Additive manufacturing of hydroxyapatite-based functional scaffolds via vat photopolymerisation for bone tissue engineering

Abstract

The requirement for functional scaffolds made from hydroxyapatite-based materials is gaining importance in bone tissue engineering (BTE) due to their ability to provide mechanical behaviour similar to that of native tissue. Additive manufacturing (AM) via vat photopolymerisation (VPP) has been demonstrated to be a superior choice for fabricating FS, as it offers enhanced mechanical strength to support intricate structures with high printing resolution compared to other 3D printing methods. The current work presents a detailed review of different VPP AM processes to manufacture hydroxyapatite-based scaffolds for bone tissue engineering. The roles of various monomers, photoinitiators, and dispersants, as well as their impact on printability and the resulting scaffold properties, were elucidated. In addition, the debinding and sintering processes of scaffolds are elaborated. This review helps researchers and practitioners understand the latest advancements in VPP AM processes, photocurable resin technologies, and selection strategies for developing HAp scaffolds for BTE.

Graphical Abstract

This is a visual representation of the abstract.

Introduction

Hydroxyapatite (HAp) with a chemical structure of Ca₁₀ (PO₄)₆(OH)₂, ¹ and Ca/P ≈1.67, is found in the bones and teeth as a primary mineral component. ² HAp accounts for 65−70% weight of the human bones, and about 70−80% weight of dentin and enamel. ³ Owing to the chemical similarity of HAp with the bone mineral composition, it has been widely used in bone tissue engineering. ⁴ HAp is used in scaffolds for bone repair and growth, ⁵ coating material for orthopedic implants, ⁶ and as a component in surgical screws. ⁷ Bone regeneration employs porous scaffolds as frameworks to facilitate cell attachment, growth, differentiation, and tissue development. ⁸ Bone is a heterogeneous tissue composed of intricate structures of type I collagen (organic component) and inorganic HAp.^8,9 Most properties of the bone tissues are gradient-based for specific functionality. In this context, functional scaffolds (FS) are promising for tailoring mechanical and biological properties according to the requirements for bone tissue engineering (BTE). ¹⁰ By imitating the hierarchical, gradient, and porous architectures of natural bone, researchers create scaffolds that mimic the structural and functional properties of native tissue. ¹¹ Furthermore, creating a scaffold with a gradient porosity and interconnected pore networks enhances nutrient diffusion and supports vascularisation, both of which are indispensable for effective bone regeneration. ¹² Traditional methods, including thermal-induced phase separation, freeze drying, and solvent casting, are widely used in tissue engineering to fabricate scaffolds. ¹³ Nevertheless, the application of these fabrication techniques to FGS construction is hindered by constraints in achieving optimal pore interconnectivity, precise pore size distribution, and the ability to replicate complex intrinsic geometries.^13,14 In this context, additive manufacturing plays a substantial role in fabricating gradient scaffolds from HAp in a layer-by-layer fashion for tissue engineering. The AM methods suitable for fabricating the scaffold from the HAp are extrusion, light, and power-based processes. ¹⁵ Material extrusion (MEX),^16,17 direct ink writing (DIW), ¹⁸ powder bed fusion, ¹⁹ and stereolithography ²⁰ based AM processes are used for fabricating scaffolds from ceramic-based materials. MEX requires a filament (often a composite of HAp and thermoplastic polymer) or slurry made from HAp. The filament is extruded through a heated nozzle and deposited layer by layer to form the desired scaffold structure. ²¹ Researchers^22,23 created MEX filaments from HAp and polylactic acid (PLA) to 3D print the scaffolds. However, the MEX printers have lower resolution and a significant staircase effect, leading to poor surface roughness. Many scaffolds require precise pore architecture to allow cell migration and nutrient diffusion, which MEX struggles to achieve. Achieving highly porous structures with uniform pore size and interconnectivity is difficult. Direct ink writing (DIW) involves the extrusion of powder-loaded inks/ suspensions through a nozzle. ²⁴ The requisite rheological behaviour of the inks for DIW can be attained by tailoring the binder to the ceramic powder ratio. One notable benefit of the DIW technique is its ability to achieve a high solid loading slurry, facilitating easier densification following the drying and sintering processes. Nevertheless, achieving homogenisation and preventing agglomeration in slurries with such high solid loading can be challenging, potentially leading to issues with small-sized nozzles clogging and negatively impacting the overall printing quality. ²⁵ The speed and accuracy of the DIW are inadequate for manufacturing the components on a large scale. ²⁶ Additionally, 3D-printed ceramic parts experienced significant shrinkage of ∼ 30–40% during the debinding stage. ²⁷ Selective laser sintering (SLS) and selective laser melting (SLM) are the power bed fusion-based AM methods that fabricate fully dense components without the requirement of post-sintering. Due to ceramics’ poor thermal shock resistance, it is more challenging to manufacture ceramic parts using SLM than SLS, as the high thermal gradients involved in the process lead to high thermal stress and resulting cracking/distortion. ²⁸ In contrast, additive manufacturing (AM) via vat photopolymerisation (VPP) methods can provide higher mechanical strength to support the intricated structures with better printing resolution. ²⁹ The broader aspects of the AM or 3D printing technologies are mainly focused in the existing reviews for biomedical applications. The present review provides the detailed overview on advancements in the vat photopolymerisation AM processes to manufacture the scaffolds for the bone tissue engineering from the hydroxyapatite. Different photocurable resins, and their preparation were reported in detail. In addition to this, the performance of the scaffolds fabricated from the VPP 3D printing process was compared after the debinding and sintering stage with the scaffolds fabricated from the other 3D printing methods.

Additive manufacturing via vat photopolymerisation

The additive manufacturing process through VPP operates on the principle of photopolymerisation, where liquid monomers or oligomers are cured when exposed to a light source of a specific wavelength, forming a solid thermoset network.^30,31 It selectively cures the liquid resin layer by layer, based on the geometry, through controlled exposure to the light source, which enables the precise manufacturing of complex 3D objects with high resolution.^32,33 Most photoreactive biomaterial systems rely on free-radical chain-growth polymerisation triggered by light exposure to produce a photo-crosslinked network structure. ³⁴ The most common photocrosslinking reactions involved in photopolymerisation-based 3D printing include free radical polymerisation (FRP), photo-mediated redox (PMR), and thio-lene photocrosslinking (TLP). FRP occurs in the three stages: initiation, propagation, and termination. ³⁵ During initiation, light decomposes the photoinitiator into free radicals through photolysis, which occurs at a specific wavelength and then reacts with monomers to form covalent bonds. In propagation, these radicals continue reacting with monomers, sustaining polymer growth as shown in Figure 1. Termination occurs when radicals combine, halting the reaction and yielding the final crosslinked polymer network. ³⁶ TLP reactions happen when the thiol (R-SH) and alkene ((R-CH = CH₂) groups are exposed to light, which causes them to react and form covalent bonds. ³⁷ PMR crosslinking happens in polymers functionalised with phenol groups, where a photoredox catalyst initiates a redox reaction between reactive moieties. This reaction leads to the formation of covalent bonds, resulting in crosslinking of the polymer network.^35,36 A mixed mode of polymerisation is based on the combination of TRP and FRP, utilising the associated functional groups in the photocurable resin. ³⁵

Figure 1.

Schematic representation of the photocurable resin polymerisation. ³⁸

VPP 3D printing systems utilise two primary configurations: bottom-up and top-down approaches. In the bottom-up (constraint-surface) configuration, the photomask is illuminated from beneath the resin tank. In contrast, the top-down approach involves exposing the photomask from above the resin surface. ³⁹ The bottom-up configuration minimises the initial resin volume required in the vat, thereby allowing component fabrication with reduced material consumption. ⁴⁰ Large-scale objects can be fabricated, as the container height remains independent of the printed object dimensions.

Additionally, the constrained interface between the transparent vat bottom and the build platform facilitates high Z-axis resolution when combined with a recoating mechanism. ³⁹ However, for most photosensitive materials, a separation step is required to detach the cured layer from the vat surface after each exposure. ⁴⁰ This separation process introduces several challenges, including difficulty in handling high-viscosity resins, contamination from residual cleaning agents, and a high risk of part damage during detachment from the separation film. ⁴¹ In the top-down approach, the build plate is wholly immersed in the resin bath, and light is incident on the resin surface. This configuration inherently eliminates separation forces during printing and offers higher spatial resolution. ⁴² This setup is also suitable for processing high-viscosity slurries, as it commonly employs a doctor blade-based recoating system to ensure uniform resin spreading between layers. ⁴¹ However, it requires a large volume of resin. ⁴² Additionally, the height of the resin bath restricts the maximum printable part height, and controlling vibrations at the resin–air interface is challenging. ³³ In contrast, the bottom-up method is more suitable for mass production, although its resolution is relatively lower than that of the top-down approach. Owing to its low resin consumption and high levelling rate, the bottom-up configuration is widely adopted. ⁴³

Stereolithography (SLA), digital light processing (DLP), masked stereolithography (mSLA), Liquid Crystal Display (LCD) 3D printing, continuous liquid interface production (CLIP), and two-photon polymerisation (2PP or TPP) are widely used VPP 3D printing methods. Figure 2 illustrates the schematic representation of various photopolymerisation-based 3D printing methods. SLA 3D printing uses a mirrored system to direct a computer-controlled laser that cures liquid resin layer by layer in a bottom-up, vector-by-vector manner. The build platform is immersed in a resin vat and moves up or down (depending on bottom-up and top-down configurations) after each layer is cured based on the STL file design. ⁴⁴

Figure 2.

