Calcium Phosphate Microcapsules as Multifunctional Drug Delivery Devices

More challenging active pharmaceutical ingredients are entering the market, spurring the introduction of novel drug delivery strategies that necessitate a paradigm shift from exhausted excipients to materials with combined actions and multiple functionalities. In this study, an inorganic calcium phosphate microparticle with a hollow internal structure is introduced as a biocompatible and multifunctional microcapsule: the template inverted particle (TIP). A robust process is presented to create a unique particle geometry, which is characterized by a particle size of 20 µm and a hollow cavity enclosed by a specially engineered porous shell. This study focuses on the characterization of TIP as an excipient for the design of solid dosage forms. The cavities in the particle centers serve as an encapsulation space, resulting in boosted water uptake capacity of 5.3 cm3 g−1. Benefiting from the material's high wettability and water uptake rates, TIP tablets immediately disperse in the oral cavity. Mechanistic studies reveal a viscoelastic behavior of empty TIP microcapsules in accordance with the Kelvin–Voigt model of a parallel spring‐dashpot configuration. The unique particle geometry is maintained during compaction thanks to its exceptional structural integrity. This study demonstrates how multifunctional TIP microcapsules can be applied as a pharmaceutical drug delivery device.


Introduction
Inert particulate drug carriers offer promising opportunities in oral drug delivery. [1][4] Examples include organic DOI: 10.1002/adfm.202303333[7][8] Our previous work highlighted multiple applications for FCC. [2,9,10]These porous carriers have excellent compatibility (i.e., an ability to keep desired properties after the application of compressive force) [11] and form tablets extremely resistant to crushing. [12]Despite their hardness, these tablets disintegrate (i.e., disperse) within seconds when exposed to water.The rapid disintegration results from the high porosity and significant specific surface area (SSA) of the micrometer-sized drug carriers. [13,14]This facilitates rapid liquid uptake by capillary forces and weakens the van der Waals forces, which hold the particles of the tablet together.The reported short disintegration times of under 10 s outperform many tablet formulations made of traditional excipients like microcrystalline cellulose (MCC) and are comparable to the reconstitution time of lyophilisates. [2][17][18] Rapid disintegration within the oral cavity leads to prolonged buccal mucosa exposure, enhancing oral bioavailability and an immediate onset of therapeutic action. [19,20]Orally dispersible tablets (ODTs), made of FCC, have an excellent mouthfeel and are well accepted by children. [21,22]Moreover, concerning dosage form administration, solid dosage forms like tablets have several advantages over liquid dosage forms such as syrups.[25] There are only a few porous drug carriers, such as modified anhydrous dibasic calcium phosphate (Fujicalin), FCC, or amorphous magnesium aluminometasilicate (Neusilin), which support direct compaction at a large scale and offer cost-effective manufacturing process. [6,26,27]With these components, no filler or diluent is required resulting in minimal amounts of additional excipients (i.e., a few percentages of a superdisintegrant and the lubricant magnesium stearate) and small tablet sizes.
Despite the features mentioned earlier, porous inorganic carriers, including FCC, share a common limitation: their limited loading capacity.Insufficient pores interlink, as found in many inorganic carriers, makes efficient drug loading difficult. [28,29]ub-optimal loading results in drug deposition on the carrier's surface instead of filling the internal structures.Consequently, the carrier loses its multifunctionality, specifically its ability to produce tablets that disintegrate quickly and with high mechanical strength. [30]iven these limitations, we have explored in the present study the properties of a novel inorganic carrier material, namely calcium phosphate-based microcapsules. [31]These microcapsules, designated as template inverted particles (TIP), have a uniform size and inner geometry characterized by a single large cavity enclosed by a porous calcium phosphate shell.The shell's microcapillary structure was engineered and manufactured to facilitate drug loading by solvent-based methods.Particles consist of pure calcium phosphate in the form of hydroxyapatite, which can be considered to be safe since it is widely used as a supplement and food additive.][34] Hydroxyapatite is a naturally occurring mineral known to be a major component of bone. [35]It is listed as a compendial material in the United States Pharmacopeia (USP) and European Pharmacopeia (Ph.41] The present study aims to confirm TIP's chemical and structural composition and visualize its morphology and internal geometry.It describes the physical-chemical properties of TIP such as pore size distribution, intrusion volume, and water uptake capacity.In addition, this study determines if the hollow TIP particles withstand compressive forces applied during compaction.

