In at least one aspect, the present invention is related to compositions that can function as bone or enamel substitute materials.
Calcium phosphates (CaP) are important biomaterials in the fields of tissue engineering and drug delivery, because of their low toxicity, excellent biocompatibility, and osteoconductivity. For example, nanostructured CaP materials have been used in biomedical products such as scaffolds for bone regrowth,1 bioactive coatings and composites,2 carriers for drug delivery,3 and bioactive fillers for the occlusion of exposed dentin tubules.4 However, the application of conventional CaP materials as hard tissue substitutes has been largely restricted by their brittleness and low strength. Progress to reinforce their mechanical properties through incorporation of high-strength materials in the CaP matrix has been incremental.5,6 Therefore, there is a need for an effective alternative strategy to boost the mechanical properties of synthetic CaP materials in a way to broaden their application in the areas of repair of multiple fractures of long bones, or vertebral body substitution.7
The hierarchical structure of biological composites has inspired scientists to develop high-performance materials with superior properties and functionalities.8-12 Natural structural materials such as nacre, bone, and tooth exhibit excellent mechanical performance associated with their composition and unique hierarchical structures. For instance, the lamellar architectures in nacre and bone are well-known structures that provide these biocomposites with mechanical properties that far exceed those of their constituent materials.13 Another prime example, tooth enamel, consists of densely packed arrays of elongated apatite crystals organized into an intricate interwoven structure.14 Because of its unique architecture and dense mineral components, mature enamel possesses remarkably high elastic modulus and hardness, which are rarely achieved in artificial CaP materials.15,16 Thus, mimicking the structural features of such natural composites is a promising route to developing new structural materials with superior mechanical performance.13,16
Over the past decade, efforts to synthesize CaP materials with bioinspired structures have relied on methods such as freeze casting,17 vacuum-assisted filtration,18 and biomimetic mineralization.19,20 In one such study, a bioinspired hydroxyapatite/poly(methyl methacrylate) composite with a nacre-mimetic lamellar structure was prepared by a bidirectional freezing method,17 and in another, Li et al. used a vacuum-assisted-filtration method to combine CaP plates with amyloid fibrils to generate a hybrid nanocomposite with bone-mimetic features.18 Bioinspired CaP materials with bone-mimetic or enamel-like structures have also been synthesized through matrix-regulating mineralization of apatite nanocrystals with the assistance of peptides,21,22 proteins,23-25 protein-inspired polymers,26,27 block copolymers,28,29 self-assembled liquid crystals,19,20 and other organic additives.30,31 These promising studies illustrate steps toward the formation of intricate structures at limited scale, although synthesis of bioinspired CaP materials with the architectural features over multiple length scales remains a great challenge. Another promising route to achieve hierarchical structures in CaP materials involves the use of a precursor crystal as a template. For example, based on the structural resemblance between monetite (dicalcium phosphate anhydrous, DCPA, CaHPO4) and hydroxyapatite (HAp, Ca5(PO4)3OH) crystals, monetite was shown to be an effective precursor in the preparation of various nanostructured hydroxyapatite crystals.32-34 Using macrosized crystals as a template, Liu et al. synthesized ordered HAp structures via a phase transformation from monetite.35 This result demonstrated the feasibility of mimicking hierarchical designs of natural structures via a phase transformation from precursor crystals. However, the time-consuming preparation of the macro-sized monetite precursor (3 to 6 months) challenged its suitability for broader applications. Generating multilevel hierarchy is still a huge challenge in the fabrication of HAp from the direct transformation of a single monetite crystal.
Accordingly, there is a need for new materials that can functionally substitute for bone or enamel.
The present invention solves one or more problems of the prior art by providing in at least one embodiment, a method for forming a composite material. The method includes a step of dissolving chitosan, a dicarboxylic acid, a phosphate salt (e.g., NaH2PO4) and a calcium salt (CaCl2.2H2O) in water to form a reaction solution. The reaction solution is heated to form a layered monetite/chitosan/dicarboxylic acid composite (i.e., a first composite comprising monetite, chitosan, and a dicarboxylic acid). The layered monetite/chitosan/dicarboxylic acid composite is transformed into a calcium apatite/chitosan/dicarboxylic acid composite (i.e., a second composite comprising calcium apatite, chitosan, and a dicarboxylic acid).