Vat photopolymerisation 3D printing technologies: (a) stereolithography, (b) digital light processing, (c) liquid crystal display 3D printing, (d) continuous liquid interface production, and (e) two-photon polymerisation.^45,46

The critical parameters that influence the efficacy and accuracy of the SLA process are laser power (PL), exposure time (TE), concentration of polymer, type, and amount of photoinitiator.^39,40 In DLP, the entire layer cures at once through light exposure using the digital light projector beneath a transparent resin vat, resulting in increased printing speed compared to SLA.^47,48 In the case of LCD 3D printing or mSLA, the light source consists of LEDs connected in series, which emit UV light through the LCD. The UV light passes through the LCD, enabling the simultaneous curing of an entire layer, which results in better throughput than the SLA. The LCD functions as a dynamic mask, selectively blocking light in regions of the cross-sectional image that should remain uncured. ⁴⁹ Giudice et al. ⁵⁰ investigated the dental model's 3D printing using the entry-level mSLA and SLA methods, and their accuracy was compared. It was reported that the SLA 3D printers were found to be more precise than the entry-level mSLA printers. Similarly, Tsolakis et al. ⁵¹ compared the LCD and DLP 3D printed dental models, and proposed that the DLP printers have better accuracy. Nevertheless, both printers can be used to print orthodontic appliances accurately.^51,52

Similar to DLP systems, CLIP technology projects patterned light composed of small pixels onto a photocurable resin. Nevertheless, instead of a traditional digital projector, CLIP employs advanced laser or LED-based projectors. ⁵³ A key innovation in CLIP is the use of an oxygen-permeable layer, which creates a dead zone above the vat base, enabling continuous rather than layer-by-layer printing, as shown in Figure 2(d).^53,54 This layer introduces oxygen, which reacts with photopolymerisable monomers to form stable peroxy radicals that inhibit polymerisation in the dead zone.^53,55 As a result, resin flows uninterrupted beneath the curing object, allowing for rapid and continuous polymer growth. This process accelerates printing speeds by 25–100 times compared to SLA and DLP systems, though it requires low-viscosity resins (up to approximately 2.5 Pa·s^56,57) for optimal performance. ⁵⁸ TPP uses a laser with a longer wavelength to trigger the photopolymerisation of resin through the two-photon absorption. ⁵⁹ The resolution of TPP 3D printing is approximately 10–50 nm. ⁶⁰ Figure 3(a) compares the resolution of the different 3D printing methods. Stereolithography (SLA) is a light-based 3D printing method that demonstrates better resolution, approximately 25 μm, a superior surface finish, and mechanical properties, as well as sintered densities, similar to those attained through conventional manufacturing methods. ⁶¹ Figures 3(b), (c), and (d) compare the different VPP 3D printing methods in terms of resolution, cost, and printable area.

Figure 3.

(a) Comparison of the resolution of 3D printing methods, ⁶¹ comparison of the different vat photopolymerisation 3D printing methods in terms of (b) resolution, (c) capital cost, ⁶² and (d) printable area ⁶³ with respect to the minimum feature size.

Hydroxyapatite-based photocurable resin

The primary component of polymerisation-based 3D printing methods is the photocurable resin, which comprises monomers/oligomers, photoinitiators, and dispersants. Monomers/oligomers polymerise through photoinitiation to form crosslinked constructs. The dispersant molecules help enhance the homogeneity and stability of the HAp particles in the resin. ⁶⁴ HAp slurry is prepared by mixing powder particles with photocurable resins, along with a suitable dispersant, light absorber, and defoamer. The HAp slurry should have an appropriate viscosity based on the 3D printing method adopted, and it should exhibit shear-thinning behaviour. For instance, at a shear rate of 30 s⁻¹, the viscosity of the HAp slurry should be less than 3 Ns/m² for the DLP 3D printing.^65,66 A low viscosity of the HAp slurry is preferred to ensure that the scraper of the SLA 3D printer can uniformly and smoothly spread suspensions layer by layer. ⁶⁷ The slurry should have a higher solid loading to minimise dimensional shrinkage during debinding and subsequent shrinkage.^68,69 However, it also leads to increased viscosity and reduced reactivity of the UV-curable system. Therefore, striking an optimal balance between solid loading and curing reactivity is crucial. The slurry should possess high stability, ⁷⁰ sufficient cured depth, and narrow cured width. ⁶⁷ Figure 4 illustrates a typical procedure for the preparation of the HAp photocurable slurry using the polyethylene glycol diacrylate (PEGDA) polymer and Irgacure 2959 (I2959) photo initiator made by Chen et al. ⁷¹ The authors first dissolved 0.5% (w/v) of the photoinitiator I2959 in the prepolymer PEGDA by stirring with a magnetic stirrer at 45°C.

Figure 4.

Preparation of polyethylene glycol diacrylate (PEGDA) and hydroxyapatite (HAp) photocurable slurry. ⁷¹

Effect of the dispersant

Dispersants interact with ceramic particles through various intermolecular forces, including hydrogen bonding, electrostatic interactions, and van der Waals forces.^72,73 Once the dispersant is adsorbed onto the particle surfaces, these molecules provide steric hindrance by extending their long chains or bulky groups into the surrounding medium.

This steric barrier helps prevent direct particle-to-particle contact and minimises agglomeration, as illustrated in Figure 5(a). ⁷² In addition to steric effects, some dispersants possess charged functional groups that generate electrostatic repulsion between similarly charged particles. This electrostatic force stabilises the suspension by preventing particle aggregation.^72,74 As the dispersant concentration increases, more ceramic particles become coated with dispersant molecules, enhancing stabilisation. ⁷⁵ These excess molecules can also form a three-dimensional network in the slurry, serving as physical barriers to particle collisions. This phenomenon is known as depletion stabilisation, where unanchored polymer chains generate repulsive forces that prevent particle approach.^72,73 The influence of the dispersant on the viscosity and stability of the ceramic slurry is illustrated in Figure 5(b). When a ceramic photocurable slurry contains a lower dispersant concentration, the dispersant covers only a small portion of the particle surface. This incomplete surface coverage fails to reduce interparticle attractions sufficiently, resulting in a tendency to cluster together and form irregular agglomerates. These agglomerates contribute to increased viscosity and poor overall dispersibility of the suspension.

Figure 5.

Particle dispersion stabilisation mechanisms, and dispersant influence on the stability and viscosity, (a) mechanisms for the HAp particles stabilisation in a photocurable resin, and (b) variation of viscosity and stability with the concentration of dispersant. ⁷²

On the other hand, if the particle surfaces are fully saturated, the excess dispersant remains free in the medium. This surplus can interact adversely with the particle surfaces, disrupting their structural integrity. Such disruption promotes flocculation, leading again to high viscosity and poor dispersion. However, when the dispersant concentration is optimal, particles become uniformly and completely coated. This generates a stable repulsive barrier, minimises agglomeration and flocculation, and results in a ceramic suspension with low viscosity and excellent dispersibility. ⁷⁵ Liu et al. ⁷⁶ investigated the effect of the BYK−111 dispersant on the rheology and curing behaviour of the HAp slurry. Steric stabilisation is the dispersion mechanism of BYK-111 dispersant. Hydrogen bonding between the −OH groups of the HAp and the −COOH of BYK−111 occurs and anchors onto the particle surfaces of the HAp. The hydrophobic chain of BYK−111 is compatible with the acrylate-based photosensitive resins owing to its ester and alkane segments. This dual interaction enhances the dispersion, reduces the agglomeration, and sedimentation of the HAp particles. Figure 6 illustrates the effect of the BYK−111dispersant on the rheology, stability, and cure behaviour of the hydroxyapatite slurry.

Figure 6.

Influence of the dispersant on the rheology, stability, and cure behaviour of the photocurable hydroxyapatite slurry. ⁷⁶

The viscosity of the slurry first decreases and then increases with increasing BYK−111 dispersant concentration. The 4% BYK−111 dispersant was observed to be the optimal concentration for improved stability. The curing process of photocurable resin-based ceramic slurry in the UV irradiation follows the Beer-Lambert law. ⁷⁷ The curing depth $({\mathrm{C}}_{\mathrm{D}})$ of the single layer can be expressed mathematically in Equation 1.^78,79 $${\mathrm{C}}_{\mathrm{D}}={\mathrm{D}}_{\mathrm{P}}(\ln {\mathrm{E}}_{\mathrm{i}}-\ln {\mathrm{E}}_{\mathrm{c}})$$ (1)

Where ${\mathrm{D}}_{\mathrm{P}}$ represents the penetration depth (or depth sensitivity). Ei and Ec signify the incident energy of UV light and the critical energy required for curing the resin, respectively.

