0.5</sub>Sr<sub>0.5</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>, predicts a Ca/P ratio of 1.58. EDS analysis (Figure 2b) shows a measured Ca/P ratio of 1.51, which falls within acceptable limits given the sensitivity of EDS. Mapping in Figure 3b confirms a homogeneous distribution of elements on the PLA/HASr scaffold surface.

Figure 3 presents SEM images of PLA/PDA and PLA/HASr/PDA scaffolds, clearly showing the deposition of PDA microparticles. PLA/HASr/PDA scaffolds exhibit a visibly greater quantity of PDA microparticles compared to PLA/PDA (agglomerate particles indicated by arrows). The surface morphology of PLA/HASr (as seen in Figure 2a) displays sharper and more defined curves, potentially providing a larger surface area for PDA deposition. Moreover, the presence of PDA in PLA/HASr/PDA scaffolds probably increased even more the surface area of the devices.

## 3.2 | Chemical Analysis

### 3.2.1 | Wettability

Figure 4 shows the contact angle values of the printed pieces and the appearance of droplets on the flat surface of the material. The incorporation of HASr reduces the contact angle, enhancing the hydrophilicity of the devices. PLA typically exhibits a contact angle of 88.3° ± 0.4°, whereas PLA/HASr shows a reduced contact angle of 64.4° ± 0.1°. This trend is consistent with findings by Ge et al. [20], who reported contact angles of 89.3°

for neat PLA and 67.2° for PLA/HASr scaffolds fabricated using selective laser sintering.

The hydrophilicity of scaffolds significantly increases with the addition of PDA. Contact angle values for PLA/PDA and PLA/HASr/PDA, 40.6° ± 1.6° and 31.5° ± 5.8° respectively, show no statistical difference. The increase in standard deviation may be attributed to intentional non-uniform deposition of PDA microparticles, enhancing hydrophilicity while also facilitating ion release. In addition to PDA's chemical properties (catechol and amino groups promoting hydrophilicity), SEM images reveal increased surface area for PLA/PDA and PLA/HASr/PDA (Figure 4), potentially transitioning from Young's to Wenzel's wetting states [33].

Hydrophilicity is a critical property in scaffold applications for BTE. However, consensus on the optimal contact angle for scaffold applications is lacking, as proteins tend to adsorb more on hydrophobic surfaces [31], crucial for cell adhesion and proliferation processes. Intermediate contact angle values typically align well with both in vitro and in vivo results. Meng et al. [34] explored hydrophilic surface modifications through alkaline treatment, achieving water contact angles of ~60° on PLLA scaffolds after a 2-h treatment, resulting in favorable morphology and mineralization with KUSA-A1 cells.

### 3.2.2 | FTIR

Figure 5a presents the FTIR spectrum of the scaffolds and of the dopamine. This technique allows the confirmation of the PDA coating and HASr presence. All scaffolds showed typical bands at approximately 1750 cm⁻¹ (carboxyl group vibration band), 1180 and 870 cm⁻¹ (ether group vibration bands). These bands are related to PLA chemical groups. The scaffolds PLA/HASr and PLA/HASr/PDA presented bands at 604 and 566 cm⁻¹ related to PO⁴ vibrational modes, indicating the presence of HASr [15].

The Figure 5a presents N–H and C–N groups in dopamine FTIR spectra, at approximately 3400 and 1500 cm⁻¹ [35]. But only the most intense band (C–N group) is also present in PLA/PDA and PLA/HASr/PDA scaffolds. This result indicates the presence of PDA coating in these scaffolds.

