Chapter 20 - Nanotechnology: a challenge in hard

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NANOTECHNOLOGY: A CHALLENGE IN HARD TISSUE ENGINEERING WITH EMPHASIS ON BONE CANCER THERAPY

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Denisa Ficai, Anton Ficai, Alina Melinescu, Ecaterina Andronescu University of Bucharest, Bucharest, Romania

CHAPTER OUTLINE 1 Introduction....................................................................................................................................513 2 Representative Materials for Hard Tissue Engineering.......................................................................516 2.1 Calcium Phosphates.................................................................................................516 2.2 Oxides (Alumina, Zircone, Bioglass)............................................................................519 2.3 Silicates..................................................................................................................521 2.4 Carbonaceous Materials.............................................................................................522 2.5 Composite Materials.................................................................................................524 3 Drug Delivery Systems Designed for Hard Tissue Engineering............................................................525 3.1 Calcium Phosphates as Drug Delivery System in Bone Cancer Therapy...........................525 3.2 Oxides as Drug-Delivery System in Bone Cancer Therapy..............................................527 3.3 (Mesoporous) Silica and Silicates in Bone Cancer Therapy............................................528 3.4 Carbonaceous Materials as Drug Delivery System in Bone Cancer Therapy......................529 3.5 Composite Materials as Drug-Delivery System in Bone Cancer Therapy...........................530 4 Conclusions....................................................................................................................................532 Acknowledgments..................................................................................................................................533 References............................................................................................................................................533

1 INTRODUCTION The revolution in medicine began during the 21st century. The convergence of researchers on molecular biology, chemistry, genetics, physics engineering, and medicine led to an exponential growth rate of discovering and applying medical technology. Biomedical applications of nanotechnology are the direct result of such convergences (Freitas, 1999). Since 1950, the evolution of biomaterials for bone grafting has been amazing. Starting with metals and alloys, the 1st generation of biomaterials for bone tissue engineering, new materials were developed: ceramics and polymers (2nd generation), composite and nanocomposites (3rd generation), Nanostructures for Cancer Therapy Copyright © 2017 Elsevier Inc. All rights reserved.

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and tissue-engineered nanocomposites (4th generation) which can contain biological materials or even cells (Murugan and Ramakrishna, 2005). Certainly, these classes can be further improved by special design and the addition of certain third components. Some examples based on collagen (COLL) and hydroxyapatite (HA) composite materials and highlight the correlation between composition and morphology of these materials with the specific properties/characteristics of the materials are presented in Table 20.1. Table 20.1 COLL/HA Composite Materials With Specific Properties/Characteristics Composition

Specific Characteristics

References

1.

COLL/HA with different morphology

The morphological changes can be exploited in designing the mechanical and biological properties of these materials

Ficai et al. (2009, 2010a,c)

2.

COLL/HA-Ions

Different ionic species can be used for tailoring the desired properties, in many cases these properties being induced due to the morphological changes while other are induced by the presence/delivery of the ions

Ficai et al. (2010b)

3.

COLL/HA-antibiotics

The presence of antibiotics can assure the delivery of the antibiotics required for the treatment of different diseases of bone or surrounding tissues but also to prevent infections

Martins and Goissis (2000); Nandi et al. (2009)

4.

COLL/HA-BMP

The role of bone morphogenetic proteins is to assure an enhanced, local bone formation and thus a faster healing

Cha et al. (2014); Hu et al. (2003); Taniyama et al. (2015)

5.

COLL/HA-Fe3O4

Magnetite is many times exploited as Andronescu et al. (2010) a hyperthermia generator and thus is used as an antitumoral agent which can be exploited in many forms of cancer, including bone cancer

6.

COLL/HA-cisplatin; COLL/HA-Fe3O4-cisplatin

Different drug-delivery systems based on collagen and hydroxyapatite can be designed for bone cancer treatment. The delivery rate, drug loading capacity, the nature of the antitumoral agent, etc., can be controlled and thus the antitumoral activity can be tailored accordingly.