HAp slurries with improved stability promote deeper UV light penetration by maintaining uniform particle distribution, which has been noticed for the 4% dispersant. In contrast, poorly dispersed slurries, such as those with 3% dispersant, suffer from HAp particle agglomeration and settling, which blocks UV light and limits its penetration. The authors observed slight improvements in the C_D for 5% and 6% dispersant concentrations. C. Feng et al. ⁸⁰ evaluated the influence of the solsperse 17000 dispersant at the different concentrations (1wt%, 2 wt%, and 3 wt%) by keeping the solid loading as 45 vol%. The author noticed that 2 wt% was the optimal concentration of Solperse 17000 dispersant. A high solid loading HAp slurry with improved stability can be achieved through surface modification. Li et al. ⁸¹ investigated the influence of the surface modification of the HAp particles on the sedimentation rate using the three surface modifiers, such as octadecenoamide, stearic acid, and (3-aminopropyl) triethoxysilane, on the stability of the HAp slurry over time. The authors observed that the octadecenoamide modifies the surface, promoting better wettability with the resin and stability compared to the other two, with a sedimentation rate of 74.5%–76.5%, as shown in Figure 7. A summary of the different photopolymers, photoinitiators, and dispersants used in the preparation of HAp photocurable slurry is provided in Table 1.

Figure 7.

Influence of surface modification of hydroxyapatite on the resin contact angle, and sedimentation rate. ⁸¹

Table 1.

Details about photopolymer, photoinitiator, and dispersant used in the preparation of HAp photocurable slurry.

Photopolymer	Photoinitiator	Dispersant	Reference
PEG600DMA	2,4,6-Trimethylbenzoyl-diphenyl oxide phosphine (TPO)	γ-(Methacryloxy) propyl trimethoxysilane (KH570)	82
1,6-Hexanediol diacrylate (HDDA)–HEMA–TMPTA	TPO	Solsperse 17000	80
poly (ethylene glycol) diacrylate (PEGDA)	Phenylbis (2,4,6-trimethylbenzoyl) phosphine Oxide (PPO)	Triton X-100	83
HDDA, HEMA, and EA	TPO	BYK111	76
Urethane acrylate and PEGDA-400	TPO	BYK-2155	84
HDDA and trimethylolpropane triacrylate (TPGDA)	TPO	BYK111	85
HDDA	TPO	BYK	86
Acrylamide (AM) and N-N′ methylenebisacrylamide (MBAM)	UVI-6976	Ammonium polyacrylic acid (PAA−NH4)	67
Amine-modified polyester acrylate resin (AMPA)	Ethanone, 2,2-dimethoxy-1,2-diphenyl	Phosphate ester	87
HDDA, TMPTA	TPO	Solsperse KOS163	88
Genesis Development Resin Base from Tethon3D (Omaha, Nebraska, USA)	-	Sodium dodecyl sulphate	89
Acrylic resin	TPO	Polyacrylate dispersant (CPM-D-1) and alkanolamine salt dispersants (CPM-D-2)	90
PEGDA-700	Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP)	Ammonium polycarbonate and polyethylenimine	91

Effect of the solid loading

Zeng et al. ⁹² 3D-printed a compact HAp scaffold containing 30 wt% HAp with a 50-μm layer thickness, and showed the presence of HAp and β-TCP phases after sintering at 1200°C. In a subsequent study, a higher solid loading was employed to examine the physical and mechanical performance of DLP 3D-printed HAp scaffolds. ⁹³ The scaffold fabricated with 45 wt% solid loading and a 100-μm layer thickness exhibited a compressive strength of 15.25 MPa. Scaffolds with high solid loading produce dense structures with better mechanical behaviour, and such high solid-loading scaffolds can be achieved by incorporating an appropriate dispersant into the slurry. ⁹⁴ Santos et al. ⁹¹ investigated the effect of the solid loading on the rheological behaviour of the Hap photocurable slurry with the incorporation of the dispersant (Targon 1128). Suspensions with HAp solid loadings up to 40 vol% exhibited shear-thinning behaviour, though higher loadings caused a significant rise in viscosity, as shown in Figure 8(a). As the solid loading increases, the number of ceramic particles in the slurry also rises, reducing the proportion of resin. Consequently, the internal friction within the HAp–resin slurry intensifies, increasing viscosity. ⁸⁰ The addition of Targon 1128 enhanced electrostatic stability, and viscometry at 40 vol% (10 s⁻¹) showed a minimum viscosity plateau near 0.85 wt%, as depicted in Figure 8(d). The polyelectrolyte layer is sufficiently thick at this concentration to counteract Van der Waals forces and prevent agglomeration. From the photorheology (Figure 8(f)), the authors noticed that upon UV exposure, both storage (G′) and loss (G″) moduli increased rapidly, confirming effective pre-polymer crosslinking, and Figure 8(g) represents the Jacob's working curve for the developed HAp slurry. ⁹¹ A similar study developed the UV-curable 50% HAp slurry, which exhibits shear-thinning behaviour.

Figure 8.

Influence of solid loading on the rheological and curing behaviour of the HAp slurry (a) effect of solid loading, (b) influence of dispersant on the viscosity of the slurry, (c) effect of the polymer concertation at the different concertation of HAp, (d) effect of the solid loading with the dispersant on the shear thinning behaviour of the slurry, (e) influence of the HAP amount on the viscosity of the slurry, (f and g) photorheological behaviour of the slurry rate, ⁹¹ and (h and i) effect of HAp. ⁹⁵

As shown in Figure 8(h) and (i), the shear stress of the dispersed HAp–resin slurry increases with rising shear rate, and higher HAp content results in greater shear stress at the same shear rate. ⁹⁵ Table 2 summarises the resin composition, including the amounts of HAp, viscosity, and particle size-related data from previous studies.

Table 2.

Details about the HAp photocurable slurry from the previous studies.

Resin composition	Amount of HAp	Viscosity (Pa·s)	Particle size (µm)	Reference
1,6-Hexanediol diacrylate	50 wt%	0.133 (at 100 s−1)	0.15	95
Commercial resin (68 vol%)	32 vol%	-	-	96
Commercial resin	30 wt%	> 5	12	93
3,4-epoxycyclohexylmethyl-3, epoxycyclohexane carboxylate	60 wt%	< 3 (at 100 s−1)	0.30	97
AM and MBAM	52 wt%	< 3 (at 40 s−1)	1	67
AMPA	50 vol%	1.7	0.3	87
Commercial resin	50 wt%	0.55 (at 100 s−1)	8.94	89
DSM's Somos WaterShed XC 11122	30 wt%	-	0.02	92

Effect of the HAp powder particle size and shape

The size of the HAp powder particles in the photocurable slurry is an important factor. The particle size (PS) should be less than the layer thickness (L_T), typically in the range of 25– 100 µm. PS from 0.05 to 10 µm is suggested for the photopolymerised-based 3D printing. ⁷⁸ Studies have reported that the ceramic particle size significantly influences stability and curing behaviour. The stability of the photocurable slurry is enhanced with the supplementation of ceramic powders with nanoparticles of a size. For SiC slurry, Feng et al. noticed a decrease in curing depth ⁹⁸ with the increase in the amount of fine powder. Liu et al. made a similar observation ⁹⁹ for the Si₃N₄ slurry. With the increasing mass ratio of coarse particles, the decreased curing depth was observed by Liu et al. ¹⁰⁰ for the BaTiO₃ slurry. It is owing to the upsurge in the effect of light scattering that occurs when the particle size in the ceramic slurry is close to the wavelength of the curing light. Liu et al. ¹⁰¹ investigated the influence of the particle size on the stability and curing depth of photocurable HAp slurry by considering the powders with the micro-sized (denoted with M) to nano-sized (represented with N) particles. Mass recording results indicated that particle size gradation had a significant influence on slurry stability. As the proportion of fine particles increased, the settling rate decreased notably, as shown in Figure 9. This behaviour can be attributed to the nanoscale size of the ceramic particles, which results in a high number of unsaturated atoms and -OH groups on the surface, thereby increasing surface energy and Brownian motion. ⁹⁹ These effects collectively reduce settling, in agreement with Stokes’ law, which states that settling velocity is proportional to the square of the particle size as expressed in equation 2. ¹⁰¹ $$\mathrm{Settling}\mathrm{velocity}=\mathrm{g}\times {\mathrm{d}}^2\left[{\displaystyle \frac{\mathrm{Density}\mathrm{of}\mathrm{ceramic}\mathrm{powder}-\mathrm{Density}\mathrm{of}\mathrm{resin}}{18\times (\mathrm{viscosity}\mathrm{of}\mathrm{the}\mathrm{slurry})}}\right]$$ (2)

Figure 9.

Effect of the particle size on the curing depth, (a) measurement of the stability, (b) mass of the hap particles settled on the disc, (c) curing depth and curing width influenced by the particle size, and (d) curing depth variation with the ln (exposure of energy (E)). ¹⁰¹

Where d and g represent the particle size of the HAp powder and gravitational acceleration, respectively.

A decrease in cured depth and an increase in critical energy were observed with the rise in HAp nanoparticle content in the slurry, as shown in Figure 9(e). The results are aligned with Griffith and Halloran (Equation 3). ¹⁰² $${\mathrm{C}}_{\mathrm{D}}=\left[{\displaystyle \frac{2d_m}{3{\mathrm{S}}_e}}\right]\left[{\displaystyle \frac{{\gamma }_m^2}{{({\gamma }_c-{\gamma }_m)}^2}}\right](\ln {\mathrm{E}}_{\mathrm{i}}-\ln {\mathrm{E}}_{\mathrm{c}})$$ (3)

The terms $d_m$ , ${\mathrm{S}}_e$ , ${\gamma }_c$ , and ${\gamma }_m$ represent the mean particle size, scattering efficiency, refractive index of the ceramic, and refractive index of the medium, respectively. Navarrete-Segado et al. ¹⁰³ investigated the stability behaviour of the photosensitive slurry with different HAp particle sizes. The authors obtained the different particles through the planetary ball mill (PBM) and stirred bead mill (SBM) processes.