### 3.2.3 | XRD

Figure 5b presents the X-ray diffractograms of the scaffolds obtained from the PLA/HASr composite filaments and of the HASr. The scaffold without filler, PLA, has a characteristic amorphous halo around 2θ = 16° [4] revealing that the polymer is predominantly amorphous. The diffractograms of the scaffolds containing HASr show the characteristics of both peaks of PLA and HASr. The sharp peaks at approximately 2θ = 32.5° (peak related to the crystalline structure of HASr, which corresponds to the plane [211]) and 2θ = 40.5° (peak possibly related to the plane [310]) are observed in both HASr, PLA/HASr, and PLA/HASr/PDA curves. These results are consistent with those observed in the literature [36] and these results collaborate with those obtained by FTIR, Figure 5a. It
<figure><img src="image_8.png" /><figcaption>**FIGURE 5** | (a) FTIR spectrum of the scaffolds and dopamine, (b) XRD diagram of the scaffolds and of the strontium-substituted hydroxyapatite, (c) Thermal gravimetric analysis with simultaneous differential thermal analysis curves for the scaffolds and (d) DSC results for the first heating cycle of the scaffolds samples. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<table><caption>**TABLE 2** | Values determined from the TGA curves obtained through simultaneous TG/DTA analyses of PLA/HASr filaments and scaffolds.</caption><thead><tr><th>Sample</th><th>T<sub>onset</sub>(°C)</th><th>Residual mass (%)</th></tr></thead><tbody><tr><td>PLA—filament</td><td>344.1</td><td>—*</td></tr><tr><td>PLA/HASr—filament</td><td>346.1</td><td>6.4</td></tr><tr><td>PLA—scaffold</td><td>344.4</td><td>—*</td></tr><tr><td>PLA/HASr—scaffold</td><td>345.6</td><td>5.6</td></tr></tbody></table>

can also be noted that no phase transformation occurred as a consequence of the PDA coating, as PDA is an amorphous structure.

### 3.3 | Thermal Analysis

Simultaneous thermogravimetric and differential thermal analyses (TGA/DTA) were conducted on the materials, as shown in Figure 5c. The DTA curve allows for the determination of the glass transition temperature (Tg) (indicated by a baseline change), crystallization temperature (Tc) (related to an exothermic peak), melting temperature (Tm) (an endothermic peak following Tc), and peak temperature (Tpeak), which represents the temperature at which the degradation rate is highest (also an endothermic peak). The TGA curve provides the degradation onset temperature (Tonset) and residual mass of each sample. These thermal properties are detailed in Tables 3 and 4.

Table 2 indicates that there are no significant differences in Tonset, suggesting that HASr does not influence the thermal stability of the composite. The residual mass indicates substantial

<footnote>*No significant residual mass.</footnote>
<table><caption>**TABLE 3** | Values determined from the DTA curves obtained through simultaneous TG/DTA analyses of PLA/HASr filaments and scaffolds.</caption><thead><tr><th>Sample</th><th>T<sub>g</sub> (°C)</th><th>T<sub>m1</sub> (°C)</th><th>T<sub>m2</sub> (°C)</th><th>T<sub>c</sub> (°C)</th><th>T<sub>peak</sub> (°C)</th></tr></thead><tbody><tr><td>PLA—filament</td><td>60.9</td><td>153.5</td><td>—</td><td>116.0</td><td>368.2</td></tr><tr><td>PLA/HASr—filament</td><td>58.3</td><td>154.7</td><td>148.8</td><td>114.7</td><td>371.0</td></tr><tr><td>PLA—scaffold</td><td>59.3</td><td>152.7</td><td>—</td><td>119.8</td><td>367.4</td></tr><tr><td>PLA/HASr—scaffold</td><td>61.1</td><td>154.1</td><td>149.4</td><td>111.0</td><td>370.8</td></tr></tbody></table>

Note: T<sub>g</sub>, glass transition temperature; T<sub>m</sub>, melting temperature; T<sub>c</sub>, crystallization temperature; T<sub>peak</sub>, peak temperature.