Andronescu et al. (2013); Ficai et al. (2015)

1 Introduction

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The challenges for scientists and engineers working in nanotechnology are extraordinarily complex and often “quite multidisciplinary” in nature. Biomedical nanotechnology is a rapidly emerging and varied field of applied nanotechnology, developed from advanced research work on novel nanoscale. Fundamental to continued promotion of biomedical nanotechnology domain is the foundation and support of the staff’s effort from the researcher groups in complementary areas. Such cooperation should take place not only at a particular domain level, but also internationally. Successful development and implementation of increasing international collaboration, global perspective on research brings together the best researchers in the world for the benefit of all mankind. As a result of international cooperation, nanotechnology has the potential for rapid development and subsequently a profound global impact on science, technology, and society. In September 2009, a more advanced definition was introduced: “A biomaterial is a substance that has been engineered to take a form which, alone or as part of a complex system, is used to direct, by control of interactions with components of living systems, the course of any therapeutic or diagnostic procedure, in human or veterinary medicine” (Dorozhkin, 2015). The basic role of biomaterials in tissue engineering is to provide temporary mechanical support and mass transport to encourage cell adhesion, proliferation, and differentiation, and to control the size and shape of the regenerated tissue. The novelty of the applications refers to diagnostics and treatments of diseases starting to develop in relation to the total progress in the fundamental science of life, as characterized by the biological interactions at the nanometer level, and integration of the nanotechnology and synthetic biology products. Biomedical nanotechnology presents many revolutionary opportunities in the struggle against all types of cancer, neurodegenerative disorder, infections, and other illnesses. There are clear applications of biomedical nanotechnology in medicine (medical diagnostics), drug delivery, delivery of proteins and peptides, as well as imagistic and therapeutic diagnosis and technical tissues (Gener et al., 2016; Gutierrez et al., 2016; Hernando et al., 2016; Jackman et al., 2016; Ko, 2016; Xu et al., 2015). A wide range of options exist for designing a specific biomaterial to be used as a matrix template, including natural biomaterials, synthetic biomaterials, and composites composed of two or more material types/classes. Ideally, a cellular scaffold, in addition to being biocompatible, should be a biomaterial device with physical and mechanical properties that match those of the target tissue and that contain a multitude of cytokines, growth factors, and cell adhesion molecules that can promote a regenerative microenvironment for appropriate cell populations and induce their behavior (Chen and Liu, 2016). The uses of nanoparticles and nanostructured materials in bone-related diseases are increasing and cover pure regenerative but also curative aspects including pain management, infections, or cancer (Tautzenberger et al., 2012). Bone cancer is a group of diseases involving bony tissue. There are two different forms of bone cancer, some specific to bone (developed in bone) and called bone sarcomas (osteosarcoma and Ewing’s sarcoma being the most important representative) and some nonspecific, derived from other cancer, most usually from breast and prostate. Distant organ metastasis is the most important cause of morbidity and mortality; about three of four patients with advanced breast cancer will develop bone metastasis (Sasaki et al., 2016). Based on more studies, the locoregional delivery can bring some advantages compared with the systemic administration,

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the most important being improved efficiency and lower toxicity. In the case of primary bone cancers, the risks of recurrences are very high, and this is why the tumoral cells must be completely destroyed. For this reason, the long-term action was highlighted in some papers and the use of different antitumoral agents is recommended because some of them can assure a long-term antitumoral activity against the tumoral cells (Teodor et al., 2016). Based on the actual trends, the use of alternative antitumoral therapies are recommended; among these, the use of antitumoral nanoparticles along with the species able to induce hyperthermia or photothermia are extensively studied (Ficai et al., 2015; Kim et al., 2016; Sonmez et al., 2015; Swierczewska et al., 2016; Tseng et al., 2007; Zou et al., 2016). According to the EU Report on Nanotechnology, the global market for smart biomaterials reached $47 billion in 2009 and will rise to $113 billion by 2025 (Holzapfel et al., 2013). Currently, more than 600,000 hip and a million knee replacements are performed annually worldwide (Jones, 2013). For instance, three of the most commonly diagnosed types of cancer tend to metastasize in bone and so, during the therapeutic protocol bone defects appear and bone grafts are needed (Alemany-Ribes and Semino, 2014; Grasman et al., 2015).

