Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

References 519

a ﬁ xed - bed, continuous - ﬂ ow reactor using the PZBAAr catalyst pretreated in a

stream of O

2

at 400 ° C, followed by H

2

at 400 ° C. In order for the reaction to take

place, a temperature above 250 ° C was required, and the methanol conversion

reached 100% at 350 ° C, giving H

2

in a yield of 2.52% (deﬁ ned as: (mol of H

2

)/(mol

of CH

3

OH), theoretical value: 2.75). The reaction at 350 ° C, however, also afforded

3% CO that was not the OSRM product. Both XPS and IR studies of CO adsorption

implied the presence of metallic Pd on the surface, which could promote CO for-

mation reactions such as the decomposition of methanol and the reverse water

gas - shift reaction. The authors stressed that the high H

2

yield at 350 ° C with the

PZBAAr catalyst was unusual for such low active metal loadings.

15.4

Conclusions and Outlook

Today, mesoporous alumina is becoming a relatively “ common ” catalyst and cata-

lyst support, with several synthetic methods using various types of structure -

directing agents having been developed. Nonetheless, the search continues to

produce mesoporous alumina with better structural qualities. Whilst mesoporous

alumina itself exhibits a unique catalytic performance, as seen in the base catalysis

in scCO

2

, it serves as a much better support for metal - loading compared to con-

ventional alumina, owing to its intrinsic high surface area. Interestingly, “ molecu-

lar sieving, ” which is well established in zeolite catalysis chemistry, has also been

observed in several oleﬁ n - metathesis reactions promoted by mesoporous, alu-

mina - supported Re catalysts. It is expected that these fundamental studies and

future investigations in this area will lead to the practical use of mesoporous

alumina as both catalyst and catalyst support in large - scale production processes

in the chemical industry.

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Nanoceramics for Medical Applications

Besim Ben - Nissan and Andy H. Choi

523

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

16

16.1

Introduction

“ Nanostructured materials ” refer to certain materials which have delicate struc-

tures and sizes that fall within the range of 1 to 100 nm. As a consequence of this

size, an extensive development of nanotechnology has taken place in the ﬁ elds of

materials science and engineering during the past decade. Yet, such developments

have not come as a surprise, when it is appreciated that these nanostructured

materials have the ability to be adapted and integrated into biomedical devices.

This is possible because most biological systems, including viruses, membranes

and protein complexes, exhibit natural nanostructures.

The microstructure and properties of nanostructured materials depend in

an extreme manner on the method of their synthesis method, as well as on

their processing route. Consequently, it is extremely important to select the

most appropriate technique when preparing nanomaterials with desired proper-

ties and property combinations. The synthesis techniques most commonly used

for the production of advanced ceramics include pressing, as well as wet chemical

processing techniques such as co - precipitation and sol – gel, all of which have

been used to produce nanoparticles, nanocoatings, and nanostructured solid

blocks and shapes.

In modern ceramics technology, pressing is accomplished by placing the powder

into a die and applying pressure to achieve compaction. Hot pressing ( HP ) and

hot isostatic pressing ( HIP ) are the most common methods used to produce

bioceramics. HIP can induce the higher densities and small grain structures

required by bioceramics, whereby heat and pressure are applied simultaneously

and the pressure is applied from all directions via a pressurized gas such as helium

or argon. In contrast, ﬂ at plates or blocks and nonuniform components are rela-

tively easily produced using HP.

Sol – gel processing is unique in that it can be used to produce different forms,

such as powders, platelets, coatings, ﬁ bers and monoliths of the same composi-

tion, merely by varying the chemistry, viscosity, and other factors of a given solu-

tion. The advantages of the sol – gel technique are numerous:

524 16 Nanoceramics for Medical Applications

• it is of the nanoscale

• it results in a stoichiometric, homogeneous and pure product, owing to mixing

on the molecular scale

• high purity can be maintained as grinding can be avoided

• it allows reduced ﬁ ring temperatures due to the small particle sizes with high

surface areas

• it can be used to produce uniform, ﬁ ne - grained structures

• it allows the use of different chemical routes (alkoxide or aqueous - based)

• it is easily applied to complex shapes with a range of coating techniques.