As shown in Figure 10, the backscattering (BSTG) profiles of HAp-filled suspensions over 180 min displayed that HAp−SBM exhibited the highest stability, with negligible changes throughout the analysis. In contrast, HAp−Initial and HAp−PBM slurries displayed sedimentation at the bottom and clarification at the top, though less pronounced for HAp−PBM. These stability differences arise from particle size and suspension viscosity, since smaller particles and higher viscosity slow down sedimentation.^103,104 Among the three suspensions, HAp−initial showed the lowest stability (turbiscan stability index (TSI) ≈ 2.75), while HAp−SBM, with the tiniest particles, maintained the highest stability with an almost unchanged TSI. HAp−PBM exhibited intermediate stability, reaching a TSI of about 1.48 after 180 min. ¹⁰³

Figure 10.

Influence of the particle size on the sedimentation of the HAP slurry, (a) morphology of the different HAp particles, (b) profiles of backscattering (BSTG) for the HAP slurries with the different particle size, and (c) variation of turbiscan stability index (TSI) of the HAP slurries with the time. ¹⁰³

Particle shape strongly influences suspension rheology by modifying fluid–particle and particle–particle interactions. A suspension can accommodate the highest particle fraction before jamming when the particles are spherical (equant), and this maximum packing fraction decreases progressively as the particle shape becomes more oblate or prolate. ¹⁰⁵ As a result, suspensions of equant particles exhibit lower viscosity than those containing anisometric particles at the same volume fraction. Non-spherical particles are orientable, and their rheological effect depends on their alignment with the flow. Anisometric property intensifies particle–particle interactions because rotating non-spherical particles sweep a larger interaction volume than spheres. HAp powders typically comprise needle-like nanocrystals with lengths of ∼ 25–50 nm and thicknesses of ∼ 5 nm. ¹⁰⁶ The crystallite size strongly depends on the mineralisation route and pre-treatment conditions.^106,107 In needle-like particles of HAp, the anisotropic particles or their aggregates tend to align under shear, which reduces flow resistance. Consequently, HAp suspensions commonly exhibit pronounced shear-thinning behaviour. ¹⁰⁸ C-axis-oriented HAp powders with cylindrical particles exhibited both shear-thinning and dilatant behaviour, observed not only in flocculated slurries but also in highly concentrated dispersed slurries. ¹⁰⁹

Influence of the monomer

Resin monomers containing the polymerisable functional groups, often called reactive diluents, can be divided into mono-, bi, and multifunctional monomers. Due to variations in molecular weight, structure, and functional groups, these monomers exhibit different rheological and photopolymerisation characteristics. ¹¹⁰ Previous studies have reported that the performance of slurries with varying viscosities significantly influences printability, with higher viscosity adversely affecting the process and contributing to pore defect formation in the ceramic structure. ¹¹¹ The monomer's properties and the photocuring efficiency play a crucial role in determining the quality of the 3D-printed component during the green stage. Therefore, selecting a suitable monomer is essential to ensure optimal curing behaviour and structural integrity. Camargo et al. ¹¹² compiled various monomers and their properties from previous studies utilised in the formulation of ceramic slurries for VPP 3D printing. Most of the reported monomers are associated with zirconia- and alumina-based ceramic slurries, with a limited number related to silica, and very few applied in preparing hydroxyapatite (HAp) slurries. The widely used monomers include acrylamide (AM), ⁶⁷ cyclic trimethylopropane formal acrylate (CTFA), ¹¹³ 2-hydroxyethyl methacrylate (HEMA), ¹¹⁴ 1,6-hexanediol diacrylate (HDDA),^114,115 trimethylolpropane triacrylate (TMPTA),^114,116 tripropylene glycol diacrylate (TPGDA), ¹¹⁵ polyethylene glycol (400) diacrylate (PEGDA400), ¹¹⁷ urethane acrylate (UA), ¹¹⁸ cycloaliphatic epoxy, ⁹⁷ isobornyl acrylate (IBA), and propoxylated neopentoglycol diacrylate (PNPGDA), ¹¹⁹ 2-phenoxyethyl acrylate (PHEA), caprolactone acrylate (CA), trimethylolpropane (3EO) triacrylate (TMP3EOTA), and TMP9EOTA. ¹²⁰ For uniform and efficient recoating of successive layers during printing, the self-leveling behaviour of UV-curable resins plays a crucial role. Therefore, it is essential that the suspension in the resin tank can spontaneously spread and re-level after each printed layer is completed. ¹²¹ VPP 3D printing requires ceramic slurries with high solid content and low viscosity to minimise the risk of defects, such as cracks, pores, warping, and other structural imperfections, in the final sintered ceramic components. ¹²² Achieving low viscosity is essential for formulating high solid content ceramic slurries; however, maintaining low viscosity remains challenging, especially when aiming for effective self-leveling.^122,123

Li et al. ¹²⁴ suggested that the selecting the monomers with the lower molecular weight and particle grading are crucial for accomplishing the low viscosity slurry. Figure 11(a) and (b) represents the properties of the different monomers. CA and PHEA demonstrated low elastic moduli, with a notable difference in toughness. In contrast, HDDA, HEMA, and TPGDA exhibited relatively higher elastic moduli. Among these, HEMA showed the highest toughness, while HDDA had lower toughness than TPGDA. However, to emphasise the toughness differences while minimising the influence of high viscosity, HDDA was preferred over TPGDA. ¹²⁰ Figure 11(c) illustrates the shear thinning behaviour of the HAp slurries (40 vol% solid loading) prepared using the four monomers (CTFA, PHEA, HDDA, and HEMA). All the slurries exhibited the shear-thinning behaviour. However, for the same volume percentage of the solid loading, CTFA exhibited higher viscosity, followed by the PHEA. HDDA and HEMA showed nearly similar viscosity and shear-thinning behaviour. Figure 11(d) represents the curing behaviour of the four HAp slurries. All the slurries exhibited an augmented curing depth that was linearly proportional to the logarithm of the energy dose. ¹²⁵ The critical energy and penetration depths for the four monomers were observed as CTFA < PEHA = HDDA < HEMA, and CTFA < HDDA < PEHA < HEMA, respectively. The difference among monomers is mainly due to their difference in molecular weight and structure. ¹¹⁰

Figure 11.

Properties of resin and HAp slurries, (a) photocurable properties of the resin, (b) toughness and elastic modulus of the cured monomers, (c) shear thinning behaviour of the HAp slurries, and (d) HAp slurry photocuring properties. ¹²⁰

Wang et al. ¹¹⁰ considered the nine monomers (three monofunctional, three bifunctional, and three multifunctional monomers), and evaluated their performance for selecting the better monomer, and the properties of the nine polymers are provided in the table. The authors introduced a new scoring system that can be used to evaluate the performance of different monomers quantitatively. The scores for each property of the monomer were calculated using Equations 4 and 5, and then averaged to obtain the overall score. $${\mathrm{S}}_{\mathrm{P}}={\displaystyle \frac{{\mathrm{P}}_{\mathrm{maximum}}-{\mathrm{P}}_{\mathrm{experimental}}}{{\mathrm{P}}_{\mathrm{maximum}}-{\mathrm{P}}_{\mathrm{minimum}}}}$$ (4) $${\mathrm{S}}_{\mathrm{Q}}={\displaystyle \frac{Q_{experimental}-Q_{minimum}}{Q_{maximum}-Q_{minimum}}}$$ (5)

Here, S_P and S_Q represent the scores of individual monomer properties P and Q, respectively. P corresponds to the properties such as viscosity and shrinkage, whereas Q corresponds to tensile strength and curing depth of the monomer. The notation “P_experimental” signifies the experimental values of the properties for the individual monomer. Notations “P_maximum” and “P_minimum” indicate the maximum and minimum value among monomers for the considered property, respectively. The same notation applies to Q, with the suffixes “experimental,” “maximum,” and “minimum” defined analogously. Table 3 summarises the properties of commonly used mono, di, and multifunctional monomers. The scores for each monomer were calculated for all four properties, and the corresponding average value was determined. For example, the viscosity, shrinkage rate, tensile strength, and curing depth scores for the ACM monomer were 0.92, 0.81, 0.93, and 1.00, respectively, resulting in an average score of 0.92. Similarly, scores were calculated for all monomers, and their average scores were determined. Monomers with higher average scores were considered more suitable for SLA-based 3D printing of HAp, as they provide a better balance of rheological, mechanical, and curing characteristics. Achieving the required overall performance with a single monomer is quite challenging. Hence, most researchers combined two or more monomers to create a photosensitive resin system with superior overall performance.

Table 3.