2 REPRESENTATIVE MATERIALS FOR HARD TISSUE ENGINEERING The term bioceramic is a general term used to cover glasses, glass–ceramics, and ceramics that are used as implant materials. The name “Bioglass” was trademarked by the University of Florida for the original 45S5 composition. It should therefore only be used in reference to the 45S5 composition and not as a general term for bioactive glasses (Jones, 2013). Biomaterials in general, and the oxide ones, in particular, are a category of materials indispensable for improving the quality of life and its prolongation. The first material used for reconstructive surgery was pasty plaster in 1892, while in 1920 tricalcium phosphate was used to treat injured bones. Traditional dentures, polymer- and metal-based, were gradually replaced by other materials with improved properties. Ceramic materials have attracted special interest because of their better chemical stability compared to metals, due to biocompatibility and excellent tribological properties (Kokubo and Honmachi, 2000; Muster, 1992). Biomaterials researches undertaken at present are focused not only on replacement, but also on tissue regeneration. For centuries, when tissues were sick or degrading, surgical intervention was used to remove the harmful part, with obvious limitations. It is essential to recognize that there is no material suitable for all applications of biomaterials. In terms of classes of materials, ceramics, glasses and glass–ceramics are generally used to repair or replace the connecting hard skeletal tissues. The success of their use depends on the ability to form a stable bond with the tissue. Tissue attachment mechanism is directly related to the type of tissue response to the implant–tissue interface. There are four types of response (Table 20.2) and four different ways to attach the prosthesis to the bone system.

2.1 CALCIUM PHOSPHATES The mineral phase of bone tissues in vertebrates is composed mainly of CaPs, which explains why these CaP materials have chemical properties suitable for bone-remodeling kinetics. On the basis

2 Representative Materials for Hard Tissue Engineering

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Table 20.2 Types of Implant-Tissue Response (Hench, 1991; Shirtliff and Hench, 2003) If the material is toxic, surrounding tissue dies. If the material is nontoxic and inactive biologically (substantially inert), forms a fibrous tissue of variable thickness. If the material is nontoxic and biologically active (bioactive), forms a connection interface. If the material is not toxic and dissolves, the surrounding tissue replaces it.

of composition, synthetic CaPs presently used as biomaterials are classified as calcium hydroxyapatite (HA), Ca10(PO4)6(OH)2; tricalcium phosphate (α- or β-TCP), Ca3(PO4)2; biphasic calcium phosphates (BCPs) for mixtures of HA and β-TCP; and unsintered apatites or calcium-deficient apatites (CDAs). Solubility and biological properties of these CaP materials depend strongly on crystal size, ionic impurities, specific surface area, and both macroporosity and microporosity. All these parameters also have a specific influence on the bioceramics’ final mechanical properties (Verron et al., 2010). The calcium phosphates are closest to the mineral part of the calcified tissue (bones, teeth). Therefore, these materials can replace bone tissue in the human body. Essential constituents required by normal metabolism are Ca and P, along with numerous other elements in very small amounts (Table 20.3) (Legeros, 1994). Depending on the Ca/P ratio we can distinguish different hydrated and nonhydrated calcium phosphates, according to Table 20.4 (Yamamuro and Hench, 1990). Currently, two compounds based on calcium phosphates are indicated for making tough implants: HA and TCP, the best results being obtained for the Ca/P ratio = 1.60. Of calcium phosphates mentioned, HA phase is stable at human body temperature and in saline and internal environment, that is, at pH >4.2. Instead, at pH 99.5%) is used in hip prostheses capable of handling weights, and in dental implants due to its excellent resistance to corrosion, good compatibility, wear resistance, and high tensile strength (Christel, 1986). The results of aging and fatigue, respectively, indicate that it is essential that Al2O3 implants be manufactured to the highest standards of quality assurance, especially if they are used as orthopedic prostheses in younger patients. The implant of alumina on the surfaces subjected to wear loads, such as the prosthetic hip, must have a very high degree of sphericity, which is produced by milling the appropriate precursor oxide and polishing the two surfaces in contact with the implant. Other clinical applications of alumina prostheses, reviewed by Hulbert (1987, 1993), included prosthetic knee, bone screws, alveolar layers and maxillofacial reconstruction, keratoprostheses (replacement of the cornea), replacement of segments of bone, blades, screws, and dental implants. However, Christel (1986) notes that load distribution due to high elasticity modulus of alumina may be responsible for bone atrophy and loosening of the acetabular cup in elderly patients with osteoporosis or rheumatoid arthritis. It is therefore essential that the patient’s age, the nature of the joint disease, and reconstructive biomechanics be carefully considered before using prostheses, including ceramic alumina (Hench, 1991).