Sol – gel coatings also have the added advantages that the costs of the precursors

are relatively unimportant, owing to the small amounts of materials required.

Shrinkage up to a number of coatings, depending on the chemistry, is fairly

uniform perpendicular to the substrate, and the coatings can dry rapidly without

cracking. Shrinkage becomes an important issue, however, in monolith ceramic

production.

At present, the most common materials in clinical use are those selected from a

handful of well - characterized and available biocompatible ceramics, including

metals, polymers, and their combinations as composites or hybrids. These unique

production techniques, together with the development of new enabling technolo-

gies such as microscale, nanoscale, bioinspired fabrication (biomimetics) and

surface modiﬁ cation methods, have the potential to drive at an unprecedented rate

the design and development of new nanomaterials useful for medical applications.

The current focus is on the production of new nanoceramics that are relevant

to a broad range of applications, including: implantable surface - modiﬁ ed medical

devices for better hard - and soft - tissue attachment; increased bioactivity for tissue

regeneration and engineering; cancer treatment; drug and gene delivery; treat-

ment of bacterial and viral infections; delivery of oxygen to damaged tissues;

imaging; and materials for minimally invasive surgery. A more futuristic view,

which could in fact become reality within two decades, includes nanorobotics,

nanobiosensors, and micronanodevices for a wide range of biomedical

applications.

A biomaterial , by deﬁ nition, is a nondrug substance that is suitable for inclusion

in systems that augment or replace the function of bodily tissues or organs. A

century ago, artiﬁ cial devices made from materials as diverse as gold and wood

were developed to a point where they could replace the various components of the

human body. These materials were capable of being in contact with bodily ﬂ uids

and tissues for prolonged periods of time, while eliciting little, if any, adverse

reactions.

When these synthetic materials are placed within the human body, the tissues

react towards the implant in a variety of ways. The mechanism of tissue inter-

action at a nanoscale level is dependent on the response to the implant surface,

and as such three terms which describe a biomaterial, with respect to the

tissues ’ responses, have been deﬁ ned, namely bioinert , bioresorbable , and bioactive

(Figure 16.1 ):

16.1 Introduction 525

• Bioinert refers to any material that, once placed within the human body, has a

minimal interaction with its surrounding tissue; examples include stainless

steel, titanium, alumina, partially stabilized zirconia, and ultra - high - molecular -

weight polyethylene.

• Bioactive refers to a material which, upon being placed within the human

body, interacts with the surrounding bone and, in some cases, even soft

tissue.

• Bioresorbable refers to a material that, upon placement within the human body,

begins to dissolve or to be resorbed and slowly replaced by the advancing tissues

(e.g., bone).

During the early 1970s, bioceramics were employed as implants to perform sin-

gular, biologically inert roles. The limitations of these synthetic materials as tissue

substitutes were highlighted with the increasing realization that the cells and

tissues of the body perform many other vital regulatory and metabolic roles. The

demands of bioceramics have since changed, from maintaining an essentially

physical function without eliciting a host response, to providing a more positive

interaction with the host. This has been accompanied by increasing demands on

medical devices that they not only improve the quality of life but also extend its

duration. Most importantly, nanobioceramics – at least potentially – can be used as

body interactive materials, helping the body to heal, or promoting the regeneration

of tissues, thus restoring physiological functions.

The main factors in the clinical success of any biomaterial are its biocompat-

ibility and biofunctionality , both of which are related directly to tissue/implant

interface interactions. This approach is currently being explored in the develop-

ment of a new generation of nanobioceramics with a widened range of medical

applications. The improvement of interface bonding by nanoscale coatings, based

Figure 16.1 Classiﬁ cation of bioceramics

according to their bioactivity. (a) Bioinert

(alumina dental implant); (b) Bioactive,

hydroxyapatite (Ca

10

(PO

4

)

2

(OH)

2

) coating on a

metallic dental implant; (c) Surface - active,

bioglass or A - W glass; (d) Bioresorbable

tricalcium phosphate implant [Ca

3

(PO

4

)

2

].