Properties of commonly used mono, di, and multifunctional monomers. ¹¹⁰

	Monomer	Viscosity (Pa·s) × 10−3	Tensile strength (MPa)	Shrinkage (%)	Curing depth (µm)
Monofunctional	Cyclic trimethylolpropane formal acrylate (CTFA)	8.13	0.64	7.1	168
	Hydroxyethyl acrylate (HEA)	9.27	11.23	8.72	136
	Acryloyl morpholine (ACM)	12.7	31.23	8.29	720
Difunctional	1,6-hexanediol diacrylate (HDDA)	5.75	18.35	13.45	314
	Propoxylated neopentyl glycol diacrylate (PONPGDA)	13.9	20.3	9.41	292
	Tripropylene glycol diacrylate (TPGDA)	16.8	17.88	9.49	220
Multifunctional	Trimethylolpropane triacrylate (TMPTA)	92	33.52	12.22	372
	Trimethylolpropane ethoxylated triacrylate (TMPEOTA)	51.8	26.7	9.78	309
	Propoxylated trimethylolpropane triacrylate (POTMPTA)	87.3	25.3	10.36	328

Influence of the photoinitiator concentration

The efficiency and rate of the photopolymerisation are affected by the amount of the photoinitiator added to the photocurable slurry. The optimal amount of photoinitiator needs to be determined for the better performance of the components. When the photoinitiator concentration exceeded the optimal level, excessive light absorption occurred at the surface of the HAp slurry, which instigates the incomplete photopolymerisation.

Chen et al. ⁷¹ investigated the influence of the photoinitiator concentration on the cell viability of the 3D-printed HAp components, as shown in Figure 12. PEGDA and Irgacure 2959 are the photosensitive polymer and photoinitiator used to prepare photocurable HAp slurry. Authors evaluated cell viability on SLA 3D-printed components by considering the amount of photoinitiator as 0.1 (w/v)%, 0.25 (w/v)%, 0.5 (w/v)%, and 0.75 (w/v)% of PEGDA. For the 0.5 (w/v)% and 0.75 (w/v)%, higher cell viability was observed due to complete curing, indicating non-toxicity and encouraging cell proliferation. At lower concentrations, the incomplete curing of PEGDA promoted a toxic environment for the cells, resulting in decreased cell viability. Considering both dimensional accuracy and cell viability, the author has selected 0.5% (w/v) as the optimum amount of the photoinitiator. A similar study reported on the GelMA and HAp scaffolds. ¹²⁶ The different photoinitiators used in preparing the photocurable HAp slurries are listed in Table 2, as reported in previous studies.

Figure 12.

Effect of photoinitiator concentration on the cell viability, (a) cured PEGDA/HAp ⁷¹ and (b) cured GelMA/HAp. ¹²⁶

SLA 3D printing of HAp

During SLA 3D Printing, when the HAp photosensitive slurry is spread on the cured surface, its fluidity allows for close interfacial contact and diffusion, forming a slurry-cured diffusion interface due to physical adsorption and van der Waals forces. ⁶¹ The physical adsorption during the paving process is enhanced due to the large number of molecules. When exposed to UV light, photoinitiated radical polymerisation forms microgels that grow into a macrogel. The interfacial tensile strength increased between the layers due to increased exposure of the slurry surface.

The surface grooves in the cured layer are filled with the HAp slurry, and the curing results in the enhanced mechanical bond between the layers.^61,127 The dative bonds formed by the active Hydrogen with the active group are shown in Figure 13 by the blue dotted lines. Finally, chemical bonding, characterised by the succession of covalent bonds (R-CH2-CH2-R′ and R-CH2-CO-R′), contributed to strong interlayer bonding. Figure 13(c) and (d) illustrate the influence of the laser power and scanning speed on the cured width and cured depth of the monolayer disc. Increasing UV curing power from 50 to 100 mW results in higher surface energy and wider curing widths. However, increasing scanning speed from 3 to 5 m/s reduces the energy that reaches the surface of the photocurable slurry, thereby narrowing the curing width. The cured depth gradually increases with power, although its growth is more stable and less pronounced than the variation in width. Notably, when the power increased from 85 to 90 mW, a slight decrease in cross-sectional area occurred due to laser-induced material removal. ⁶¹ Chen et al. ⁷¹ evaluated the cured width of a single line (µm) (C_WS) for different combination of the process parameters, and solid loading as shown in the Figure 14. The effect of laser power (LP), scanning speed (SS), and solid content (SL) on the single line cured width was analyzed by the regression analysis, and equation 6 is provided, which relates the LP, SS, SL, and C_WS. $$C_{WS}=10.151(LP)-1493.31(SS)+46.923(SL)-0.008(LP{)}^2+169.272(SS{)}^2-0.886(SL{)}^2-1.961(LP\times SS)+6.349(SS\times SL)+1760.783$$ (6)

Figure 13.

SLA 3D printing of HAp, (a) schematic of the interlayer bond mechanism, (b) designed path for the monolayer disc, (c and d) 3D-printed and cured monolayer disc, (e) influence of the laser power on the width and depth of the of cured line, and (f) effect of scanning speed on the width and depth of the cured layer. ⁶¹

Figure 14.

Influence of the SLA 3D printing process parameters on the curing depth, (a) cured line obtained for the different experimental conditions, (b, c, and d) effect of laser power, scanning speed, and solid loading on the single line cured width, respectively, (e, f, g, and h) influence of scanning space, laser power, scanning speed, and solid loading on the curing depth, respectively, ⁷¹ SLA 3D-printed components, (i) disc-shaped porous scaffold, ⁷¹ (j) hemi spherical cup, ⁸¹ and (k) square grid. ⁶⁷

It is observed that the parameter LP is found to be dominant among all, with a regression coefficient of 0.597 on the C_WS. SS was noticed as the second dominant parameter on the C_WS. The C_WS variation with the SL at different LPs is plotted as shown in Figure 14(d). It was noted that the regression coefficient of 0.099 for SL signifies a less influential parameter than the LP and SS on the C_WS. The authors observed that the scanning space or hatch spacing (HS) had a more significant influence on the monolayer cured depth (MCD) than the LS, SS, and SL.

A large MCD was observed at the lower value of HS, as shown in Figure 14(e), and it may be attributed to the enhanced energy accumulation in the regions where scan lines overlap. Due to the overlapping of scan lines during the hatching process, the local energy density on the surface becomes higher than the energy delivered by a single laser pass. ¹²⁸ As shown in Figure 14(j), Li et al. ⁸¹ fabricated hemisphere shell, while wang et al. ⁶⁷ a square grid-like structure (Figure 14(k)) from HAp photocurable resin for the bone tissue engineering applications. Previous works have reported the different SLA 3D printers used for 3D printing the HAp photocurable resin. For instance, 3D CERAM (France), ¹²⁹ SPS450B (Shaanxi Hengtong Co. Ltd, China), ⁶⁷ SLA-250 (3D System, Valencia, CA). ⁹⁷ Chen et al. ¹²⁹ evaluated the toxic effects of the photocurable HAp slurry and SLA 3D-printed HAp scaffolds. Cytotoxicity of the scaffolds was assessed for the 3D-printed green bodies, debinded, and sintered samples using L929 cells, as depicted in Figure 15. The morphology of the cells in the photocurable HAp slurry and green body aligns with that of the positive control group, indicating a toxic nature. The debinded and sintered samples exhibited better cell viability, indicating they are non-toxic to the L929 cells.

Figure 15.

Evaluation of cytotoxicity: morphology of L929 cells, and optical density (CCK8 method) of the photocurable HAp slurry, printed green body, debinded, and sintered samples for 24 h. ¹²⁹

3D printing of HAp through the DLP method

DLP 3D printing can effectively decrease printing time in manufacturing intricate bone grafts, owing to its photoinduced process. ¹³⁰ Unlike SLA's point-by-point scanning, it cures entire layers simultaneously, resulting in quicker build times, especially for larger and more intricate structures. Cho et al. ¹¹⁴ investigated the influence of the exposure time on the DLP printed components with different pore sizes from the HAp photocurable slurry (solid loading of 43 vol%) as depicted in Figure 16.

Figure 16.

Study of pore size and exposure time influence on the DLP 3D printability, (a) images of the designed pore size, (b) exposure time of 6.5 s, (c) exposure time of 7.0 s, (d) exposure time of 7.5 s, (e and f) optical images of DLP printed components, (h and j) live and dead stain images of the osteoblast-like saos-2 cells at 14 days, and (k) viability of the cells (kit-8 assay). ¹¹⁴

The breaking of the scaffold strands was observed at an exposure time of 6.5 s, whereas pore clogging was noticed at 7.5 s (1 mm pore size). At 6.5 s of exposure time, the weak crosslinking due to insufficient exposure results in strand breakage. At the higher exposure time (7.5 s), the longer exposure likely caused over-curing, leading to excessive polymerisation and spreading of cured resin into the pore regions, which resulted in clogged pores. The authors determined the optimal exposure time and pore size to be 7 s and 1 mm, respectively, for improved printability. The surface morphology of the scaffolds after sintering, and cell viability, are depicted in Figure 16(e)-(g) and (h)-(j), respectively. A decrease in the size of the strand and pores of the scaffold was observed during sintering. The percentage of porosity after sintering for the HAp bulk, porous HAp, and surface-modified biomimetic HA_P scaffolds (S-HAp) was observed as 40.9 ± 1.5%, 61.1 ± 2.4%, and 61.3 ± 0.8%, respectively. Nearly 50% of the porosity increased for the porous HAp scaffold. Due to biomimetic mineralisation, no significant change in porosity was observed in the S-HAp scaffold, which also exhibited enhanced cell viability compared to the other two scaffolds. ¹¹⁴ Various studies fabricated the HAp scaffolds for bone tissue engineering using the DLP 3D printing technique. For instance, Martines et al. ⁸⁹ fabricated the porous HAp scaffolds with the solid loading up to 50%, noticed the 36.5% and 13.8% of the shrinkage in the X–Y and Z directions, respectively. Liang et al. ⁸⁶ prepared the nano HAp slurry with the HDDA monomer, TPO photo initiator, and surface-modified nano HAp powder. TPMS (Triply periodic minimal surface), BCC (body-centered cubic), and CPS (cubic pore-shaped) scaffolds were fabricated with a porosity of 70%. Among the three, CPS exhibited the highest compressive strength of ∼22.5 MPa. The CPS scaffold exhibited better cell metabolism, which is attributed to its larger pore size and lower substrate curvature than the other two scaffolds. Similarly, various researchers reported the DLP 3D printing of the HAp components, and their summary is provided in Table 4.