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Porous alumina can be used as a synthetic bone tissue scaffold material or a porous ceramic prosthetic device. The success of a porous scaffold depends on its ability to provide a functional balance between mechanical strength, pore size, interconnectivity of the porous structure and properties of osteoconductivity. Porosity is an important design characteristic to satisfy an application as a synthetic bone scaffold material. An optimal pore size 300 mm is required for bone formation and vascularization. Scaffold porosity of at least 90% is required to provide high surface area for cell interaction and sufficient space for extracellular matrix regeneration (Soh et al., 2015). ZrO2-based ceramics have gained attention as a biomaterial. Stabilized ZrO2-based ceramic is less brittle than the Al2O3-based. Zirconia (ZrO2) is used also as a ball joint in total hip prostheses (Purchio and Zavaglia, 1995). The potential advantages of zirconia in loaded prostheses are its low modulus of elasticity and high strength (Hench, 1991). There are insufficient data to deduce whether these properties will determine a higher clinical success over long periods of time (>15 years) (Hench, 1991). Zirconia has several characteristics that are superior, such as a high affinity for bone tissue, noncarcinogenic properties, and the absence of an oncogenic effect. This makes it a good choice in many implant applications. A further advantage brought by zirconia is that zirconia grain has been shown to serve as a nucleation site for the development of calcium-based minerals. The use of zirconia as a biomaterial has largely been restricted to hip joint replacement. Lately, zirconia has also found use as a restorative material in dental applications. However, despite the superior wear performance of zirconia over alumina, one of the major issues of zirconia ceramics in implantation is that aging of zirconia happens due to the presence of water, which promotes the transformation of tetragonal to monoclinic phase, and eventually leads to surface roughening and cracking. Partially stabilized zirconia has been introduced to prevent the transformation to a monoclinic phase. One example of this is yttria stabilized zirconia, in which yttria is added to stabilize the tetragonal or cubic phase. There has been an increased interest in yttria-stabilized tetragonal zirconia polycrystals, which is highly used in clinical application, due to their higher fracture resistance and flexural strength (Denry and Kelly, 2008). The first bioactive glasses have been developed by Hench (1991). and were subsequently tested in terms of biocompatibility. SiO2 and P2O5 form a glassy, amorphous network and the presence of Na2O and CaO (glass network modifiers) modify the network such that the Ca/P ratio becomes close to that of bone. The extraction of Na, close to the surface, moves the thermodynamic equilibria so that the oxygen bridges of the vitreous matrix are broken, the diffusion rate increases, and the Ca and P ions propagate in this superficial area where a CaO and P2O5 rich layer forms. Ca/P ratio can be adjusted according to corrosion and stability of the vitreous matrix. As the corrosion is difficult to adjust, one can try to influence the matrix properties through the content of SiO2 (Hench, 1991; Hulbert, 1987; Łączka et al., 2016; Schmidt, 1999). If it exceeds 60 mol%, the matrix is sufficiently stable, so the layer rich in CaO and P2O5 could not be formed until after several weeks. These glasses are not bioactive. There are also glass–ceramics, such as Ceravital type, with low concentration of SiO2 in favor of a major concentration in CaO and P2O5. As in the case of high SiO2 bioglass, one expects the formation of a carbon-rich layer of hydroxyapatite (calcium), which will adsorb the collagen, thereby causing mineralization of bone. The primary requirements for bioglasses/glass–ceramics to serve as biomaterials are their good biocompatibility, the ability to form hydroxyapatite layer as a result of contact with simulate body fluid (SBF), the lack of cytotoxicity or immunogenicity, and mechanical properties that prevent any