526 16 Nanoceramics for Medical Applications

on biomimetics, has been of worldwide interest during the past decade, and

today several companies are in early commercialization stages of new - generation,

nanoscale - modiﬁ ed implants for orthopedic, ocular, and maxillofacial surgery,

as well as for hard - and soft - tissue engineering.

Biomimetic processing is based on the notion that biological systems store and

process information at the molecular level, and the extension of this concept to

the processing of nanocomposites for biomedical devices and tissue engineering,

such as scaffolds for bone regeneration, has been brought out during the past

decade [1] . Several research groups have reported the synthesis of novel bone

nanocomposites of hydroxyapatite ( HAp ) and collagen, gelatin, or chondroitin

sulfate, through a self - assembly mechanism. These self - assembled experimental

bone nanocomposites have been reported to exhibit similarities to natural bone in

not only their structure but also their physiological properties [2] .

The term nanocomposite can be deﬁ ned as a heterogeneous combination of two

or more materials, in which at least one of those materials should be on a nanom-

eter - scale. By using the composite approach, it is possible to manipulate the

mechanical properties such as strength and modulus of the composites closer to

those of natural bone, with the help of secondary substitution phases. For example,

HAp – polymer composites have been shown to have an elastic modulus close to

that of bone.

The fabrication of a nanocomposite can be achieved by physically mixing or

introducing a new component into an existing nanosized material, which allows

for property modiﬁ cations of the nanostructured materials and may even offer

new material functions. For example, some biopolymers and biomolecules, such

as poly(lactic acid) ( PLA ), poly(lactic - co - glycolic acid) ( PLGA ), polyamide, collagen,

silk ﬁ brin, chitosan, and alginate have been reported to mix into nanohydroxyapa-

tite ( nanoHAp ) systems.

Another form of nanocomposite which has been developed for biomedical appli-

cations is the gel system . For this, nanostructured materials can be entrapped in to

a gel (a three - dimensional ( 3 - D ) network immersed in a ﬂ uid), such that the prop-

erties of the nanomaterials can be improved and tailored to suit the speciﬁ c needs

of certain biomedical devices. A nanogel , which is a nanosized, ﬂ exible hydrophilic

polymer gel [3] , is an example of a gel that can be used in drug delivery carriers.

These nanogels can bind and encapsulate spontaneously (through ionic interac-

tions) any type of negatively charged oligonucleotide drug. A key advantage of

nanogels is that they allow for a high “ payload ” of macromolecules (up to 50 wt%),

a value which normally cannot be approached with conventional nanodrug carriers

[4] . Recently, a novel intracellular biosensor has been fabricated by entrapping

indicator dyes into an acrylamide hydrogel [5, 6] , whilst a carbon nanotube ( CNT )

aqueous gel has been developed as an enzyme - friendly platform for use in enzyme -

based biosensors [7] .

This aim of this chapter is to provide information relating to the use of biocer-

amics for medical applications, including tissue engineering and regeneration

using scaffolds and liposomes, nanoHAp powders for medical applications,

calcium phosphate nanocoatings and other surface modiﬁ cation techniques, simu-

16.2 Tissue Engineering and Regeneration 527

lated body ﬂ uids, and nano - and macrobioceramics for use in drug delivery and

radiotherapy.

16.2

Tissue Engineering and Regeneration

16.2.1

Scaffolds

In the past, the development of bone tissue engineering has been directly related

to changes in materials and nanotechnology. While the inclusion of materials

requirements is standard in the design process of engineered bone substitutes, it

is also critical to incorporate clinical requirements such that clinically relevant

devices can be engineered.

Multiple clinical reasons exist for the development of bone tissue - engineering

alternatives, including a need for better ﬁ ller materials when reconstructing large

orthopedic defects, and for orthopedic implants that are mechanically more suit-

able to their biological environment. The traditional biological methods of bone -

defect management include autografting and allografting .