In another embodiment, a composite formed by the method set forth herein is provided. Typically, this composite is a multilayer structure that functions as a bone or enamel replacement material. The multilayer structure includes a plurality of organic matrix layers and a plurality of calcium phosphate-containing layers. The organic matrix layer includes chitosan and a dicarboxylic acid. Characteristically, each calcium phosphate-containing layer is interposed between a pair of organic matrix layers.
b. (A) XRD and (B) EDS spectra of the organized HAp-chitosan-MAc composite. All of the diffraction peaks can be readily indexed to hexagonal phase hydroxyapatite (JCPDS 09-0432) crystals. EDS revealed the presence of calcium, phosphate, carbon and oxygen ions in the HAp-chitosan composite. The Ca/P molar ratio is measured to be 1.6, which is lower than the stoichiometric Ca/P ratio for hydroxyapatite, indicating an incomplete phase transformation consistent with HRTEM result in
Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.
Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the term “polymer” includes “oligomer,” “copolymer,” “terpolymer,” and the like; molecular weights provided for any polymers refers to weight average molecular weight unless otherwise indicated; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.
It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.
It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.
The term “comprising” is synonymous with “including,” “having,” “containing,” or “characterized by.” These terms are inclusive and open-ended and do not exclude additional, unrecited elements or method steps.
The phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When this phrase appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.
The phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter.
With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.
Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.
“CaP” means calcium phosphates.
“CPC” means calcium phosphate cement.
“HAp” means hydroxyapatite.
“Hydroxyapatite” means calcium apatite usually approximated by the chemical formulae Ca5(PO4)3(OH) or Ca10(PO4)6(OH)2.
In at least one embodiment, a multilayer structure that functions as a bone replacement material is provided. With reference to
In some variations, the organic matrix layers have an average thickness greater than, in increasing order of preference, 10 nm, 20 nm, 30 nm, 50 nm or 100 nm. In another variation, the organic matrix layers have an average thickness less than, in increasing order of preference, 2 microns, 1 microns, 500 nm, 300 nm, 200 nm, or 100 nm. In a refinement, the organic matrix layers have an average thickness from about 50 nm to about 1 micron. In another refinement, the organic matrix layers have an average thickness from about 50 nm to about 200 nm. Similarly, in some variations, calcium phosphate-containing layers have an average thickness greater than, in increasing order of preference, 10 nm, 20 nm, 30 nm, 50 nm or 100 nm. In another variation, calcium phosphate-containing layers have an average thickness less than, in increasing order of preference, 2 microns, 1 microns, 500 nm, 300 nm, 200 nm, or 100 nm. In a refinement, the calcium phosphate-containing layers have an average thickness from about 50 nm to about 1 micron. In another refinement, the calcium phosphate-containing layers have an average thickness from about 50 nm to about 200 nm.
Another feature of the present invention is the number of layers in the multilayer structure. In a variation, the multilayer structure has, in increasing order of preference, greater than or equal to 2, 3, 4, 5, or 6 organic matrix layers. In a refinement, the multilayer structure has, in increasing order of preference, less than or equal to 50, 30, 10, 8, or 5 organic matrix layers. In a particular refinement, the multilayer structure has from 3 to 30 organic matrix layers and from 3 to 30 calcium phosphate-containing layers.
In another refinement, a method for making the multilayer structure set forth above is provided. The method includes a step of dissolving chitosan, a dicarboxylic acid, a phosphate salt (e.g., NaH2PO4) and a calcium salt (e.g., CaCl2.2H2O) in water to form a reaction solution. In a refinement, urea is included (i.e., added to) in the reaction solution. In a variation, the reaction solution includes 0.5 to 3 weight percent phosphate salt, 0.5 to 5 weight percent a calcium salt, 0.5 to 4 weight percent chitosan, and 0.1 to 1 weight percent of the C2-6 dicarboxylic acid. In a refinement, the reaction solution includes 0.5 to 5 weight percent urea.