521

structural failure during handling the implant and normal patient’s activity. Moreover, for bone engineering and scaffold production, bioglass should display controlable three-dimensional structure and interconnected porosity for cell proliferation and vascularization. Class A bioactive biomaterial (i.e., second generation) refers to bioglasses of very reactive surface capable to form a stable interface with bone and inducing surface reactions involving the dissolution of critical amounts of soluble Si, P, Na, and Ca ions, leading to cellular responses promoting osteogenesis (osteoproduction). Class B includes biomaterials capable of forming a stable interface between the implant and bone tissue (i.e., osteoconductive biomaterials) (Łączka et al., 2016). From a compositional viewpoint, bioactive glasses can be basically divided into three groups, depending on the representative former oxide present in the formulation, that is, SiO2-based (silicate), B2O3-based (borate), and P2O5-based (phosphate) systems. The first group comprises a wide range of glass formulations, including 45S5 Bioglass (46.1SiO2-24.4Na2O-26.9CaO-2.6P2O5 mol.%); borate glasses are characterized by higher reactivity than silicate materials, which results in faster bioactive kinetics; phosphate glasses are resorbable materials and their dissolution rate can be tuned according to their oxide composition. In this regard, emerging fields of research for bioactive glasses include neuromuscular repair (fibrous constructs formuscle and nerve regeneration), artificial cornea, orbital implants, epithelial and cardiac tissue engineering, treatment of gastric ulcers and nonosseous cancer therapy (Baino et al., 2016). In 1992, researchers (Kokubo, 1991; Ohura et al., 1992) explored a new way of using biovi troceramics and bioglasses: cancer treatment. In the first case, a SiO2-Al2O3-P2O5 glass–ceramic, containing lithium ferrite LiFe5O8 and hematite or α-Fe2O3, exhibits good biocompatibility and magnetic properties. The introduction of this powder surrounding the tumor and applying an alternating magnetic field allows increasing the local temperature up to 43°C and destroys cancer cells by hyperthermia. Liver cancer can be treated using Al2O3-SiO2-P2O5 glass microspheres in which the 89Y isotope is activated in 90Y and 31P into 32P isotope. These glasses emit β radiation by neutron bombardment and destroy cancer cells (Bîrsan et al., 2004; Kawashita, 2005).

2.3 SILICATES Many reports have suggested that silicon would be essential for the metabolic processes associated with growth and skeletal development and integrity of the extracellular matrix, with direct effects of Si on the biomineralization process. Many other reports have proved that the incorporation of silicon into a synthetic HA structure improves its bioactivity. This research evaluated the biocompatibility effects of Si HA (Si content so 0, 1, 3, and 5 mol%) in vitro with human osteoblasts. A high Si content (5 mol%) appears to promote rapid bone mineralization (Supova, 2015). Silica mesoporous materials are ordered porous structures of SiO2, characterized by a high pore volume, large surface area (hundreds of m2/g) and narrow pore size distribution. This fascinating class of nanomaterials are synthesized by the self-assembly of silica-surfactant composites in which inorganic species (silica precursors) simultaneously condense, giving rise to the formation of mesoscopically ordered composites. In 2004, was pioneered the synthesis of mesoporous materials in the multicomponent SiO2-CaO-P2O5 system, which paved the way for developing multifunctional bioactive glasses exhibiting drug uptake/release ability, as well as superior bioactivity compared to conventional meltderived and sol–gel materials (Baino et al., 2016).