Design and Crystallographic Elucidation of TIP
As shown in Figure 1a, TIP manufacturing consists of three main steps, namely, activation of calcium carbonate (CC) with orthophosphoric acid to TIP Stage 1 (TIP S1), followed by calcination resulting in the formation of TIP Stage 2 (TIP S2).Subsequent pH-controlled washing removes the template core, resulting in the final TIP.The latter resulted in a high initial pH of the washing solution (pH 12.6) and gradually decreased with additional washing cycles to a pH below 10.5.Using X-ray powder diffraction (XRPD), we identified the involved materials and their crystalline structure contributing to the particle formation.In Figure 1b, we identified the XRPD pattern of the CC template with characteristic peaks at 23.03°, 29.38°, 35.96°, and 43.16°.Activated TIP S1 reveals the formation of an octacalcium phosphate (OCP) shell with characteristic peaks at 4.68°and 9.41°surrounding the CC template.This notion is supported by scanning electron microscopy (SEM) analysis of a cross-section of embedded TIP S1 particles, which reveals a formation of a porous shell (Figure 1c).Following calcination at 700 °C, TIP S2 is formed.The XRPD spectrum in Figure 1d displays no carbonate-specific patterns, demonstrating the complete decomposition of the CC core into CaO with characteristic peaks at 30.98°, 37.31°, and 53.81°.SEM analysis underlined the decomposition, revealing a porous core.TIP S2 patterns additionally confirm a crystalline change in the particle shell from OCP to pentacalcium phosphate (i.e., hydroxyapatite), with the OCP peaks disappearing and the characteristic hydroxyapatite peaks appearing at 10.93°, 25.98°, 31.86°,32.29°, 32.97°, and 34.15°(Figure 1d).Next, we washed the calcined material and removed the decomposed calcium oxide CaO core to obtain the final TIP.As shown in Figure 1e,f, TIP material is consistent with the peaks of the tricalcium phosphate reference material (TCP), which is identical to hydroxyapatite, indicating that all impurities and intermediate products, such as the CaO and Ca(OH) 2 , have been completely removed.To confirm that TIP is composed entirely of pure hydroxyapatite, its XRPD spectrum was compared to that of and -TCP.The latter has characteristic peaks at 10.92°, 13.64°, 27.84°, 31.04, and 34.38°, while the alpha form is defined by its characteristic peaks at 12.04°, 22.08°, 22.90°, 24.26°, and 30.76°.The absence of noticeable similarities between the spectra of TIP and -/-TCP demonstrates the purity of the material (Figure S1, Supporting Information).In Figure S2, Supporting Information, the Fouriertransformed infrared (FTIR) spectrum of TIP is compared to spectra of and -TCP and TCP reference material in the form of hydroxyapatite.
The FTIR spectra of TIP and TCP reference material displayed three sharp peaks at 631, 601, and 561 cm −1 , characteristic of hydroxyapatite.The FTIR peaks at 3287 and 631 cm −1 present in TIP and TCP reference material only are attributed to OHstretching and libration bands in hydroxyapatite.The most intense peaks were found between 611-503 and 1220-913 cm − 1 , corresponding to the stretching vibrations of the phosphate groups (PO −3 4 ).

Morphology and Internal Geometry of TIP
Next, we studied the surface morphology and elucidated the internal particle geometry.Figure 2 displays SEM and focused ion beam (FIB)-SEM images of activated calcium carbonate particles (TIP S1), particles after calcination and template removal (TIP), and ivermectin-loaded TIP particles.Despite the change in the crystalline structure of TIP S1 undergoing calcination (Figure 1ce), no morphological differences in the particle surface were observed between the SEM images of TIP S1 and TIP.Both particle stages revealed a porous surface covered with delicate lamellar structures (Figure 2a).A FIB-SEM analysis was carried out to explore the impact of calcination and washing on the internal geometry of the particles.FIB-SEM results of TIP S1 and TIP particles show that calcination followed by washing has removed the template core, leaving behind a cavity and fundamentally changing the internal particle geometry (Figure 2b).Notably, TIP S1 particles reveal an electron-dense core surrounded by a thin, porous shell.In contrast, TIP particles show an epoxy resin-filled core, confirming the presence of a manifold cavity surrounded by a porous shell.A few particles (emphasized with a white arrow) exhibit remaining Ca(OH) 2 depositions within the core (Figure 2c).Ivermectin-loaded TIP displays surface morphologies similar to those of pure, not-loaded TIP.No drug depositions on the surface of loaded TIP particles were observed, indicating that the drug is loaded into the internal structures of the TIP particles.This statement is consistent with the FIB-SEM analysis of drug-loaded TIP particles (Figure 2b).Compared to the not-loaded TIP particles, the FIB-SEM images of the ivermectin-loaded TIP show drug deposition in the pores of the particle's shell and reveal a partially filled cavity.The finding of the FIB-SEM analysis was further supported by the particle cross-sections, which demonstrated a fully loaded core.