Bone regeneration requires four components: (i) a morphogenetic signal;

(ii) responsive host cells that will respond to the signal; (iii) a suitable carrier of

this signal that can deliver it to speciﬁ c sites then serve as scaffolding for the

growth of the responsive host cells; and (iv) a viable – and most importantly – a

well - vascularized host bed [8 – 10] . A scaffolding material is used either to induce the

formation of bone from the surrounding tissue, or to act as a carrier or template

for implanted bone cells or other agents.

The process of bone regeneration is common to the repair of fractures. The incor-

poration of bone grafts, the skeletal homeostasis and the cascading sequence of

biological events are often described as the remodeling cycle . Stem cells have been

incorporated into a range of bioceramics and when implanted, can combine with

mineralized 3 - D scaffolds to form highly vascularized bone tissue. These nanos-

cale cultured cell – bioceramic composites can be used to treat full - thickness gaps

in long bone shafts, providing an excellent integration of the ceramic scaffold with

bone, and a good functional recovery (Figure 16.2 ).

Peroglio et al . [11] have produced and characterized polycaprolactone ( PCL ) -

coated alumina scaffolds to validate the concept of polymer – ceramic composites

with increased fracture resistance. Alumina scaffolds were sintered using a foam

replication technique, and the polymer coating was obtained by inﬁ ltrating the

scaffold with either a PCL solution or nanodispersion.

Bioactive glasses are amorphous, silica - based materials that are biocompatible,

bioactive, osteoconductive and even osteoproductive. Sol – gel - derived bioactive

glasses were developed by Hench et al . [12] to produce 3 - D bioactive scaffolds

with hierarchical interconnected pore morphologies similar to trabecular bone.

The scaffolds consist of a pore network with macropores in excess of 500 μ m

528 16 Nanoceramics for Medical Applications

that are connected by pore windows with diameters in excess of 100 μ m, this

being the minimum pore diameter required for tissue ingrowth and vasculariza-

tion in the human body. The scaffolds also have textural porosity in the mesopore

range (10 – 20 nm). The compressive strength reported was in the range of that

for trabecular bone. In conclusion, Horsch et al . [12] stated that, by combining

these properties with those inherent to sol – gel - derived bioactive glass, these

scaffolds would have high potential as scaffolds for bone tissue engineering

applications.

Considerable attention has also been paid during the past two decades to bioac-

tive composite grafts that consists of a bioactive ceramic ﬁ ller in a polymeric

matrix. These bioactive composite grafts are designed essentially to achieve

interfacial bonding between the graft and the host tissues. HAp/collagen,

HAp/ polyethylene ( PE ), and HAp/Ti - 6Al - 4V are notable examples of bioactive

composite grafts [13, 14] .

Of particular interest is the combination of HAp with collagen as a bioactive

composite, as this appears to be a natural choice for bone grafting [15] . Skeletal

bones comprise mainly collagen and carbonate - substituted HAp, both of which

are osteoconductive components; consequently, an implant manufactured from

such components is likely to behave in similar fashion. A composite matrix, when

embedded with human - like osteoblast cells, showed better osteoconductive prop-

erties compared to monolithic HAp, and produced calciﬁ cation of an identical

bone matrix. In addition, collagen – HAp composites proved not only to be biocom-

patible in both humans and animals [16] , but also to behave mechanically in a

superior fashion to the individual components. Moreover, the ductile properties

of collagen helped to increase the poor fracture toughness of HAps.

The reconstruction of bone tissue using nanocomposite bone grafts with struc-

ture, composition, physico - chemical, biomechanical, and biological features that

Figure 16.2 Bone regeneration therapy using

marrow mesenchymal cells for bone - graft

applications for increased bioactivity. (a) The

marrow cell/hydroxyapatite composite scaffold

is cultured in an osteogenic medium;

(b) After two to three weeks of culture, the

cultured bone graft/hydroxyapatite composite

is inserted into a bone defect.

Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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