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Silicon-based ceramics, such as silicon carbide have also been evaluated clinically as antithrombogenic coatings on vascular stents. Recently, silicon nitride were approved by the FDA for spinal fusion applications and hip arthroplasty. However, in all these cases, the biomaterial was designed to be relatively bioinert, but with excellent biodurability. Silicon-substituted ceramics have gained a strong reputation within the biomaterials community for their enhanced osteoinductive properties, while porosified silicon and silica have been developed as biodegradable materials for a number of therapeutic applications, including drug delivery, dietary “nutraceutical” supplementation, and in orthopedic composite biomaterials (Henstock et al., 2015). Resorbable silicates have found their way into wider commercial healthcare, being included as bioactive glass particles in toothpastes designed to combat tooth sensitivity and enamel loss, and as a nutraceutical, although the bioavailability of many of these colloidal silica suspensions is rarely reported in the marketing material.

2.4 CARBONACEOUS MATERIALS More recently, a range of carbon-based nanomaterials, such as carbon nanotubes (CNTs), graphene (G), and nanodiamonds (NDs) have also been explored for bone tissue engineering (Cross et al., 2016). Carbonaceous materials exist in a wide variety of forms including carbon, graphite, diamond, highly oriented pyrolytic graphite (HOPG), fibers, such as ThermalGraph, carbon foam, and CNTs. Modern carbon-based composites represent a potential advancement in UHMWPE biomaterials. CNTs and graphene are one-dimensional and two-dimensional carbonaceous materials that have drawn great attention from research over the last decade, both for the purpose of discovering their underlying fundamental science and to exploit their potential in novel device applications. Many researchers describe the state of the art of CNT/UHMWPE and graphene/UHMWPE composites with regard to the three facets of the current UHMWPE paradigm: achieving enough wear resistance to avoid or delay osteolysis, ensuring the oxidative stability of the material, and preventing an appreciable loss in mechanical behavior. The mechanical properties of graphene can reinforce the structural behavior of scaffold biomaterials in tissue engineering and regenerative medicine, in addition to enhancing cell adhesion, proliferation, and differentiation. UHMWPE used in total joint replacements and polymethyl-methacrylate (PMMA) cement used for fixing these prostheses to bone are the polymers that have been most widely investigated as a matrix for graphene/CNTs reinforced composites. Mechanical properties of these carbonaceous composites are strongly conditioned by the consolidation process and particularly by the dispersion of the nanofillers (Puertolas and Kurtz, 2014). The mechanical properties of carbonaceous materials, such as their low compressive strength and friability, make it difficult to shape and join them and thus require new, innovative techniques in design and fabrication that differ from those currently employed with metals (Wang et al., 2012). Carbon nanotube-based magnetic hydroxyapatite composite materials (HA-f-MWCNTs) were already developed and tested (Afroze et al., 2016). Based on their results, these innovative multiwall carbon nanotube/hydroxyapatite nanocomposites exhibit hard ferromagnetic properties with the best coercivity of 2985Oe and saturation magnetization of 0.23 emu/g. The hysteresis loss was found to be 0.44 kJ/m3. Based on the overall properties (magnetic and biologic, especially) these materials


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are expected to be able to selectively target the desired organs and tissues and to be used in bone cancer treatment. The synthesis of the HA-f-MWCNTs magnetic nanocomposite was done starting from multiwalled carbon nanotubes MWCNTs. For this purpose, the MWCNTs were oxidized with H2SO4:HNO3 in 3:1 ratio at 70°C for 4 h under stirring, followed by 12 h without stirring, and additional 2 h with stirring, with all steps at same temperature and then, after a purification step, the hydroxyapatite is synthesized by a traditional method using precipitation with Ca(OH)2 and H3PO4. By this procedure, the mineralization occurs on the MWCNTs surface and is assisted by the presence of the carboxylate groups, as we highlight in Fig. 20.2.