Table 4.

Summary from the previous studies on the DLP 3D printing of the HAp components.

References	Summary of the work
Y. Zeng et al. 92	When the HAp was about 30 wt%, it had better light-curing properties with good spreading ability. After sintering, the shrinkage in X-Y and Z-direction was noticed as X–Y was about 36.5% and 13.8%, respectively. Nearly 65% of volume shrinkage was observed. The phase composition in the sintered components is mainly of HAp and tricalcium phosphate.
Z. Liu et al. 93	The HAp photosensitive slurry with 45% solid loading was prepared, which is suitable for 3D printing using the DLP method at 50°C. Optimal delamination thickness, intensity, and exposure time were noticed as 100 μm, 10,000 μW/cm2, and 4 s, respectively. After sintering, nearly 49.8% of porosity and pore sizes ranging from 300 to 600 μm are observed, which are favourable for bone cell attachment and proliferation. Biological evaluation showed that the HAp scaffolds could induce calcium deposition in MC3T3-E1 preosteoblasts, indicating good osteogenic potential.
Yongxia et al. 90	HAp slurry with a solid loading of 40 vol% was developed. The slurry has a viscosity of 3.7 Pa·s at 10 s−1. After 3D printing and sintering, a dense ceramic with a relative density of 95.85% can be obtained at a sintering temperature of 1300°C.
Chengwei et al. 80	The HAp slurry was prepared with 2 wt% of the dispersant and solid loading up to 50 vol%. The influence of the sintering temperature on the different solid loadings of HAp was investigated. Relative density, shrinkage rate, and compressive strength increase with the increase in sintering temperature, and the optimal sintering temperature is 1250°C based on the flexural strength of the scaffolds.
Martines et al. 89	A commercial resin (Genesis Development Resin Base) from Tethon3D (Omaha, Nebraska, USA) with a solid loading of 40–50% of HAP. After two-step sintering, a volumetric shrinkage 60% was noticed.
Martinez et al. 131	LithaBone HA400 slurry with 46% of HAp Cell proliferation and its morphology on the 3D-printed HAp coupons are analogous to tissue culture polystyrene plates. Demonstrated the indication for osteogenesis.
Yao et al. 82	Scaffolds with the HAp powder slurries with 67% solid content were successfully 3d-printed and sintered. Microporous structure with a median pore diameter of 336.01 nm. Compressive strength of 11.61 MPa and a compressive modulus of 1.26 GPa.
Mehdi et al. 132	The authors investigated the influence of the average particle size (APS) (0.3–2.7 µm) and curing depth to layer thickness ratio on the structural defects and mechanical properties of the scaffold. A minimum of 0.9 µm APS is required to attain substantial flexural strength. The progressive vanishing of the major defects was observed by the authors when the curing depth to layer thickness ratio varied from 1.4 to 3.3 at APS of 2.7 µm.
Liu et al. 76	The authors investigated the dispersion mechanism of BYK111 and its effects on HAp slurry's rheology, stability, and curing behaviour. The low concentration of BYK111 (3 wt%) results in insufficient dispersion, leading to high viscosity, particle settling, and light scattering, which reduces UV penetration, slows polymerisation, and decreases printing precision. The authors optimised the BYK111 concentration (4 wt%) for better printing and surface quality.
Bingyan et al. 133	Diethylene glycol diacrylate (DEGDA)-based slurry containing 0–2 wt% HAp was used to 3D-print the structures, which exhibited a compressive strength of 2.9–5.8 MPa. Surface modification of HAp particles with chitosan enhanced the compressive strength by ∼53.88% over the same content of unmodified HAp. 3D-printed structures maintained good biocompatibility, with a biodegradation rate of approximately 12% within 30 days.

LCD 3D printing of HAp

LCD (mSLA) 3D printing is a cost-effective, relatively fast, and defect-free method, as it cures each layer in a single exposure rather than building it point-by-point compared to conventional SLA 3D printing.^134,135 In DLP systems, the light source and projection optics must be precisely aligned and focused to maintain image sharpness across the entire build area. In DLP printing, the resolution is inherently determined by the pixel count of the digital micromirror projector, which sets an upper limit on achievable detail beyond a certain threshold. ¹³⁶ However, the LCD system features a motorised z-positioning stage, the only moving mechanical component. It eliminates the need for precise laser beam focusing on the resin surface. ¹³⁷ Roohani et al. ¹³⁷ investigated the influence of the particle size and its refractive index on the printing accuracy of the scaffolds made from the HAp and Baghdadite (BGD) through the LCD 3D printing. Figure 17 illustrates the designed and LCD 3D-printed images created from the BGD and HAp. From the comparison of DLP- and LCD-printed pictures obtained using the BGD, it can be inferred that the DLP-printed components exhibit lower resolution than those produced by LCD 3D printing. HAp components were printed using the different 50^th percentile particle sizes (d₅₀) of 9.0 µm (HAp9), 2.6 µm (HAp2.6), and 0.5 µm (HAp0.5). Decreasing the particle size from the 9µm to 0.5 µm, the enhancement in the surface finish, and the ability to achieve smaller feature sizes were attained. ¹³⁷ Mondal et al. ¹³⁵ reported influence of the solid loading of the HAp on the printability. Photocurable resin was prepared from the varied composition of acrylated epoxidised soybean oil (AESO), PEGDA, and nano HAp (nHAp) (rod shape with length of ∼120 nm), with the nHAp content varied from 0 to 15 vol% (SP0, SP5, SP10, and SP15). In the SP notation, S denotes AESO, and P denotes PEGDA monomer, respectively, and the corresponding proportions are shown in Figure 18. The increased viscosity was noticed with the addition of nHA_P to the AESO/PEGDA photocurable resin. Except for SP15, all other inks exhibited zero shear yield strength (SYS). Since SYS can lead to defects, SP15 was excluded from the printability tests, while the remaining three inks (SP0, SP5, and SP10) were evaluated for printability. Indeed, printability decreased with increasing nHAp content as depicted in Figure 18, likely due to UV light being blocked and scattered by the nanoparticles. The human femoral diaphysis model was 3D-printed from the SP5 slurry and exhibited satisfactory printability. ¹³⁵

Figure 17.

Designed and 3D-printed features, (a) designed mask model, (b) DLP printed models from the BGD, (c) LCD 3D-printed features from the BGD, LCD 3D-printed features from HAp with the different particle size, (d) HAp9, (e) HAp2.6, and (f) HAp0.5. ¹³⁷

Figure 18.

mSLA 3D printing of HAp slurries, (a) 3D-printed components, (b) printability of the different resins, (c and d) 3D-printed human femoral diaphysis using SP5, (e) viability of MC3T3-E1 osteoblasts for the 7 days, ¹³⁵ (f) 3D printing in different orientations, and (g–i) SEM images of the printed scaffolds. ¹⁰³

The addition of 10 vol% nHAp (SP10) increased tensile strength by 58%, yield strength by 90%, Young's modulus by 144%, and plane-strain fracture toughness by 41% compared to pure AESO/PEGDA photocurable resin. These enhancements are attributed to efficient nanoscale load transfer between phases, facilitated by the high surface-area-to-volume ratio of the nanoparticles. SP0, SP5, and SP10 exhibited better cell viability of MC3T3-E1 osteoblasts on the 7th day, enhanced with the increasing HAp content (SP0 < SP5 < SP10) but lower than that of HAp. Chen et al. ⁹⁵ submicron-sized HAp particles were crucial for maintaining slurry stability and preventing sedimentation during LCD 3D printing. The authors investigated the influence of the solid loading (30 wt%, 40 wt%, and 50 wt%) on the properties of the scaffold. The scaffold printed from the 30 wt% HA slurry most closely matched the design, achieving a pore size of 605 μm and a layer thickness of 49.81μm. After sintering, the linear shrinkage increased with decreasing HAp content, resulting in pore sizes of 390–450 μm, which are ideal for bone tissue engineering.

Mondal et al. ¹⁰³ examined the anisotropic effect of printing orientation by fabricating scaffolds with 1000-μm macropores at 0°, 45°, and 90° orientations as shown in Figure 18 (f)–(i). No significant differences were observed in pore size, porosity, or relative density across the printing orientations. However, debinding–sintering shrinkage was found to depend on the printing orientation, with greater shrinkage occurring perpendicular to the printed plane. Mechanical property analysis revealed a slight anisotropic effect, with the highest compressive strength (4.9 ± 0.3 MPa) achieved by scaffolds printed at a 45° orientation.