Characterization of TIP
Upon activation, SEM image analysis revealed intact particles with functionalized surfaces showing a characteristic lamellar surface structure of TIP S1 and TIP.Compared to the CC template material, TIP S1 and TIP samples do not contain a fine particle fraction (Figure 3a-c).In agreement with the SEM images, the size distribution of CC confirmed the presence of a small particle fraction and thus increased the polydispersity index (0.47 ± 0.03) (Figure 3d).This fraction becomes smaller after activation, resulting in a decreased polydispersity index (0.32 ± 0.01) and a narrower particle size distribution of TIP S1.However, TIP S1 particles are bigger (47.09 ± 1.45 μm) than the template CC material (18.98 ± 1.82 μm), indicating that activation results in particle growth (Figure 3e).After calcination and subsequent washing, the resulting TIP particles exhibit a smaller average particle size of 20.05 ± 0.12 μm, the particles are polydisperse, and their size distribution reveals a bimodal distribution (Figure 3f).
Table 1 summarizes the physicochemical properties of the template material, the TCP reference material, the final product TIP, and the TIP precursor particle stages.Surface functionalization by activation with phosphoric acid significantly impacted the measured physical properties.The SSA, capillary constant, and water uptake rate were significantly greater in TIP S1, TIP S2, and TIP than in the non-activated CC template material.Meanwhile, calcination and consequent template removal by pH-controlled washing had the most striking effect on the water uptake capacity and skeletal density.In contrast, the SSA slightly decreased compared to TIP S1.Notably, capillary constant and water uptake rates are very low for the non-porous CC and TCP reference materials.
To illustrate the impact of the calcination and template removal, Figure 4 compares the pore size distribution, water uptake capacity, and mercury intrusion volumes of CC, TIP S1, and TIP.As shown in Figure 4a, TIP S1 and TIP have a bimodal pore size distribution.TIP and TIP S1 powders were not densely packed in the penetrometer, causing the formation of interparticular voids.Larger pore sizes of TIP S1 in the range from 2 to 14 μm are assigned to inter-particular voids.In the case of TIP, the peak at larger pore sizes is assigned to the diameter of the particle cavity since it has been observed that TIP absorbs mercury at atmospheric pressure.
In contrast, the pore size distribution between 10 nm and 2 μm represents the intra-particular pores.Notably, the pore size distributions of TIP and TIP S1 differ significantly.TIP has a narrower intraparticular pore size distribution than TIP S1, while the larger pore size distributions reveal a significantly larger area under the curve and occur earlier.In contrast, CC template material reveals a unimodal pore size distribution due to the absence of intraparticular pores.Figure 4b compares the measured mercury intrusion volumes of TIP S1 and TIP tablets with the theoretically calculated intrusion volumes based on particle crosssections and tablet porosities.TIP's measured mercury intrusion volumes perfectly align with the calculated intrusion volumes.The most striking finding from Figure 4b was the consistently more than twofold higher intrusion volumes of TIP compared to TIP S1.Intrusion volumes of TIP and TIP S1 calculated based on tablet porosities (0.33 and 0.15 cm 3 g −1 , respectively) perfectly aligned with the results measured using the mercury   porosimetry (0.33 and 0.14 cm 3 g −1 , respectively) and calculated SEM cross-sections (0.33 and 0.11 cm 3 g −1 , respectively).Encouraged by these data, we tested the water uptake capacity of our particles.Surface functionalization of TIP S1 resulted in an increased water uptake capacity (2.28 cm 3 g −1 ) compared to the not activated template material (0.39 cm 3 g −1 ).In agreement with the mercury intrusion data, TIP further enhanced the water uptake capacity (5.27 cm 3 g −1 ), highlighting that removing the template creates enough space for encapsulation and water uptake (Figure 4c).Particle cross-sections, shown in Figure 4d-f, visualize the structural differences of the particles.The image analysis corroborates pore size distribution, mercury intrusion, and water uptake capacity data.

Compaction of TIP
Figure 5a shows the tensile strength as a function of the compressive pressure.CC template material did not form any compacts up to compressive pressures of 300 MPa.Still, CC tablets exhibit very low tensile strengths above that pressure.Surface functionalization is associated with a significant increase in tensile strength resulting in hard tablets.TIP S1 showed a steep and nearly linear ascent in tensile strength, unlike the other compacted particles.Calcination and template removal reduced the tensile strength of TIP tablets compared to their precursor.However, tensile strength increased twofold when compressive pressures exceeded 300 MPa.Noticeable was the color change of the tablets after calcination.TIP tablets were significantly whiter than CC and TIP S1 tablets.Figure 5b shows the disintegration times of different tablets, highlighting that TIP tablets have the shortest disintegration times (5.4 ± 0.8 s) compared to CC (7.4 ± 0.8 s), TIP S1 (12.3 ± 1.4 s) and TCP reference material tablets (18.3 ± 0.9 s).We visually analyzed TIP before and after compaction to rule out particle fragmentation as a reason for the decreased tensile strength.The SEM images in Figure 5c display a primary TIP particle before compaction (lamellar surface, top) and after compaction and subsequent disintegration in water (smooth surface, middle).Figure 5c (bottom) displays a cross-section of a TIP particle after tablet disintegration.It reveals an intact particle geometry and a resin-filled cavity highlighting the preserved porosity and functionality of the shell.
Figure 6 shows a schematic representation of the underlying mechanisms responsible for the change in compactibility of precursor TIP S1 (characterized by its calcium carbonate core) and TIP (characterized by an empty cavity).The first presented mechanism in Figure 6 is the change of the crystalline lattice sys-tem upon calcination.Three crystal water molecules leave the lattice system, which is associated with a change of the crystal lattice system from a triclinic OCP to a P6 3 /m hydroxyapatite shell, as illustrated in Figure 6, Mechanism 1. Morphological differences between TIP S1 and TIP microstructures were analyzed using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM).After calcination, particles revealed lamellae with rounded edges and intra-lamellar perforations, whereas TIP S1 lamellae exhibited characteristic sharp and angular edges (see Figure S3, Supporting Information).HRTEM images of the surface microstructures are illustrated in Figure 6.The hydroxyapatite lamellae of TIP revealed ordered crystal lattice structures (P63/m), and no ordered crystal lattice systems were detected in the OCP lamellae (triclinic P-1) of TIP S1.Further, HRTEM images of TIP and TIP S1 microstructures are shown in Figures S4 and S5, Supporting Information.A second mechanism contributing to the change in compactibility is described by the Kelvin-Voigt model of a viscoelastic material displayed in Figure 6, Mechanism 2. [42] Particles are represented as a system where a viscous dashpot is connected in parallel with an elastic spring.During the compaction of TIP S1, the viscous dashpot component is predominant.After removing the calcium carbonate template, the hollow TIP gains elasticity and exhibits dominant spring behavior.