FIGURE 20.2 Synthesis of HA-f-MWCNTs Nanocomposite Starting From MWCNT Via Functionalization (Oxidation) and Mineralization

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2.5 COMPOSITE MATERIALS Composites are multiphase materials composed of at least two phases: matrix and reinforcement. The composites comprise almost all types of biomaterials: almost inert, bioactive, resorbable. To provide properties analogous to the bone, at least two materials with different mechanical properties are combined, so that the resulting composite has the qualities of elasticity and tenacity that are close to real bone. The difficulty of making the composite is to choose a fiber that is connected in a manner that is neither too tight nor too loose to the matrix, which must have a coefficient of thermal expansion adapted to that of the matrix to maintain the convenient matrix–fiber cohesion during their development cycle. Another undesirable effect determined by the most reinforcements is the increase of the modulus of elasticity. The difference in elasticity moduli for the implant and subsequently for the bone becomes larger and, therefore, the cured bone strength is lower. Composite bone scaffolds developed so far have been used only with nonload bearing and low load-bearing applications, such as bone defect filler, biomolecular delivery, and maxillofacial treatments. The field of bone tissue engineering has progressed rapidly. Use of natural polymers which are inherently biocompatible seems promising in bone regeneration process (Basha et al., 2015) (Fig. 20.3). All these nanostructured materials, as well as their derived composite materials and core@shell structures were exploited as grafting materials as well as drug delivery systems for the treatment

FIGURE 20.3 Schematical Representation of the Synthesis of the Multifunctional Ceramic Graft and Its In Vivo Functionality (A) Porogen embedded uniaxial pressed ceramic precursor(s); (B) porous ceramic graft; (C) biological active agent (drug)-loaded porous ceramic bone graft; (D) visualization of the ceramic bone graft implanted into the bone defect with highlighting the surrounding tissue and the locoregional release; (E) visualization of the healed bone.

3 Drug Delivery Systems Designed for Hard Tissue Engineering

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of bone-related diseases (Andronescu et al., 2013; Hirabayashi and Fujisaki, 2003; Vinay and Kusumdevi, 2016).

3 DRUG DELIVERY SYSTEMS DESIGNED FOR HARD TISSUE ENGINEERING Many of the aforementioned materials are also used as drug delivery support of a wide variety of active agents, from antimicrobial to chemotherapeutic agents, and from pure, regenerative to curative agents.

3.1 CALCIUM PHOSPHATES AS DRUG DELIVERY SYSTEM IN BONE CANCER THERAPY It is well known that calcium phosphates, in both ceramic and cement form are widely used in hard tissue engineering (orthopedic and dental applications). These materials can be obtained in various conditions, some of these strategies allowing the incorporation of the active agents and thus can act as a locoregional delivery system (Arcos and Vallet-Regi, 2013). Calcium phosphate-based bioceramics and cements are used in large bone defects and osteoporotic fractures healing, as well as in therapy for infections and tumors. It is important to mention that locoregional delivery is extremely important, especially in the case of the toxic biological active agents like chemotherapeutic agents (Andronescu et al., 2013). Calcium phosphates can be used as granules, pastes, cements, or coatings. The active agents can be added during the synthesis or postsynthesis of CaPs. The loading is more facile in the case of cements because in the case of ceramic the biological active components is usually destroyed at the firing temperature necessary to consolidate the ceramic material and thus, the loading step occurs after the sintering of the ceramic graft. Low temperature processing is also possible: precipitation sol-gel, biomimetic routes present interesting alternatives because the loading should occur in the same step as the synthesis. One of the most common ways to obtain drug delivery ceramic grafts with both regenerative and curative capacity is presented in the following sentences. The first step is devoted for the synthesis of the porous ceramic support/graft. The consolidation of the ceramic can be done without restriction at the normal firing temperature. If necessary, porogens (pore-forming agents) can be used (Del Real et al., 2002). The pore size, shape, distribution, and connectivity, as well as their specific number (number of pores/volume) are essential in optimizing these multifunctional bone grafts. Usually, the pore size should be lower than 10 µm because larger pores do not assure a controlled delivery (Arcos and Vallet-Regi, 2013; Del Real et al., 2002; Hing et al., 2005). In the second step, the porous ceramic graft can be loaded with the biological active agents. Unfortunately, the presented route involves the use of porous ceramic materials and in this case the mechanical properties are weak compared with dense ceramics. So, these drug delivery systems can be used, especially for filling small bone defects and defects of bones not exposed to high mechanical loading. CaPs cements are used as synthetic bone grafts that can be easily loaded with biological active agents because of the low-temperature self-setting that occurs in vivo, usually after their injection in the bone defect (Ginebra et al., 2012). The setting mechanism, hydrolysis or acid-base reaction, involves mild reaction conditions that do not destroy the active agents. The microstructure of the final cements, as well as the delivery are significantly influenced not only by pH and temperature but also particle

