Biomimetic NanocompositesA New Approach

It should be noted that the above studies clearly indicate that single-phase components do not always provide all the properties necessary for bone grafting, thus considerable attention has been paid to composites. The term composite can be defined as a heterogeneous combination of two or more materials, differing in morphology or composition on a microscale, in other words, a microcomposite

[97]. Composite grafts are a good choice among the synthetic bone grafts owing to the possibility of incorporating most of the favorable features of various single-phase components and to eliminate or minimize the problems associated with the monolithics as discussed in previous sections. The composite, therefore, can be treated as the only material that uniquely exhibits a range of properties equivalent to bone, and thus it could be possible to eliminate some of the problems associated with current synthetic bone grafts. Some of the bone grafts used in the last few decades for clinical bone therapy are shown in Figure 13.9, including composites.

Recently, an extensive and informative review written by Ramakrishna et al.

[98] described the recent trends and potential applications of those composites in various biomedical applications, including bone reconstruction. As bone is a dynamic living tissue, it should be noted that unless we make bioactive bonelike composites with intact favorable osteoinductive and/or osteogenic components, synthetic bone grafts would only have limited survivability. Accordingly, much attention has been paid to the manufacture of bioactive composites over the past two decades. An important advantage of the bioactive composite lies in its strong interfacial bonding ability with host tissue and its improved mechanical strength. It is, therefore, possible to match certain properties of the bioactive composites to natural bone.

Considerable attention has been paid in the past two decades to bioactive composite grafts that consist of bioactive ceramic filler in a polymeric matrix. In 1980s, Bonfield et al. developed a bioactive composite based on HA and PE [99,100]. It has been demonstrated that it has the capability of promoting extensive bone-bonding functions upon implantation. Later, it was also commercialized under the name of HAPAX™ after a thorough clinical test. Unfortunately, it is not a good biodegradable graft, which limits its wider usage in clinical medicine. It is primarily being used as a middle ear implant, which is one of the promising bioactive composite grafts used for such an application. The exciting news is that the clinician could readily shape it during surgery with respect to the size and shape of the bone defect.

Cranial Bone Defects

HA, HA-collagen, bioactive glass, alumina, autogenic bone, allogenic bone, DBM

Maxillofacial Reconstructions

HA, bioactive glass, alumina, zirconia, HA-PE, bioglass-PE, PTFE-carbon, autogenic bone, allogenic bone

Cranial Bone Defects

HA, HA-collagen, bioactive glass, alumina, autogenic bone, allogenic bone, DBM

Maxillofacial Reconstructions

HA, bioactive glass, alumina, zirconia, HA-PE, bioglass-PE, PTFE-carbon, autogenic bone, allogenic bone

PE: Polyethylene; PET: Polyethylene terephthalate; PLA: Polylactic acid; PLGA: Poly(lactide-co-glycolide); PMMA: Polymethyl methacrylate; PTFE: Polytetrafluoroethylene; PU: Polyurethane.

Figure 13.9. Commonly used bone grafts in clinical bone therapy.

PE: Polyethylene; PET: Polyethylene terephthalate; PLA: Polylactic acid; PLGA: Poly(lactide-co-glycolide); PMMA: Polymethyl methacrylate; PTFE: Polytetrafluoroethylene; PU: Polyurethane.

Figure 13.9. Commonly used bone grafts in clinical bone therapy.

Simultaneously, bioresorbable composites are developed and investigated as bone grafts. For example, Laurencin et al. [101] developed a composite containing HA and PLGA and demonstrated its cellular-compatibility suitable for bone tissue regeneration. It was found that the composite highly supported osteoblast proliferation, differentiation, and deposition of calcium phosphate minerals. The cells were proliferated for up to 21 days and formed a mineralized layer on the composite. However, some studies report that the physical properties of the composite, made of the same ingredients, are not completely satisfied upon implantation because of the conventional mode of processing technology [102]. Perhaps there may be a reason for their inadequate properties in that their constituents are dissimilar to the ingredients of natural bone.

We have also explored the possibility of utilizing HA, both at the micro- and nanolevel, in conjunction with natural and synthetic polymers in the form of composites suitable for bone grafting and bone drug delivery [103-107]. It should also be noted that most of the above composites differ from host bone either compositionally or structurally. These features are highly essential for the appreciation of better functioning of grafts with host tissue upon implantation. Of particular interest in this context is the combination of HA with collagen into a bioactive composite form, which appears a natural choice for bone grafting; that is, it mimics the ingredients of a real bone.

A composite based on HA and collagen has been developed and investigated [108]. It shows great promise because of its resemblance to natural bone in terms of biocompatibility and bioactivity. For example, the bone grafts made of HA and collagen demonstrated a more enhanced osteoconductivity [109] and other properties (e.g., biomechanical) than did their constituent phases upon implantation. Accordingly, choosing HA and collagen as precursors to design osteo-conductive composite bone grafts is perfect and competent, and can also be utilized as a carrier system for the delivery of osteoinductive and osteogenic factors. Nevertheless, most HA/collagen composites are conventionally processed by anchoring HA particulates into the matrix of collagen. Using this method, it is quite difficult to obtain a uniform or a homogeneous composite graft. Furthermore, most of the HA used in this process are sintered and large-size crystallites, which is in contrast to the natural bone apatite; therefore it may take a longer time to remodel into bone tissue upon implantation. In addition, some of the composites processed by this method exhibit very poor mechanical properties [110,111], probably owing to the lack of a strong interfacial bonding between their constituents.