<table><caption>**TABLE 4** | Values determined from the DSC curves of PLA and PLA/HASr filaments and scaffolds.</caption><thead><tr><th>Sample</th><th>ΔH<sub>m</sub> (J/g)</th><th>Xc (%)</th></tr></thead><tbody><tr><td>PLA scaffold</td><td>24.29</td><td>26.11</td></tr><tr><td>PLA/HASr scaffold</td><td>28.73</td><td>30.89</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>**FIGURE 6** | (a) Compressive stress–strain curves (mean ± SD, with shaded area representing standard deviation) and (b) compressive modulus of the scaffolds. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

material loss during processing (approximately 36% lost during filament processing and 44% during 3D printing). Despite the potential of HASr for bone tissue engineering, its particle size poses challenges for processing. Pedrosa et al. [37] reported similar mass losses with zinc-substituted hydroxyapatite in processing PCL electrospun membranes.

Table 3 shows similar T<sub>g</sub> values for the materials, indicating that addition of HASr may not have affected the amorphous part of the material. The observed decrease in crystallization temperatures of the composite filaments and scaffolds containing HASr, approximately 20°C, suggests that the filler may have acted as a nucleating agent, aiding in the organization of polymeric chains [38]. This underscores the unique characteristics of HASr when combined with PLA. There was no significant variation in melting temperature, indicating that HASr does not adversely affect crystal quality.

A shoulder observed in samples with HASr around 150°C (Figure 5c) may be attributed to the formation of crystals in the α' phase of PLA, identified in Table 4 as T<sub>m2</sub> [39, 40]. This phase could be attributed to the presence of HASr that may have acted as a nucleating agent for this crystal phase.

Figure 5d shows the results of the DSC tests carried out on the scaffolds. With these results it is possible to obtain the crystallinity index using the melting enthalpy in the melting process. Equation (6) is used to obtain the results presented in Table 4.

The results in Table 4 finally confirm that HASr influences in the crystallinity of the material. It is possible that the hydroxyapatite crystals were nucleating agents that help the formation of simple crystals (PLA alpha as showed in the shoulder in Figure 8, and the peak around 150°C in the Figure 9 in the PLA/HASr scaffolds), which has a direct impact on the properties of the material [40–43].
<figure><img src="image_10.png" /><figcaption>**FIGURE 7** | Cumulative Ca²⁺ (left) and Sr²⁺(right) concentration release profile for the scaffolds PLA/HASr and PLA/HASr/PDA for 14 days. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**FIGURE 8** | Ion diffusion hypothesis for the PDA coating in comparison with literature and this work. [Color figure can be viewed at wiley-onlinelibrary.com]</figcaption></figure>

## 3.4 | Mechanical Behavior

The scaffolds' mechanical behavior is a pivotal property for the application in BTE as the material must provide a provisional mechanical bone-like structure. Figure 6 shows the results of the mechanical compressive test applied to the scaffolds. The stress-strain curves (Figure 6a) reveal distinct differences in mechanical performance among the tested materials. The PLA sample exhibits the lowest stress values across the entire strain range, indicating its relatively poor mechanical performance compared to the modified composites. The addition of HASr leads to an increase in compressive strength, demonstrating its reinforcing effect. A more pronounced enhancement is observed in the PLA-PDA sample, suggesting that the PDA coating improves interfacial adhesion between the polymer matrix and the filler. The PLA/HASr-PDA composite exhibits the highest stress values, indicating a synergistic effect between HASr and PDA in reinforcing the material [44–46].

The Figure 6b presents the compressive modulus values for each material. The PLA/HASr-PDA composite displays the highest modulus, confirming its superior stiffness among the tested formulations. The presence of HASr alone enhances

the modulus compared to pure PLA, but the additional PDA coating further improves the mechanical performance, likely due to better load transfer between the matrix and the filler. Despite the inherent variability, as indicated by the error bars, the trend clearly suggests that the combination of HASr and PDA significantly enhances the compressive properties of PLA-based composites.