Fabrication of HAp components through the CLIP

The function of CLIP technology is similar to DLP, but it incorporates an oxygen-permeable window. The oxygen generates a dead zone of uncured resin between the growing part and the window, allowing fresh resin to flow in continuously. This enables uninterrupted printing at speeds exceeding hundreds of mm/h, surpassing similar light-based 3D printing methods.^138,139 CLIP enables layerless, monolithic fabrication, producing prints with exceptional resolution at remarkably high speeds.^138,140 Thus, it effectively eliminates the common staircasing effect seen in traditional 3D printing, which arises from the layer-by-layer curing and stacking approach used to build 3D objects. Deng et al. ¹¹⁷ investigated the development of the nano HAp-filled PEGDA-based bioactive and osteoconductive scaffolds. The authors observed an enhanced compressive strength of approximately 342% compared to pure PEGDA scaffolds. Klein et al. ¹⁴¹ fabricated HAp-filled (5 wt%) polypropylene fumarate (PPF) based composite scaffold for the bone tissue engineering applications using thiol cross-linker. The 3D-printed pure PPF and PPF with HAp-filled scaffolds are shown in Figure 19. The mechanical properties of the 3D-printed scaffolds were tested with varied post-processing parameters, including curing time and temperature, as shown in Figure 19(c) and (d).

Figure 19.

CLIP 3D-printed components, (a) polypropylene fumarate (PPF) scaffold, (b) HAp-filled (5 wt%) PPF-based composite scaffold, and (c and d) effect of the post-processing parameters on the scaffold's mechanical properties. ¹⁴¹

The ultimate tensile and compressive strength of the samples augmented with the increase in curing time and the number of drying days. With alternation of the post-processing parameters, elastic modulus in tension for the pure PPF, and PPF with the HAp filler scaffolds was enhanced from 72.9 ± 15.7 to 316.5 ± 43.3 MPa, and 69.4 ± 10.0 to 319.2 ± 8.6 MPa, respectively. Similarly, the elastic modulus in compression was enhanced from 24.3 ± 3.1 to 148.7 ± 21.8 MPa for pure PPF and from 23.5 ± 2.4 to 171.7 ± 26.0 MPa for PPF with the HAp filler scaffold.

Fabrication of HAp scaffolds using TPP /2PP

Roohani et al. ¹⁴² 3D-printed the complex structures from the bone prenucleation nanoclusters (BPNCs) with a mean size of 5 nm. These clusters, generally consisting of sub-nanometric to nanometric calcium triphosphate ions (Ca(HPO₄)₃⁴⁻), act as precursors to amorphous calcium phosphate (ACP) granules, which subsequently transform into thermodynamically stable crystalline HAp during bone formation. ¹⁴³ The dried BPNCs were amorphous, but heat treatment at 400°C initiated HAp formation, while higher temperatures yielded crystalline β-tricalcium phosphate and β-calcium pyrophosphate. Triethylamine functionalisation stabilised BPNCs, preventing agglomeration and maintaining their amorphous state, unlike untreated BPNCs that crystallised at room temperature. These stabilised BPNCs were dispersed in photocurable resin. Acrylate-based photoresin is used to produce clean, transparent, light-yellow TPP-compatible ink. During 2PP 3D printing, BPNCs coalesce into larger, spherical ACP-like particles under laser irradiation without a change in crystallinity, driven by their inherent instability. Transient ACP often forms hollow structures via dissolution–recrystallisation, wherein surface crystallisation creates a stable shell while the hydrated core dissolves. ¹⁴⁴ This transformation increases particle size by approximately four- to tenfold, yielding mature ACP-like nanoparticles. Figure 20 depicts the versatility of 2PP in fabricating CaP lattice structures and patterns using BPNC ink, from macroscale gyroid lattices (250 µm × 50 µm × 250 µm, 850 nm wall thickness) to nanoscale octet truss lattices (unit cell: 2 µm³, strut: 360 nm). Post-sintering shrinkage ranged from 15% to 35%, influenced by the BPNC content in the photocurable resin and the sintering temperature. The relatively low shrinkage compared to other vat photopolymerisation methods highlights the structural reinforcement achieved with high BPNC loading. ¹⁴²

Figure 20.

2PP 3D printing, (a) formation of amorphous calcium phosphate through the PNCs interaction with femtosecond laser, (b and c) 3D-printed lattice structures from macro to nano scale, and (d and e) micro bone tissue model displaying osteons, and 3D-printed model, respectively. ¹⁴²

Evaluation of peeling process behaviour and cleaning methods

In VPP 3D printing, the scaffold is fabricated on the printhead/build platform and subsequently peeled off from it for further processing. ⁴⁷ The scaffold is peeled off from the printhead by exerting mechanical work against the adhesive force resisting the separation. Peeling the original porous scaffold directly from the build platform is a common and efficient approach, but in highly porous scaffolds, it often causes peeling cracks that compromise mechanical stability. Therefore, understanding the mechanism of peeling crack initiation is crucial for enhancing peeling efficiency and maintaining scaffold integrity. ¹²⁰ Liang et al. ¹²⁰ investigated the peeling behaviour of the HAp scaffolds, which are made from the four different monomers such as HDDA, HEMA, CTFA, and PHEA, using finite element analysis (FEA) as shown in Figure 21 (a) and (b). The HDDA-HAp green scaffold effectively resists peeling forces and maintains cohesion among the HAp particles due to its high modulus and toughness, thereby preventing pore formation and peeling cracks. In contrast, the HEMA-HAp green scaffold exhibits strong adhesion to the platform due to hydrogen bonding, and its limited toughness results in the initiation of peeling cracks at stress concentration sites. For the CTFA-HAp and PHEA-HAp green scaffolds, their coiled and loosely packed molecular chains undergo significant deformation during peeling, ultimately developing peeling cracks. ¹²⁰

Figure 21.

Peeling process behaviour and effect of cleaning methods, (a and b) finite element analysis of the peeling behaviour of the four different monomers, ¹²⁰ (c) scaffolds images after soaking in LithaSol 80 and DBE for 24 h at various temperatures, (d) morphology of the scaffolds processed through the different cleaning methods, and (e) compressive strength of scaffolds processed through the different cleaning methods. ¹⁴⁵

Following scaffold removal from the build platform, selecting an appropriate cleaning strategy is crucial to achieving the intended properties. Different solvents are used to clean the scaffolds. For instance, ethanol, ¹⁴⁶ dibasic ester (DBE), ¹⁴⁷ isopropanol, ¹⁴⁸ lithaSol (LS) 20, ¹⁴⁹ and 30. ¹⁵⁰ DBE exhibited excellent cleaning with less delamination. Isopropanol showed moderate cleaning and delamination, and ethanol disclosed poor cleaning, which caused delamination. ¹⁵¹ The complexity of the design will determine the selection of the cleaning method and agent. Ultrasonic cleaning was more effective for small holes but caused greater structural damage to the green body. Spray cleaning produced lower surface roughness, minimal damage, and less residual slurry. ¹⁵² Ressler et al. ¹⁴⁵ investigated the efficacy of the DBE and LS80 in combination with soaking and ultrasonic methods at different temperatures, as shown in Figure 21. LithaSol 80 did not achieve complete cleaning at any temperature within the specified timeframe, while DBE showed poor performance at 23°C but proved effective at higher temperatures in thoroughly cleaning the scaffolds. These findings suggest that elevating the cleaning temperature enhances the flowability of uncured slurry within the pores of as-printed scaffolds, thereby improving cleaning efficiency. Ultrasonic cleaning with DBE was more effective than soaking; however, DBE-treated samples exhibited greater shrinkage than those cleaned with LithaSol 80. Cracks developed after 96 h of soaking, while ultrasonic cleaning produced cracks and voids after just 2 h of treatment. Statistical analysis showed no significant difference in compressive strength for most sintered scaffolds, except for a notable reduction in those soaked for 48 and 72 h, indicating that DBE soaking introduced surface defects that caused substantial damage and compromised mechanical performance. ¹⁴⁵

Debinding and sintering

Polymer-free HAp scaffolds were obtained through a one-step debinding–sintering process, in which the cured resin of the printed HAp green bodies underwent pyrolysis. At the same time, the HAp particles were simultaneously sintered, thereby enhancing the density of the scaffolds. The microstructure and quality of the sintered parts are strongly influenced by the debinding atmosphere. ¹⁵³ Air thermal debinding is the simplest method and can be carried out in a conventional air furnace; however, oxidation of the organics can be aggressive, often leading to crack formation in the sintered parts. ¹⁵⁴ To minimise such defects, very slow heating rates and extended debinding schedules are required. Shorter schedules frequently result in partial or complete cracking, typically along the printed layers. ¹⁵⁵ In contrast, debinding in inert atmospheres (vacuum, Ar, or N2) proceeds mainly through pyrolysis, where polymer decomposition occurs without reactive species from the environment; however, this approach often leaves carbon residues. ¹⁵⁶ The carbon was predominantly accumulated on the surface of the brown bodies, which resulted in reduced delamination and fewer crack formations. ¹⁵⁷ Recent advances in debinding processes incorporate a two-stage approach involving heating in an inert environment for binder removal, followed by exposure to air to eliminate residual carbon.^154,158,159 Figure 22(a) illustrates the schematic representation of the thermal debinding process followed by the sintering of the typical ceramic components. The thermal analysis of the photocurable resin (TGA/DTA/DSC) should be performed before the debinding process to determine the suitable thermal treatment at the appropriate temperature. ¹⁶⁰ The heating rate (HR) and dwell time must be optimised to prevent blisters and cracks during the debinding process. ¹⁶¹

Figure 22.