Discussion
TIPs are produced in a multistep process including activation, calcination, and template removal.Activation is forming a calcium phosphate layer onto the surface of the calcium carbonate template particles.For simplification, this study mainly focuses on the formation of hydroxyapatite.However, we have demonstrated that the activation step leads to the formation of intermediate calcium salts such as OCP.The activation is followed by calcination above 700 °C.Calcium oxide, known as burnt lime, is alkaline and reacts immediately with water turning into calcium hydroxide, which can be washed out.As a result, the 20 μm sized hollow microcapsules of uniform size are obtained, which feature a porous shell composed of pure calcium phosphate in the crystalline form of hydroxyapatite.The dimension and geometry of the template material determine TIP's size and shape.By manipulating the template material's shapes and dimensions, it is possible to create TIP in a wide range of different desired shapes and sizes.
Characterization of the microcapsules in the present report reveals unique and unprecedented properties.We, therefore, propose that this advanced pharmaceutical material may be used  as a novel type of inorganic carrier to assist the development of patient-friendly drug delivery strategies.Conventional inorganic drug carriers, such as silica nanoparticles, FCC, or calcium phosphate-based microparticles, are characterized by a spongelike internal structure of nanometer-sized pores or channels.Such particles' suboptimal drug loading capacity is attributed to their confined inner pore volume, typically high channel tortuos-ity, and dead-end pores that prevent efficient drug deposition into the carrier. [30,43]TIP microcapsules have a fundamentally different design.Pores of the outer shell lead to an inner open space as visualized by cross-section analysis and FIB-SEM images.When the particles are in contact with liquid, the capillary forces, therefore, advance the liquid to the inner space of the particle.The excellent wettability of TIP supports this process.Indeed, TIP particles have a water uptake capacity of >5 cm 3 g −1 .The water uptake capacities do not necessarily correlate with the corresponding SSA measurements.For example, TIP (5.27 cm 3 g −1 ), with the smallest SSA of 11.99 ± 0.81 m 2 g −1 , has remarkably higher water uptake capacities compared to non-hollow calcium phosphate-based excipients such as Fujicalin (1.2 cm 3 g −1 ) and FCC (2 cm 3 g −1 ).[46] It is interesting to note that SSA does not correlate with water uptake capacity and should not be used to judge loading capacity.Taken together, we conclude that the unique particle geometry of TIP provides plenty of encapsulation space, and therefore, has the potential to solve previously described limitations. [30]Higher encapsulation capacity can be delivered by introducing the manifold cavity and not by increasing SSA as traditionally accepted. [47]Furthermore, the tortuosity of the pores in the hydroxyapatite shell decreases after calcination, reducing the pore's hydrodynamic resistance, which is beneficial for loading and encapsulation.Another promising property of the TIP material is the cavity-to-pore size ratio, which allows for active loading of the highly concentrated and viscous drug solution with subsequent solvent removal through the pores in the shell.Such a mechanism simplifies drug loading, often a limiting factor in the large-scale preparation of clinical materials or medicinal products.With this respect, we demonstrated that all embedded and subsequently segmented TIP particles were filled with the highly viscous epoxy resin (10-12 × 10 3 mPa s) [48] and thus illustrated how easily the cavities can be loaded.
Moreover, this study provides evidence for the effective drug encapsulation capability of TIP microcapsules, as demonstrated by the loading of ivermectin.The drug was encapsulated within the internal particle structures of TIP particles (i.e., in the particle's cavity) without drug deposition on surface structures.Drugs loaded into such cavities of hollow porous particles are also par-tially protected from oxygen and light, which sometimes may lead to extended products' shelf-life. [49,50]n recent years, there has been significant interest in hollow micro-particles with porous shells, which are the defining characteristics of the TIP material.Several reports suggest how to fill the hollow core with drugs while the porous shell can be used to control the release of the drugs. [51]The TIP material allows for developing a patient-friendly buccal drug delivery system capable of bypassing the first-pass metabolism effect by enabling drug absorption in the buccal and upper-esophageal region of the gastrointestinal tract.The TIP material dissolves as soon as the microcapsules are swallowed and exposed to gastric fluid.The latter effect allows for rapid drug release and potentially improves the dissolution rate of poorly soluble drugs.
The cavity of the TIP particles retains the drug in its crystalline or amorphous state, while the porous shell can be modified to have mucoadhesive properties to extend the residence time of the particles on the oral mucosa. [9]These particles can still be compacted into the ODTs or used in orally dissolving films for a wide range of patients' ages.