The reaction solution is heated to form a first composite which is a layered monetite/chitosan/dicarboxylic acid composite. As such, the first composite includes layered monetite, chitosan, and the dicarboxylic acid. The layered monetite/chitosan/dicarboxylic acid composite is then transformed into a second composite which is a calcium apatite/chitosan/dicarboxylic acid composite. In some variations, the second composite is a hierarchical composite with orderly bundle-like structures on the nanoscale (i.e., structures with dimensions of 1 to 100 nm) as well as layered organization on the microscale (e.g., layers organized with separations from 0.5 to 3 microns). As such, the second composite includes calcium apatite, chitosan, and the dicarboxylic acid. In one variation, the layered monetite/chitosan/dicarboxylic acid composite is contacted with base to form a calcium apatite/chitosan/dicarboxylic acid composite. As set forth above, examples of suitable dicarboxylic acids include, but are not limited to maleic acid, oxalic acid, fumaric acid, and combinations thereof. In a particularly useful refinement, the calcium apatite/chitosan/dicarboxylic acid composite is a hydroxylapatite/chitosan/maleic acid composite. Typically, chitosan in the themonetite/chitosan/dicarboxylic acid composite and/or the calcium apatite/chitosan/dicarboxylic acid composite (e.g., the hydroxylapatite/chitosan/maleic acid composite) is cross-linked by the dicarboxylic acid. Details regarding the number of layers in the formed multilayer structure are as the same as those set forth above.
As set forth above, the monetite/chitosan/dicarboxylic acid composite and/or the calcium apatite/chitosan/dicarboxylic acid composite (e.g., the hydroxylapatite/chitosan/maleic acid composite) can each independently be a multilayered structure having at least 3 organic matrix layers and at least 3 calcium apatite-containing layers. Typically, the organic matrix layers include the chitosan and the dicarboxylic acid. In a refinement, these composites can each independently be a multilayered structure having at least 5 organic matrix layers and at least 5 calcium apatite-containing layers. In another refinement, these composites can each independently be a multilayered structure having 3 to 30 organic matrix layers and 3 to 30 calcium apatite-containing layers. In a refinement, the organic matrix layers have an average thickness from about 50 nm to about 1 micron. In another refinement, the organic matrix layers have an average thickness from about 50 nm to about 200 nm.
The following examples illustrate the various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims.
In at least one aspect, the present invention provides a biomimetic strategy (inspired by the formation of brick-and-mortar structure in nacre) to generate a layered monetite-based composite that can further transform into a HAp-chitosan composite with a multilevel hierarchical structure. In nacre, the orderly layered structure forms under the mediation of an organic matrix composed of β-chitin, silk like proteins, and acidic glycoproteins rich in aspartic acid (Asp).36-38 Among these components, the oriented fibrils of β-chitin play an important role in regulating the crystal orientation, and the Asp-rich proteins are believed to interact both with the β-chitin substrate and the growing crystals.36 Considering the critical functions of β-chitin and acidic proteins, our strategy starts with the rational design of an organic medium composed of chitosan and cis-butenediolic acid (maleic acid, MAc) to mimic the function of nacreous matrix and to generate an ordered layered structure. Chitosan is a deacetylated derivative of chitin usually used to study the crystallization process instead of chitin, owing to its soluble properties in acidic media. Experiments have shown that chitosan scaffolds are capable of promoting the formation of randomly sphere-like, rod-like, or plate-like CaP crystals,39-41 yet no observable higher-order structure has been reported in these materials, perhaps because of the absence of orderly assembly of the pristine chitosan molecules.42 To overcome this problem, here we introduced a dicarboxylic MAc molecule that could serve as a cross-linking agent and lead to the intermolecular assembly of an organized matrix (
Analyses of Chitosan-MAc Matrix. To investigate the assembly of the layered matrix, we prepared the chitosan-MAc solution by dissolving 0.25 g of chitosan (medium molecular weight, 75-85% deacetylated, Sigma-Aldrich) and 0.1 g of maleic acid (MAc, Sigma-Aldrich) in 25 mL of deionized water containing 0.72 mL of acetic acid under magnetic stirring, and the pH value was adjusted to 3.5 with 1 M NaOH. The solid phase of chitosan-MAc matrix was obtained by lyophilization for scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) studies. For circular dichroism (CD), atomic force microscopy (AFM) and transmission electron microscopy (TEM) studies, 0.1 mL of chitosan-MAc solution was diluted with 0.9 mL deionized water. The TEM sample was prepared by depositing a droplet of the diluted chitosan-MAc solution onto a TEM grid and then staining it with a droplet of a 0.1 wt % phosphotungstic acid aqueous solution.