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size and liquid to powder ratio (Ginebra et al., 2012). For in vivo setting, the most common pH is that of the normal surrounding tissue and ∼37°C but, in certain conditions (cancer, infections, etc.) these conditions can be slightly different. If the change of temperature is not very important, the change of pH may be very important. As presented by Ginebra et al. (2012) the delivery of the active agents is also strongly influenced by the way of incorporation of the active agents into the CaP cement, the active component could be incorporated before (the active components can be dissolved into the liquid phase or can be dispersed into the solid/powder phase) or after the cement formation (the drug is added into the preset cement). The release profile closely fits the Higuchi model. In real in vivo conditions, the release mechanism can be altered because the release is not only diffusion controlled but, in real conditions, cement dissolution can occur or even a thin apatite layer can be formed and therefore can modify the delivery rate of the active agents. CaPs cements is currently used as drug delivery system of many active agents from ions (Ca2+, phosphates, Sr2+, silicate, Zn2+, Ag+, etc.) to small molecules or even molecular weight drugs: antibiotic, antiinflammatory, anticancer, or antiosteoporotic agents, as well as growth factors and other proteins. Tang et al. (2011), for instance, described a one-step synthesis route of calcium phosphate nanocarrier system of two hydrophobic drugs: ibuprofen and atorvastatin calcium. Based on their work it can see that the drug-loading capacity is dependent on the pH for both drugs. The delivery was found to be strongly dependent on the drug-loading capacity and preparation conditions (pH and concentration of the drug), the cumulative release at ∼6 h being between ∼17% and 65%. Qi et al. (2016) present the sonochemical synthesis route of hydroxyapatite nanoflowers (HAFs) with promising use in protein/drug delivery. The synthesis of the HAFs was realized starting from CaCl2, creatine phosphate disodium salt tetrahydrate in the presence of continuous ultrasound field assured by an ultrasonic device of 200 W, operated at 28 kHz for up to 90 min. The adsorption capacity of the proteins is dependent on some factors among which, the concentration of the protein, the synthesis conditions of the HAFs. The adsorption capacity (mg/g) of hemoglobin, used as model protein was tested in aqueous solution (0.2–3.0 mg/g hemoglobin), at 37°C under magnetic stirring at constant rate. In all cases the samples were maintained 4 h in contact with the protein solution, the results being presented in Table 20.5. The MTT tests show that the as-prepared HAFs exhibit excellent biocompatibility and can be good candidates for biomedical applications. Santos et al. (2009) used calcium phosphate granules as delivery system of 5-fluorouracil (5-FU). For this reason, they synthesized porous hydroxyapatite granule by spray-drying (inlet temperature 170°C, spray feed flow of 6 mL/min, concentration ∼1% apatite in water–SDG) followed by a drying step at 200, 400, 600, and 800°C–HSDG. The as-obtained samples (HSDG and SDG) were loaded with 5-FU by immersing these samples into 100 mL of 250 mg/L 5-FU for several days while the

Table 20.5 Protein Absorption Capacity of the HAFs at 37°C and 4 h of Adsorption No.

Material

Protein Concentration (mg/mL)

Adsorption Percentage (%)

Adsorption Capacity (mg/g)

1.

HAFs-40

0.2–3.0

90.3–26.1