## 3.5 | Ion Release

The in vitro degradation results in the release of Ca²⁺ and Sr²⁺ ions, which play an important role in the bone healing process, are shown in Figure 7. Initially, Ca²⁺ release is similar for PLA/HASr and PLA/HASr/PDA scaffolds, but there is an interesting trend towards greater release in the PLA/HASr/PDA system. Qi et al. [23] also reported a sustained release of Ca²⁺ ions, significantly enhancing the function of biological materials. The Sr²⁺ release graph shows no significant differences but also demonstrates sustained ion release.

Figure 8 illustrates the proposed mechanism that explains how the PLA/HASr/PDA system behaves compared to PLA/HASr. The
<figure><img src="image_12.png" /><figcaption>**FIGURE 9** | (a) SEM images of the scaffold samples after 24 h of in vitro mineralization, (b) EDS graph and maps of Ca and P presence, (c) adsorbed protein amount (µg/mL) for the samples. [Color figure can be viewed at wileyonlinelibrary.com]</figcaption></figure>

high hydrophilicity of PLA/HASr/PDA increases water intake and surface degradation of the material, as indicated by the initial burst of ions. The increased initial water diffusion could enhance degradability and ion release. Similarly, Chi et al. [47] suggest that local concentrations of 8 mg/mL Ca²⁺ ions can activate osteoblast metabolism; the scaffolds in our study, containing 0.01 g, achieve a concentration of approximately 2.5 mg/mL calcium ions in 14 days.

The physical-chemical data discussed indicate that the biomaterial shows promise for Bone Engineering applications. In vitro analysis can confirm this hypothesis. Furthermore, these preliminary results suggest the need to enhance the PDA coating with intelligent morphologies and increase the area/volume ratio to optimize trabecular geometry, as these are key factors in designing suitable scaffolds, as proposed in the methodology.

## 3.6 | In Vitro Mineralization

A simulated body fluid (SBF) solution was used to evaluate whether the produced biomaterial can form an apatite layer on its surface. The SBF replicates the ions present in human plasma, allowing the bioactivity of the biomaterial to be predicted, as this system simulates the material's contact with human blood [48–50].

Figure 9a shows the formation of apatite on the surface of the fabricated scaffolds. A greater amount of apatite is observed in the

scaffolds with the presence of PDA (PLA-PDA and PLA/HASr-PDA) in comparison to neat PLA and PLA/HASr. The apatite formation is supported by EDS analysis, Figure 9b, which shows an apatite Ca/P ratio of 1.89. The use of PDA coating to enhance the bioactivity of scaffolds is widely discussed in the literature. Huang et al. [50] also used PDA coating and observed the characteristic presence of apatite on the surface of their PLA scaffolds.

Interestingly, in the present work, SEM images reveal that within just 24 h, layers were deposited on top of each other in the PLA/HASr-PDA, as indicated by surface cracking. These results support the hypothesis that the synergistic combination of components in PLA/HASr-PDA enhances the bioactivity of the biomaterial.

## 3.7 | Protein Adsorption

Elucidating the relationship between scaffold properties and protein dynamics enables the researchers to understand how scaffolds can enhance bioactivity and regenerative potential. Before cells adhere to biomaterials, secreted proteins interact with the surface [31], and this interaction is pivotal to initial cell adhesion.

In the test of the present work, we evaluated bovine serum albumin (BSA) as a standard used in the literature [51, 52] to emulate protein adsorption in a real physiological environment. Figure 9c shows the results of BSA adsorption into the scaffolds.
It is noteworthy that a greater amount of protein is adsorbed into PDA-hydroxyapatite-containing scaffolds, demonstrating the potential of this combination to increase protein adsorption. The presence of HASr may alter the chemical composition and electric charge of scaffold surfaces, thereby increasing the quantity of active sites on the composite scaffolds for potential interactions with proteins Nga et al. [52]. Then a greater presence of PDA in PLA/HASr/PDA scaffolds increases the amount of protein adsorbed onto the surface of the scaffolds by approximately 10%, in comparison with the other samples.