Preparation of Layered Monetite-Chitosan-MAc Composite. Six grams of urea, 0.25 g of chitosan (medium molecular weight, 75-85% deacetylated, Sigma-Aldrich) and 0.1 g of maleic acid (MAc, Sigma-Aldrich) were dissolved in 25 mL of deionized water containing 0.72 mL of acetic acid under magnetic stirring. Then, 0.3 g of NaH2PO4 and 0.614 g of CaCl2.2H2O were dissolved into the solution under vigorous stirring for 2 h, and the pH value was adjusted to 3.5 with 1 M NaOH. Next, the reaction solution was heated in a water bath at 70° C. for 4.5 h, then allowed to cool down to room temperature naturally. Following the reaction, the obtained suspension was centrifuged and washed with distilled water and anhydrous ethanol several times. Finally, the sample was frozen on dry ice and lyophilized for 16 h.
Preparation of Organized HAp-chitosan-MAc Composite via Phase Transition. Layered monetite-chitosan-MAc composite (0.15 g) was dispersed in 15 mL of NaOH solution (0.1 M, pH 13) and kept in a water bath under vigorous stirring at 70° C. for 5 min. The suspension was then immediately centrifuged and washed with distilled water and anhydrous ethanol several times. Finally, the sample was frozen on dry ice and lyophilized overnight.
Preparation of Calcium Phosphate Tablets. The CaP tablets were prepared by pressing the layered monetite-chitosan-MAc or organized HAp-chitosan-MAc composite in a stainless steel mold under a pressure of 10 MPa at 130° C. for 48 h.
Characterization. SEM imaging was performed in a JSM-7001F field emission scanning electron microscope (JEOL, Peabody, Mass.) operating at an accelerating voltage of 5-15 kV. XRD patterns were recorded by a Rigaku diffractometer with Cu Kα radiation (λ=1.542 Å) operating at 70 kV and 50 mA with a step size of 0.02°, at a scanning rate of 0.1°/sec in the 20 range 10-60°. TEM images were obtained on a JEOL JEM-2100 microscope using an accelerating voltage of 200 kV. CD spectropolarimetry was performed using a J-815 spectropolarimeter (JASCO, Easton, Md.). The spectra were recorded between 190 and 260 nm with a step size of 0.5 nm and a scan rate of 50 nm/min. AFM images were obtained by using a NanoScopeIIIa scanning probe microscope system (Digital Instruments, Tonawanda, N.Y.) with a tapping mode etched silicon probe (model OTESPA-10) in air at scanning rates of 0.5-1.0 Hz for different resolutions. FTIR spectra were acquired from a Nicolet 4700 Spectrometer with a GladiATR diamond crystal accessory. Thermogravimetry (TGA) analysis was performed on a Q5000 IR (TA Instruments, New Castle, Del.) with a heating rate of 50° C./min, under an air flow of 20 mL/min from 30 to 900° C. The hardness and elastic modulus were measured at 25 test points in each sample (n=3) using a nanoindenter (AgilentMTS XP) with a Berkovich tip.
Statistical Analysis. The experiments were conducted in triplicate and the data were expressed as mean±standard deviations. Student's t test was applied to identify differences in the hardness and elastic modulus between layered monetite-based composite and hierarchical HAp-based composite. The differences were considered statistically significant at p<0.05 and highly significant at p<0.001. All the statistical analyses were carried out using Origin 8.0 (Origin lab, Northampton, Mass.) and Microsoft Office Excel 2007.