TIP is a mono-material that does not contain any impurities.In agreement with the structural analysis, TIP reveals no calcium carbonate (CaCO 3 ), calcium oxide (CaO), or calcium hydroxide (Ca(OH) 2 ) peaks in the XRPD spectra.During calcination, thermally unstable OCP is converted to pentacalcium phosphate, often called hydroxyapatite.54][55] Furthermore, it is reasonable to assume that any potential traces of organic contaminants introduced by the template material are eliminated at these temperatures.Pure hydroxyapatite is GRAS by the FDA and is a compendial compound listed in all major pharmacopeias. [36,37]Therefore, an important advantage of the proposed material is that it does not require extensive toxicological investigation if used in human or animal trials, as all materials used in the production of TIP exist in pharmaceutical grade, and their toxicological limits are well known.
Given previous uses in bone tissue engineering and its natural occurrence in our bodies, it is fair to assume that this material is well tolerated. [56]The nature of the material will find favor in future regulatory approval and is of interest to patients with dietary restrictions or cultural/religious requirements.
A very important characteristic of the proposed approach to manufacturing hollow porous microparticles is its simplicity and ease of reproducibility.These particles can be produced at the lab or pilot scale without additional changes in the described production process.The latter can be a good basis to support the research on using such materials in developing novel medicinal products and improving existing therapies by reducing side effects.
Pentacalcium phosphate, besides its biocompatibility, is also known to have excellent mechanical properties.Our compaction results show that particle geometry directly affects the tensile strength of TIP tablets.Comparing the compressibility profiles of TIP S1 and TIP, it is notable that TIP S1 forms significantly harder tablets, even at lower compressive pressures.From the presented results, we suggest that the reduction of mechanical stability is based on two mechanisms illustrated in Figure 6.
I. First, calcination induces a rearrangement of the crystal lattice systems.The triclinic P-1 OCP with an average crystal size of 13.5 ± 0.2 nm rearranges to the hexagonal HA with an average size of 16.9 ± 0.2 nm. [53,57]As a result of these changes, the mechanical stability and elasticity of the surface lamellae are likely to be affected.The different onset of the stepwise increase in tensile strength supports this statement.While the incline in tensile strength of TIP S1 tablets only occurred at high compressive pressures (>400 MPa), TIP showed an earlier onset of the stepwise increase (around 300 MPa), indicating an earlier brittle fragmentation of the lamellar surface structures. [12]Preserving the primary particle structure and functionality underlines that the TIP particles are microcapsules suitable for application as drug delivery devices.II.Second, we hypothesize that the cavity's presence increases the particle's elasticity, which partially counteracts plastic deformation during tablet compaction.Here, the Kelvin-Voigt model is used to describe the compressive behavior of TIP during compaction.The model consists of two components.
In our case, the spring element is responsible for the elasticity and is parasitic, while the dashpot element represents the anticipated plastic deformation.After calcination and removal of the template, the residual elasticity of TIP is considered to be remarkably higher than that of TIP S1.It is logical to expect that loading TIP's cavity with a drug will reduce the residual elasticity and thus improve the compatibility of the material.
These results highlight the complex interplay between the surface material lattice structure, particle geometry, and compactability.
A high particle porosity goes hand in hand with rapid liquid uptake and tablet disintegration, an important feature of the presented particles.The immediate disintegration (5.4 ± 0.8 s) into primary particles complements the combination of high drug loading and mechanical stability of TIP tablets.The latter is crucial for formulating ODTs, offering the advantage of exposure to the buccal mucosa, rapid and complete oral drug absorption, and taste masking.Therefore, enhancing patient adherence and acceptability (i.e., children and geriatric patients). [19,21,22]The performance of the TIP-ODTs prepared in this study was comparable to that of an oral lyophilisate.Disintegration data confirm that the particle geometry is responsible for the fast disintegration and not the chemical properties, as pure reference material has an entirely different water uptake capacity, contact angle, and disintegration time.It should be noted that a disintegration time range from 30 s to 3 min is reported to be achievable for ODTs. [15]s illustrated in the present study, these values are way longer than the TIP-ODT disintegration, which is faster than 6 s.