As mentioned earlier, this kind of composite truly mimics natural bone in terms of composition, but differs structurally owing to the grain size and form of the reinforcing agent, which might impede the interfacial bonding between their constituents; thereby it may lose osteointegration considerably. There is a chance of improving the osteointegration by reducing the grain size of the reinforcing agent or by activating the nucleation of ultrafine apatite growth onto the matrix, which may lead to enhanced mechanical strength with improved biological and biochemical affinity to the host tissue. Collectively, it should be noted that microcomposites have been processed with microsize inclusions; therefore it is quite difficult to achieve appropriate interfacial bonding between the constituents. They are also in contrast to biological mineralization of natural bone, which indicates that an alternative method is needed to precisely process a composite bone graft that mimics biological mineralization.

Recently, with advances of nanotechnology and biosciences, biomimetic nanocomposites have gained much interest and are perceived to be beneficial over microcomposites in many aspects as bone grafts, which, of course, opens a new arena in the field of bone grafting. Nanocomposites play a pivotal role in bone grafting as a new class of bone graft material owing to their amazing characteristics, including larger surface area-to-volume ratio and superior mechanical strength, which uses a combination of several nanoscale bone graft materials and/or in conjunction with osteoinductive growth factors and osteogenic cellular components. The latter are supplement stimuli for bone growth. The term nanocomposite can be defined as a heterogeneous combination of two or more materials in which at

Human eye resoluton / ; Ribosome

Man made • / Virus / ; Insu|in devices / Blood cells f Lipoprotein

/ Osteon / Bone minerals;

Atoms

Infinite netw°rks Large molecules Oligomers (elements)

Macro scale Micro scale Nano scale Sub-nano scale Pico scale

10' Meter r /10-1 /10-2 /10-3 / 10-5 / 10-7 /10-8 /10-9 / / io-iy 0-y

1x10' 1x108 1x10' 1x10' 1x10s 1x104 1x103 1x102 10 1 0.1 0.01

1 Adapted and re-drawn from Ref. [22]

Figure 13.10. Dimensions of various substances with highlights of biological systems.

least one of those materials should be in a nanometer-scale range. A nanometer is equal to 10-9 m. The dimensions of some of the biological systems are highlighted in Figure 13.10.

It is surprising to realize that most biological systems begin with a nanoscale mechanism that might be the basis for the success of their unbelievable characteristic functions. For example, bone is the toughest natural nanocomposite, precisely evolved by nature's proficiency in using a nanoscale hierarchical assembly with the basic ingredients of HA and collagen; therefore researchers are trying to imitate such a process, through so-called biomimetics, for making nanocomposite materials into synthetic bone grafts and perceiving these to be the right choice over microcomposites. Therefore, it is considered that engineering nanocompos-ite bone grafts through the biomimetic approach is a somewhat new class of method.

As per a literature survey, there are numerous methods for the processing of nanocomposites suitable for bone grafting. A current research trend on the HA-polymer nanocomposites in bone grafting applications is given in Table 13.7 [112-127]. It should be noted that, from the compiled data, HA/collagen is the only system so far studied up to in vivo animal studies as compared to other HA-

polymer nanocomposite systems, which further suggests their impact. In addition, they are anticipated to provide all the factors essential for bone regeneration for long-term survivability. There is also a possibility of improving interfacial bonding between the constituents of the composites and superior surface reactivity of the whole composite with the host tissues by nanostructural and biomimetic strategies. Inasmuch as this field is still in its infancy, there is as yet no standardized method for making such nanocomposite grafts. The following sections describe their impact and recently developed processing methodology with suitable illustrated examples.

Table 13.7 Current trend on HA-polymer nanocomposites for bone grafting.

Experimental studies performed

Table 13.7 Current trend on HA-polymer nanocomposites for bone grafting.

Experimental studies performed

In vitro

In vivo

Nanocomposite

Materials

cell

animal

systems

Methods characterization

culture

study

Refere

HA/Natural Polymer Nanocomposite Systems

HA/collagen

Precipitation

+

-

-

[112]

HA/collagen

Biomimetics

+

+

-

[113]

HA/collagen

Biomimetics

+

-

+

[114]

HA/collagen

Biomimetics

+

+

+

[115]

HA/collagen/PLA

Biomimetics

+

+

+

[116]

HA/collagen/alginate

Biomimetics

+

+

-

[117]

HA/chitosan

Precipitation

+

-

-

[118]

HA/gelatin

Biomimetics

+

-

-

[119]

HA/silk fibroin

Mechanochemical

+

-

-

[120]

HA/Synthetic Polymer Nanocomposite Systems

HA/PCL

Solvent-casting

+

-

-

[121]

HA/PLA

Solvent-casting

+

-

-

[122]

HA/PEG/PBT

Precipitation

+

-

-

[123]

HA/PHMA

Biomimetics

+

-

-

[124]

HA/Polyanhydride

Photo polymerization

+

-

-

[125]

HA/PHEMA

Biomimetics

+

-

-

[126]

HA/PAA

Insitu polymerization

+

-

-

[127]

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