Results and Discussion
Assembly of Organized Chitosan-MAc Matrix. Unlike the chitin in nacre matrix, chitosan alone does not form an organized structure because it lacks the acetylamide groups that contribute to hydrogen bonding during the self-assembly of chitin.42 In our study, only irregularly spherical-like morphology can be observed when examining pure chitosan (
These observations demonstrate the ordered coassembly of chitosan-MAc matrix due to the specific interaction between chitosan and MAc, which was further confirmed by FTIR. Compared with the FTIR spectra of pure chitosan or MAc, the COO— stretch vibration band shifted from 1703 to 1710 cm−1 in the spectra of chitosan-MAc mixture (
These shifts indicate a strong interaction between the charged carboxyl and amine groups.42,44,45 Indeed, under our experimental conditions (3.5<pH<4.5), the amine groups of chitosan (pKa=6.5) are protonated to form positively charged chains.46 Simultaneously, the two carboxyl ends of MAc (pKa=1.92) are fully ionized.47 As a result, the chitosan chains can be cross-linked by MAc molecules through hydrogen bonding and electrostatic interactions between the positively charged amine groups and the negatively charged carboxyl groups (
The assembly states of pure chitosan and chitosan-MAc matrix were revealed by tapping mode atomic force microscopy (AFM) and transmission electron microscopy (TEM) under the conditions described in
Assembly of Layered Monetite-Chitosan-MAc Composite. To prepare a calcium phosphate composite with an orderly layered structure, we adopted a wet-chemical precipitation method in the presence of a chitosan-MAc mixture. The resulting calcium phosphate products exhibited a brick-and-mortar structure (
This crystallographic characterization and composition of the layered composite were further confirmed by XRD and energy dispersive X-ray spectroscopy (EDS) (
To investigate the assembly process of the layered monetite-chitosan-MAc composite, the pH of the reaction system and the product morphology were monitored at different reaction times (
CO(NH)32+2HO22NH4+2OH+CO2 (1)
HPO2→4−+OH−→HPO42+H2O (2)
Ca2++HPO42−→CaHPO4 (3)
H2PO4− is dominant in the initial reaction solution with a pH of 3.5. As reaction time increases, the pH gradually increases due to the hydrolysis of urea (eq 1), leading to a conversion of H2PO4− to HPO42− (eq 2).61 When the concentration product of Ca2+ and HPO42− is larger than the Ksp of monetite, the monetite starts crystallizing in the solution (eq 3). Meanwhile, the nucleation of crystals leads to a decrease in the pH of the reaction solution because of the consumption of Ca2+ and OH− (eqs 2 and 3). When the reaction time is increased to 1 h, white precipitation of monetite is formed in the solution. The products obtained at this time consist of monetite platelets assembled in an orderly layered structure (
We can further consider the role of the chitosan-MAc mixture in this assembly mechanism. We did not observe any layered structure at any point during the reaction process in the absence of either chitosan or MAc. In a comparative experiment without chitosan and MAc, the monetite produced from precipitation formed plate-shaped structures with a width of 5-10 μm (
It is noteworthy that the pH values (3.5<pH<4.5) of the reaction solution are in between the pKa values of maleic acid (1.92) and chitosan (6.5) throughout the reaction process. As a result, the protonated amine groups and ionized carboxyl groups remain active and interact with each other, leading to a persistent assembly and function of the chitosan-MAc matrix during the formation of layered monetite composite. Similar to the formation of nacre,37,38 the chitosan-MAc layers not only function as a structural framework, but also provide a charged carboxylate surface that plays an important role in inducing nucleation and interacting with forming crystals. The negatively charged carboxyl groups of the chitosan-MAc complex are preferentially adsorbed on the calcium-rich (100) surface of crystals to stabilize and confine the formation of monetite platelets. The interaction between the organic complex and the calcium ions can be verified by the appearance of a characteristic FTIR vibration of COO—Ca at 1466 cm−1 (
Synthesis of Organized HAp-Chitosan-MAc Composite via Phase Transition. Using the layered monetitechitosan-MAc composite, we were able to fabricate an organized HAp-chitosan-MAc composite via a phase transformation in an alkali solution at 70° C. XRD and EDS results confirmed that hydroxyapatite (JSPDS Card: 09-0432) was obtained after immersing the monetite-chitosan composite in a 0.1 M NaOH (pH 13) solution for 5 min (
During the phase transformation from monetite to HAp, the ordered nanostructure can only be obtained through a topotactic solid-solid transition under highly alkaline conditions (10CaHPO4+6OH−→Ca10(PO4)6(OH)2+4PO34−+10H2O).35 Specifically, the highly alkaline conditions generate a strong driving force that compels the ions to move and rearrange inside the crystal, resulting in a spatially periodic nucleation of HAp seeds and formation of well-arranged HApnanoneedles on the monetite surface. However, the structural stress induced by dehydration and the movement of ions during the transition from monetite to HAp can cause changes to the crystal morphology and collapse of the layered structures.34,62 Thus, a major challenge associated with synthesis of hierarchical HAp in the present study was to preserve the layered arrangement on a larger-scale level during the phase transformation from monetite composite to HAp. One way to overcome this problem is to use an organic matrix that can protect the organized layered structures. For example, by using different biomacromolecules and surfactants, the crystal morphology can be maintained during the phase transformation from OCP to HAP because of the interaction between organic molecules and crystals.61 In a similar way, the chitosan-MAc matrix was expected to preserve the hierarchical architecture of crystals during the monetite-HAp transition by interacting with the crystal surface as discussed above.