Recent advances have shown that the amine groups in the PDA interact well with protein groups, increasing adsorption and also modulating the cell differentiation [53]. As can be seen in the SEM images, PDA also increases the surface area, further increasing the number of HASr active sites that can adhere from a microscopic point of view.

## 4 | Conclusions

This study investigated the influence of incorporating strontium-substituted hydroxyapatite (HASr) and polydopamine (PDA) coatings on the physicochemical properties of PLA-based scaffolds for Bone Tissue Engineering (BTE). The results present that the addition of HASr enhances scaffold hydrophilicity by reducing the contact angle, thereby potentially improving bioactivity.

The application of PDA further enhances hydrophilicity, as evidenced by the contact angle values of PLA/PDA and PLA/HASr/PDA scaffolds (40.6° ± 1.6° and 31.5° ± 5.8°, respectively). The results observed in the modified scaffolds, including apatite formation and enhanced protein adsorption (increase of ~10%) in the in vitro biomineralization study, highlight the strong potential of this material combination for bone scaffold applications. Also, SEM images indicate an increased surface area due to PDA deposition, which may facilitate ion release and cell interaction.

Overall, these results highlight the promising potential of these modified PLA scaffolds for use in bone tissue engineering. These findings suggest that surface modifications can play a crucial role in optimizing scaffold characteristics to enhance the long-term osteogenic response. Further studies involving cell culture imaging, extracellular matrix production analysis, quantitative real-time polymerase chain reaction (qPCR), and immunostaining tests could provide further clarification on this matter.

### Author Contributions

**Adones Almeida Rocha:** formal analysis (equal), investigation (equal), methodology (equal), visualization (lead), writing – original draft (lead). **Vinicius Oliveira dos Santos:** formal analysis (equal), methodology (equal), writing – review and editing (equal). **Alexandre Malta Rossi:** conceptualization (equal), project administration (equal), supervision (equal). **Anna Carla Araujo:** conceptualization (equal), formal analysis (equal), investigation (equal), methodology (equal), resources (equal), supervision (equal), validation (equal), writing – review and editing (equal). **Rossana Mara da Silva Moreira Thiré:** conceptualization (lead), data curation (equal), investigation (equal), methodology (equal), project administration (equal), resources (equal), validation (equal), writing – review and editing (equal).

(equal), project administration (equal), resources (equal), supervision (equal), validation (equal), writing – review and editing (equal).

### Acknowledgments

The authors are grateful to the Laboratório de Instrumentos e Pesquisa and the Núcleo de Desenvolvimento de Processos e Análises Químicas em Tempo Real (Institute of Chemistry/Federal University of Rio de Janeiro—UFRJ) for the thermal and ICP/OES analyses, respectively. The authors also thank the Multiuser Laboratory of Materials Characterization and the Electron Microscopy Nucleus of COPPE at Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil, for FTIR and XRD analysis and for their support with the use of the microscopes, respectively. The authors would also like to acknowledge the support of Dr. Claudio Teodoro dos Santos and Dr. Cristiane Evelise from the National Institute of Technology—INT Rio de Janeiro, and Professor Luiz Malta from the Institute of Chemistry—UFRJ. The Article Processing Charge for the publication of this research was funded by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) (ROR identifier: 00x0ma614).

### Funding

This work was supported by the following Brazilian agencies: Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—CAPES, National Council for Scientific and Technological Development—CNPq (grant: 316270/2023-7), and Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro—FAPERJ (grant: Rede NanoSaúde—E-26/210.139/2019) for financial support. The financial support for the first author's PhD joint supervision is provided by CAPES PDSE 06/2024, Finance code 0001, followed by a half-thesis scholarship from INSA Toulouse participating in the program: "Bourses de thèse sur enjeux sociétaux" (BTES).

### Conflicts of Interest

The authors declare no conflicts of interest.

### Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon request.

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