(a) structural transformation from the 3D-printed green body to sintered ceramic component in the thermal debinding followed by sintering. ¹⁶² (b) sintering cycle for the different temperatures, and (c) X-ray diffraction (XRD) patterns of the HAp powder and sintered samples. ⁸²

The debinding temperature depends on the monomer used during the preparation of photocurable hydroxyapatite resin. For instance, the characteristic temperatures of the HDDA for significant weight loss were reported as 391°C and 455°C. ¹⁶¹ The use of multifunctional monomers enhances the degree of cross-linking, resulting in an increase in the characteristic temperature for the major weight loss. For instance, HDDA combined TMPTA exhibited significant weight loss at the characteristic temperatures of 250°C, 450°C, and 550°C for the same heating rate (10°C/min). ¹⁶³ Slow HR is preferable to maintain uniform temperature distribution and prevent gas formation during debinding. Sintering follows the debinding process, during which the HAp ceramic part is subjected to heat treatment, typically starting at around 500−600°C and increasing to higher temperatures depending on the sintering requirements. This process reduces voids within the ceramic through shrinkage, enhancing its density. ¹⁶⁴

Yao et al. ⁸² sintered the 3D-printed green HAp components at the different temperatures, such as 1200°C, 1300°C, and 1400°C, as per the sintering cycle shown in Figure 22(b). Due to chemical decomposition, the density of the sintered HAp components increased from 3.04 to 3.15 g/cm³ when the sintering temperature was increased by 200°C (ΔT) from 1200°C. Similarly, the shrinkage rate increased from16.26% to 21.28%. The XRD patterns of the raw HAp and sintered HAp components are shown in Figure 22(c). The specimen sintered at 1200°C exhibits no noticeable structural transition or peak shifts, confirming the dominance of the HAp phase and the absence of significant secondary phases. However, the specimen sintered at 1400°C displays a clear structural transition, indicating that at elevated sintering temperatures, HAp undergoes decomposition and phase transformation into β-TCP. ⁸² Zhang et al. ¹⁶⁵ noticed the formation of the β-TCP phase at 1200°C and 1250°C, as compared with 1100°C, and it was attributed to the decomposition of HAp. As the sintering temperature reached from 1200°C to 1250°C, the formation of a new whitlockite phase resulted in a slight reduction in the relative density of the ceramic components. Figure 23(a) demonstrates the influences of the solid loading on the relative density and shrinkage rate of the HAp components sintered at 1200°C. It was observed that with increasing solid loading, the relative density of the HAp bioceramics increased, while the shrinkage decreased. This behaviour can be attributed to the higher concentration of ceramic particles in the slurry, which promotes densification and reduces overall shrinkage during the sintering process. As the sintering temperature increased, the number of pores within the ceramic decreased, leading to a correspondingly denser microstructure. The flexural strength initially increased and then declined with increasing sintering temperature due to the formation of cracks between particles. The compressive strength exhibited a continuous increase throughout the temperature range, due to the rise in relative density, as depicted in Figure 23(b). ⁸⁰

Figure 23.

Influence of the sintering temperature on the properties of 3D-printed HAp components, (a) variation of the relative density with the sintering temperature at the different solid loading, (b) mechanical properties at different sintering temperatures, ⁸⁰ (c) pore size variation among the green and sintered parts, (d) morphology of grains for the 50%HAp (scale bar = 1 µm), and (e) viability of MG63 cells on the scaffolds printed with different solid loading and sintered at 1200°C and 1300°C. ⁹⁵

β-TCP exhibits excellent osteoconductivity and osteoinductivity; however, its rapid degradation limits bone regeneration; therefore, it is commonly combined with the more stable HA to form biphasic calcium phosphate (BCP) with a controlled degradation rate. ¹⁶⁶ Chen et al. ⁹⁵ investigated the effect of sintering temperature on LCD 3D-printed BCP scaffolds. The β-TCP phase forms at a sintering temperature of 1200°C, while the α-TCP phase emerges upon further increasing the temperature to 1300°C. At 1300°C, the Ca/P ratios of the HAp scaffolds fabricated with 30%, 40%, and 50% solid loading were 1.07, 1.16, and 1.34, respectively. In comparison, at 1200°C, the corresponding Ca/P ratios were 2.58, 3.99, and 2.87. A Ca/P ratio below 1.6 indicates the formation of TCP phases.^95,167 Scaffolds printed with 50% HAp slurry showed the highest compressive strength. Although the strengths at 1200°C and 1300°C were comparable, the formation of α-TCP at 1300°C slightly influenced the mechanical properties. Fluorescence intensity increased with incubation time, confirming that all scaffolds supported cell growth. However, the scaffold printed with 50% HAp slurry and sintered at 1200°C showed significantly lower fluorescence than the 30% HA (1200°C) and 30–40% HAp (1300°C) scaffolds. The scaffold printed using a 50% HAp slurry and sintered at 1200°C exhibited a crystal phase composition containing more than 70% HAp. Shao et al. ¹⁶⁶ reported that BCP scaffolds with a lower HAp/β-TCP ratio (50:50) exhibited enhanced MG63 cell proliferation compared to those with a 70:30 ratio. The higher β-TCP content results in improved MG63 cell proliferation, whereas the 50% HAp scaffold, containing more than 70% HAp, yields slightly reduced fluorescence intensity.^95,166

Conclusion and future aspects

Various vat photopolymerisation 3D printing techniques for fabricating hydroxyapatite scaffolds in bone tissue engineering are discussed in detail. The roles of different monomers, photoinitiators, and dispersants, as well as their impact on printability and the resulting scaffold properties, were elucidated. Additionally, the debinding and sintering processes for scaffolds fabricated through VPP 3D printing are elaborated. The critical step in the VPP 3D printing is the preparation of the HAp slurry. The particle size selection, optimisation of the monomer, HAp, photoinitiator, and dispersant concentrations are essential for preparing the HAp slurry with improved printability. Typically, ceramic resins are recommended to have a viscosity below 3 Pa·s to facilitate proper self-leveling and recoating. ¹⁶⁰ Most studies have employed a 30–50% HAp content, which results in significant shrinkage after sintering. Future research should optimise the HAp resin composition for optimal printability, with a high solid loading content, to achieve better mechanical properties.

Although substantial progress has been made in the thermal debinding of VPP 3D-printed ceramic components, the high incidence of undesired defects and the excessively long debinding cycles remain major challenges. These limitations significantly constrain manufacturing efficiency and severely hinder the large-scale industrial adoption of such components. While prior studies have explored the relationship between resin formulations and the mechanical performance of 3D-printed parts, the behaviour of the cured green bodies remains insufficiently understood and requires further investigation. In particular, comprehensive mechanical and thermomechanical studies are necessary to evaluate the impact of photoabsorbers on mechanical properties and interlayer cohesion. Most studies focus on post-printing thermal optimisation. However, thermal shrinkage alters the original dimensions, requiring strict dimensional control. Further investigations are necessary to minimise shrinkage at the design stage. Establishing a clear relationship between shrinkage, processing temperature, and solid loading percentage is crucial for developing empirical models that can accurately compensate for shrinkage during model design. Although sintered scaffolds exhibit high compressive strength, they are inherently brittle and susceptible to microstructural defects. Additionally, the influence of defects—particularly those within the interlayer regions—requires further systematic investigation. The tensile strength of the sintered specimen is mainly influenced by interlayer bonding and the degree of polymerisation. A detailed investigation is necessary to elucidate the influence of interlayer bonding mechanisms on tensile strength, as this aspect has been explored in only a very limited number of studies.

The impact of hydroxyapatite (HAp) particle size distribution and particle shape on sintering behaviour and mechanical strength has attracted increasing attention. Notably, most studies for biomedical applications employ HAp particles with a needle-like morphology. Nevertheless, the correlations between particle size distribution and particle shape with the curing behaviour of HAp-based slurries remain incompletely understood. HAp-based slurries often suffer from poor dispersion in various monomers and limited interfacial affinity with organic resin components, which adversely affects printing fidelity and slurry stability. Slurry stability is a significant challenge, often leading to inhomogeneous properties in fabricated parts. ¹⁶⁸ Consequently, surface modification of HAp particles becomes essential to enhance their compatibility with the resin matrix and to ensure homogeneous dispersion. However, the influence of HAp surface modification on photopolymerisation behaviour during printing, binder removal during thermal debinding, sintering kinetics, and the resulting mechanical properties of the printed scaffolds has not been comprehensively explored.

Achieving reliable printing and processing of HAp components requires the right balance of dispersion medium, surfactant, and HAp loading. Utilising statistical tools such as design of experiments (DOE) ¹⁶⁹ and regression models, ¹⁷⁰ in VPP 3D printing will enhance efficiency through minimal experimental studies. The optimisation of VPP 3D printing can be achieved through machine learning applications. ¹⁷¹ The predictability of machine learning models depends on several factors, including HA_P resin composition, container–HAp resin interactions, printing parameters, and the environmental sensitivity of the HAp resin; yet, these aspects remain largely unexplored. Quality control, process traceability, and real-time analytics can help reduce variability and ensure the production of reliable products for approval. ¹⁷²