Conclusion
The good compaction behavior of TIP, combined with its large encapsulation capacity, makes TIP an excellent multifunctional excipient for oral drug delivery.Its high porosity and water uptake rate result in rapid tablet disintegration.Therefore, TIP is a platform solution for designing ODTs and age-appropriate drug delivery strategies.The preservation of defined particle geometry, a key feature of TIP, supports its use as a fully functional self-loading microcapsule for oral drug delivery.TIP is made of non-toxic, biodegradable material and is widely accepted as a food additive.Ongoing studies focus on the first clinical evaluation of drug-loaded TIP-ODTs.

Experimental Section
Materials: Calcium carbonate Ph.Eur. from Lehmann&Voss&Co.(PharMagnesia CC Type L600, Hamburg, Germany) was used as a particle template.Activation was performed with 85% ortho-phosphoric acid obtained from Carl Roth GmbH (Karlsruhe, Germany).Tricalcium-phosphate (pure, ≥34% Ca) was used as a reference standard and purchased from Carl Roth GmbH (Karlsruhe, Germany).and -TCP (>98%) were used as XRPD reference materials and were obtained from Sigma-Aldrich (Germany).Drug loading was performed using methanol (Ph.Eur.) from Carl Roth GmbH (Karlsruhe, Germany).Particle embedding was done in a mixture of Araldite Resin G2 and Aradur Hardener H2 obtained from Carl Roth GmbH (Karlsruhe, Germany).N-hexane purchased from Carl Roth GmbH (Karlsruhe, Germany) was used for contact angle measurements.Lubrication during powder compaction was assured with magnesium stearate from Novartis (Basel, Switzerland).As tablet disintegrant, crosscarmellose sodium (Ac-Di-Sol, FMC, USA) was used.Galvita AG kindly provided TIPs.
Activation: The overall process of TIP manufacturing can be divided into activation, calcination, and pH-controlled washing.Particle activation was done in a dimpled reactor (Corning, New York, USA) and is described by Equation (1).A suspension consisting of 200 g CC in 1200 cm 3 of deionized water was heated to 60 °C under constant stirring (RE162, IKA Werk, Staufen, Germany).A peristaltic pump (IPS-8, Ismatec, Zurich, Switzerland) was used to add 150 cm 3 of 4 m ortho-phosphoric acid with a rate of 1.5 cm 3 min −1 .After activation, the suspension was filtered, and the filtrate was dried at 95 °C in a drying cabinet (Thermo Fisher Scientific, Langenselbold, Germany).This material was designated as TIP S1. ( 1 ) Calcination: The calcination process is described by Equations ( 2) and (3).The dried filtrate was filled into ceramic crucibles and calcined in a muffle oven (LE6/11/R7, Nabertherm, Lilienthal, Germany) for 8 h at above 700 °C.During calcination, the core of TIP S2 was converted to calcium oxide or burnt lime.The OCP shell was converted to hydroxyapatite.This material was designated as TIP S2.
pH-Controlled Washing: Template removal by washing is described in Equation (3).After cooling to room temperature, the calcined particles were suspended in deionized water and vigorously stirred for 5 min (RE162, IKA Werk, Staufen, Germany).Subsequently, stirring was stopped, and the particles were allowed to sediment for 5 min.This step was followed by decanting the turbid supernatant.Washing and decanting were repeated until the pH was ≤10.5.This material was designated as TIP or "template inverted particles." Drug Loading by Solvent Evaporation: TIP was loaded with a drug load of 30% ivermectin.The drug load is expressed as a percentage and calculated according to Equation ( 4) where m API the mass of the API (g) and m TIP is the mass of TIP (g).Drug loading of TIP was performed by solvent evaporation.Ivermectin was dissolved in methanol.TIP particles were immersed in the ivermectin solution, and the solvent was evaporated using a rotary evaporator (Rotavap R-114 from Büchi, Switzerland).The water bath was heated to 60 °C, and the suspension was degassed at 600 mbar for 5 min.Drug loading was conducted with 5 g of TIP material at a constant rotational speed of 40 rpm.The pressure was gradually reduced to 40 mbar and maintained for 30 min to eliminate any remaining solvent.The loaded powder was sieved through a 355 μm sieve and mixed for 10 min at 32 rpm in a powder blender (Turbula T2C, Basel, Switzerland).
Scanning Electron Microscopy: The surface morphology and internal geometry of the CC, TIP Stage 1 (TIP S1), TIP Stage 2 (TIP S2), and TIP particles were studied with SEM.Samples prepared for the surface morphology assessment were coated with a 20 nm gold layer (EM ACE600, Leica, Wetzlar, Germany).Samples used to reveal the internal geometry were embedded into epoxy resin (1:10 Aradur Hardener H2, Araldite Resin G2) and sanded with a target surfacing system (EM TXP, Leica, Wetzlar, Germany).Ivermectin-loaded TIP was mixed with 50 wt% magnesium stearate for 10 min in a powder blender (Turbula, TC2, Basel, Switzerland).Powder blends were compacted using a Carver 4350L hydraulic hand press (carver, USA) fitted with a 5 mm round flat tooling.The compaction force was set to 0.1 kN, and tablets were sectioned with a microtome blade.All samples were mounted with a double-sided carbon sticker on an aluminum stub.Images were recorded using a 10 kV current on an electron microscope (TM4000, Hitachi, Tokyo, Japan) with a back-scattered electron detector (BSE).