Given all these considerations, we ascribe the formation of our hierarchical HAp to a topotactic solid-solid transformation under the mediation of the chitosan-MAc matrix. The intraparticle topotactic transition was revealed by HRTEM and FFT analysis.
Mechanical Properties of Organized CaP-Based Composites. Because the poor mechanical properties of conventional CaP materials limit their application to non- or moderate load-bearing applications, improving their strength is of great interest. From a clinical point of view, an ideal CaP bone substitute material should have mechanical properties similar or superior to those of the bone tissue being replaced. To evaluate the mechanical properties of the synthetic CaPbased composites, we prepared bulk tablets via a simple hotpressing process using layered monetite-chitosan-MAc and organized HAp-chitosan-MAc composites as the building blocks (
0.40 ± 0.17c,*
a*p < 0.05 and ***p < 0.001 compared to monetite-chitosan tablets.
bTested using an Instron testing machine.
cTested using a nanoindenter.
To determine the promise of hierarchical CaP materials as bone substitute materials, future studies are needed to optimize its structure and layers dimensions, investigate the effects of these materials on cellular functions, and study the tissue responses to them in vitro and in vivo. Another subject of further research includes the safety assessment of the ingredients within the hierarchical CaP-based composites. One of the major concerns is the toxicity of maleic acid, which has been shown to be a dermal and/or ocular irritant as a free acid.65 A possible strategy to lower the concentration of free maleic acid is to neutralize it into various maleate salts.65 The design of a pH-sensitive layer that can release the acid in a controlled dose is another strategy that could be used to reduce the toxicity during the degradation of hierarchical CaP-based composites.66
To achieve hierarchical structure in CaP materials, we devised a novel pathway to generate a layered monetite composite that can further transform into a HAp composite with a multilevel hierarchical structure. A nacre-inspired matrix composed of chitosan and maleic acid was designed to mediate the construction of an orderly layered structure. The chitosan and MAc molecules assembled into an organized complex and further guided the mineralization of monetite crystals, resulting in the formation of organized and parallel arrays of monetite platelets with a brick-and-mortar structure. By hydrolyzing the layered monetite composite in an alkali solution, hierarchical HAp with multiscale ordered structure was formed through a phase transformation under the mediation of a chitosan-MAc matrix. This hierarchical structure imparts the CaP material with improved mechanical properties and demonstrates its potential applications in hard tissue engineering. We anticipate that the approach demonstrated here will provide inspiration for biomimetic designs of advanced, mechanically robust materials for biomedical applications.
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.
This application claims the benefit of U.S. provisional application Ser. No. 62/311,626 filed Mar. 22, 2016, the disclosure of which is hereby incorporated in its entirety by reference herein.
The invention was made with Government support under Contract No. DE-020099 awarded by the National Institute of Dental and Craniofacial Research. The Government has certain rights to the invention.
Number | Date | Country | |
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62311626 | Mar 2016 | US |