Transmission Electron Microscopy and High-Resolution TEM: The lamellar microstructures of TIP S1 and TIP were assessed using TEM.The particles were suspended in water and sonicated for 1 min to detach individual lamellae from the microparticles using a Sonifier 150 (Branson Ultrasonics, Connecticut, US).Next, 15 μL of the particle suspensions were added to the copper grid and negatively stained with 5 μL of 2% uranyl acetate solution.Low-resolution images were acquired with 80 kV using a CM100 (Philips, Amsterdam, Netherlands).HRTEM images were captured using a JEM-F200 (JEOL, Akishima, Japan) with 200 kV.
Focused Ion Beam Scanning Electron Microscopy: Samples were attached on a double-sided carbon sticker and sputtered with a 30 nm gold layer (EM ACE600, Leica, Wetzlar, Germany).The analysis was performed on a FIB-SEM (Helios NanoLab 650, FEI, OR, USA) equipped with a through lens detector (TLD), in-chamber electron detector (ICD), Everhart Thornley detector (ETD), electron emission gun, and a gallium ion beam-column.The sample holder was tilted 52°, and milling was executed with a 21 nA gallium ion beam current.The size of the cross-sections was customized to the particle diameter and ranged from 10-40 μm in width, 10-20 μm in length, and 14 μm in depth.During imaging, the ion beam current was turned off.The cross-sections were directly imaged by collecting secondary electrons with a 5 kV current.
X-ray Powder Diffraction: The material composition was elucidated with thin-film XRPD using a SmartLab diffractometer system 3.1.03.(Rigaku, Tokyo, Japan) equipped with a HyPix-3000 detector and a knife edge.Samples were sieved through a 200 μm sieve and placed on a lowbackground silica sample holder.Measurements were conducted in the range of 3°-60°2-Theta at 2°min −1 with steps of 0.02°.The analysis of the diffractograms was done in the PDXL2 2.4.2.0 software (Rigaku, Tokyo, Japan).
Attenuated Total Reflection Fourier-Transformed Infrared Spectroscopy: The D-attenuated total reflection (ATR)-FTIR analysis was conducted using an IRTracer-100 (Shimadzu, Kyoto, Japan).Samples were placed directly on the diamond holder, and infrared spectra were acquired with a scan number of 20 from 400 to 4600 cm −1 .
Particle Size Distribution: The particle size distribution was determined using a laser diffractometer and a small-volume sample presentation unit (Mastersizer X, Malvern Instruments, Malvern, UK).Samples were dispersed in deionized water and analyzed in triplicates.
Mercury Porosimetry: The particles' pore size distribution and mercury intrusion volume were measured on a mercury porosimeter (Autopore IV, Micromeritics, Norcross, USA) using a 3 cm 3 powder penetrometer with a stem volume of 0.412 cm 3 and an intrusion volume of 0.387 cm 3 .Mercury intrusion volumes (n = 3) were determined using a 5 cm 3 solids penetrometer with a stem volume of 0.392 cm 3 and an intrusion volume of 0.366 cm 3 .Each intrusion volume measurement was performed with three 10 mm tablets weighing 300 mg.All tablets were compacted with 10 kN on a Styl'One single-punch tableting press (MedelPharm, Beynost, France).The low-pressure run ranged from 3.59 to 206.64 kPa, and the high-pressure run ranged from 206.54 to 206.78 MPa, while both runs used an equilibration time of 10 s.
Theoretical Intrusion Volumes by SEM Images: Theoretical intrusion volumes were determined using SEM images of particle cross-sections (n = 10).Exclusively particles with a diameter of 20 μm were included in the calculations to ensure that the particles were cut exactly in half.The following equations were used m TIP =  HA × (V Shell − V Pores ) (5) V IV SEM = 1 m TIP × V EC (7)   where m TIP is the mass of one single TIP particle,  HA is the skeletal or skeletal density of hydroxyapatite, V Shell is the volume of the shell (cm 3 ), V Pores is the volume of pores within the shell (cm 3 ), V EC is the encapsulation capacity (cm 3 ), V Cavity is the volume of the manifold cavity (cm 3 ), and V IV SEM is the intrusion volume per g of substance (cm −3 g).Theoretical Intrusion Volumes by Tablet Porosities: Theoretical intrusion volumes were determined by calculating tablet porosities and volumes.Tablets weighed 300 mg and were compacted on the Style "One" (Medel-Pharm, Beynost, France) with a 10 mm tooling and a compressive force of 10 kN (n = 3).
where ɛ is the porosity of the tablet, m is the weight of the tablet, r is the radius of the tablet (cm), h is the height of the tablet (cm),  Sk is the skeletal density of the substance (g cm −3 ), V Tablet is the volume of the

Figure 1 .
Figure 1.X-ray powder diffraction analysis of TIP particles.XRPD patterns of intermediate particle stages formed during TIP manufacturing are shown.a) Schematic representation of the TIP production process.b) XRPD pattern of calcium carbonate (CC, grey) used as a particle template material.SEM image of a CC particle cross-section.c) TIP S1 consisting of a CC core (grey) surrounded by an activated octacalcium phosphate (OCP, blue) shell.SEM image of a TIP S1 cross-section.d) TIP S2 contains a CaO core (yellow) encased by a hydroxyapatite (HA, red) shell.SEM image of a TIP S2 cross-section.e) XRPD spectrum of TIP consisting of TIP (HA, red).SEM image of a TIP cross-section.f) XRPD pattern of tricalcium-phosphate reference material (TCP) in the form of hydroxyapatite (HA, red).Scale bars of SEM images (b-e): 30 μm.See Figures S1 and S2, Supporting information, for more extensive chemical characterization of TIP.

Figure 2 .
Figure 2. Surface morphology and structure elucidation.The intermediate product TIP S1 and final product TIP were studied by SEM and FIB-SEM analysis.a) SEM images of sputtered particles.b) FIB-SEM image of a single TIP S1, TIP particle, and TIP loaded with 30% ivermectin.c) SEM images of particle cross-sections embedded in epoxy resin.TIP particles containing residual Ca(OH) 2 are indicated with an arrow.Ivermectin-loaded TIP embedded in magnesium stearate.Intraparticular drug depositions (shell and cavity) in (b) and (c) are indicated with arrows.Scale bars of (a-c): 30 μm.

Figure 3 .
Figure 3. Particle size distribution.TIP particles were analyzed by laser diffraction and visual evaluation of SEM images.a) SEM of the calcium carbonate (CC) starting material used as a template.b) SEM of the activated TIP S1. c) SEM of TIP.Scale bars of (a-c): 100 μm.d) Particle size distribution of CC. e) Particle size distribution of TIP S1. f) Particle size distribution of TIP.All particle size distributions were determined by laser diffraction and are presented as bar charts (means ± SD, n = 3).Curve plots represent cumulative volume percentages (%, v/v) (n = 3).

Figure 4 .
Figure 4. Pore size distribution, intrusion volume, and water uptake.a) Pore size distribution by mercury porosimetry of CC (grey), TIP S1 (blue), and TIP (red).b) Intrusion volumes of TIP S1 (blue) and TIP (red) tablets.SEM: Calculated intrusion volumes based on SEM image analysis of particle crosssections with an average diameter of 20 μm (n = 10).Tablet: Calculated intrusion volumes based on the tablet porosities (n = 3).MP: Measured mercury intrusion by mercury porosimetry of 10 mm tablets compacted with 10 kN (n = 3).c) Water uptake of particles (n = 3).d-f) SEM of cross-sections of TIP, TIP S1, and CC (from top to bottom) embedded in epoxy resin.Scale bars: 30 μm.Values are means ± SD.Statistical significance in (b) and (c) was assessed by one-way ANOVA and Bonferroni corrected post-hoc t-tests.*** p ≤ 0.001.

Figure 5 .
Figure 5. Compactability of particles.a) Tensile strength as a function of the mean compressive pressure of CC (grey), TIP S1 (blue), and TIP (red) (n = 3).b) Disintegration time of CC template, TIP S1, TIP and TCP reference tablets (n = 4).c) SEM of a single TIP particle before compaction and after tablet disintegration (top, middle).Cross-section of a TIP particle after the disintegration of the TIP tablet (bottom).The tablet had a diameter of 5 mm and was compacted with 458 MPa.Scale bars: 20 μm.Statistical significance in (b) was assessed by one-way ANOVA.* p ≤ 0.05,** p ≤ 0.01, *** p ≤ 0.001.

Figure 6 .
Figure 6.Crystalline lattice structure and proposed mechanisms responsible for the change in compactibility.First proposed mechanism: Schematic illustration of the crystalline lattice change upon calcination from a non-ordered triclinic octacalcium phosphate (OCP) to a hexagonal P6 3 /m hydroxyapatite shell.HRTEM images of the lamellar microstructures of TIP S1 and TIP.Scale bars of TEM images: 50 nm.Second proposed mechanism: Schematic illustration of TIP S1 and TIP viscoelastic behavior using the Kelvin-Voigt model of a parallel spring-dashpot configuration.