nanotechnology in biomaterials: nanofibers in tissue...
TRANSCRIPT
195
10Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering
Deniz Yucel, Halime Kenar, Albana Ndreu, Tugba Endogan, Nesrin Hasirci, and Vasif HasirciDepartments of Biological Sciences and Chemistry, Middle East Technical University, Ankara, Turkey
10.1 Biomaterials
Devices made of synthetic or natural materials have been used in the medical area and introduced into the human body to improve human health since ancient times. Romans, Chinese, and Aztecs used gold wires in dentistry more than 2,000 years ago. Ancient Egyptians and Greeks sutured wounds with plant !bers and animal-derived materials and used wood for prosthetic limbs. Since then, scientists have continued their research to !nd bioactive agents to cure illnesses and to improve the quality of human life by using arti!cial prostheses.
There are various de!nitions of biomaterials expressed in different ways given in the literature, but more or less they have the same meaning. One of the very early de!ni-tions came from the Clemson University Advisory Board for Biomaterials (1976), which described a biomaterial as “a systemically and pharmacologically inert substance designed for implantation within or incorporation with living systems (Park, 1981).” Another def-inition of biomaterials is “materials of synthetic as well as of natural origin in contact with tissue, blood, and biological "uids, and intended for use for prosthetic, diagnostic, therapeutic, and storage applications without adversely affecting the living organism and its components” (Bruck, 1980). Biomaterials can be de!ned as materials to be used to
CONTENTS10.1 Biomaterials ...................................................................................................................... 18510.2 Tissue Engineering .......................................................................................................... 18610.3 Importance of Nano/Micro!brous Scaffolds............................................................... 18910.4 Fabrication of Nano/Micro!brous Scaffolds ............................................................... 18910.5 Nano/Micro!bers in Tissue Engineering Applications ............................................. 191
10.5.1 Bone Tissue Engineering .................................................................................. 19110.5.2 Nerve Tissue Engineering ................................................................................ 19710.5.3 Cardiac Muscle Tissue Engineering ............................................................... 204
References ..................................................................................................................................... 208
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substitute a tissue or organ of a living system that is not fully functioning, or to support the biological system in its function while in intimate contact with the tissues in a safe, reli-able, economic, and physiologically acceptable manner (Hench and Erthridge, 1982). Black (1992) de!ned biomaterial as “a nonviable material used in a medical device, intended to interact with biological systems.” In addition, during the Second Consensus Conference on De!nitions in Biomaterials Science, a biomaterial was de!ned as “a material intended to interface with biological systems to evaluate, treat, augment, or replace any tissue, organ or function of the body” (Williams, 1992). The same de!nition is found in the Williams Dictionary of Biomaterials (Liverpool University Press, 1999).
Biomaterials are used in many application areas, such as orthopedic (total joint replace-ments, bone plates and screws, etc.), dental (tooth !llings, crowns, implants, etc.), cardio-vascular (cardiac pacemaker, arti!cial heart valve, blood vessels, etc.), ophthalmic (contact and intraocular lenses), wound healing (sutures, wound dressing, tissue adhesive, ligating clips, staples, etc.), and drug delivery (oral, inhalation injectable delivery systems, micro- and nanodevices, sheets, !bers, and sponges, etc.) systems. Biomaterials can be found in different forms, such as tubes (arti!cial veins, arti!cial trachea), !bers (sutures), !lms and membranes (wound dressings, drug delivery systems, arti!cial kidney), gels (burn covers, topical drug delivery systems), and porous scaffolds (tissue engineering), depending on the site and type of use. Metals (titanium, stainless steel, cobalt-chromium alloys, gold, silver, platinum), polymers (nylon, silicones, Te"on, dacron, polylactides), ceramics (aluminum oxide, carbon, hydroxyapatite), natural materials (collagen, hyaluronic acid, reconstituted tissue such as porcine heart valves), and composites (carbon or ceramic wires, !ber-rein-forced polymers) have been used as biomaterials in the design and production of biomedi-cal devices. Material to be used as a biomaterial must ful!ll some requirements. First, it should be compatible with the biological environment and should not cause any adverse carcinogenic or allergic tissue reaction; i.e., it should be biocompatible. Degradation prod-ucts of a biomaterial must also be nontoxic and noncarcinogenic, and have the desired bioactivity toward the cells and tissue. Physical, chemical, and mechanical properties of the biomaterial should match those of the target tissue.
10.2 Tissue Engineering
Tissue engineering was once considered a sub!eld of biomaterials, but since it has gained too much importance in the last decades, it is now considered a !eld in its own right. Tissue engineering is the process of creating living three-dimensional (3D) tissues and organs by using engineering and materials methods, and speci!c combinations of cells and cell signals, both chemical and mechanical (Grif!th, 2002). This approach aims to reproduce both the form and function of the tissue. Different from nonliving biomaterials, tissue engineering is based on the formation and regeneration of a new functional tissue by com-bining living components (cells, tissue fragments) and carrier scaffolds in a construct to be implanted into the body (Kneser et al., 2002; Guelcher et al., 2006).
There are three approaches in repairing tissues. The !rst one is the injection of tissue-speci!c viable cells directly into the damaged site of the tissue. The second approach is the delivery of tissue-inducing substances, such as growth and differentiation factors, to
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target locations. The third one is the tissue engineering approach, in which cells are grown in 3D scaffolds. The use of cell suspensions or tissue-inducing substances is considered when the defects are small, well contained, or not suitable for implanting a 3D structure. To engineer tissues with practical size and predetermined shapes, the !rst two approaches are not appropriate. The main principle of the tissue engineering strategy is to seed a porous, biodegradable 3D cell carrier (scaffold, matrix) with cells, add signaling molecules like growth factors, culture the construct in a medium for growing and proliferation of cells, and then implant it into the defect area to grow a new tissue. It is expected that the cells of the natural tissue would also attach to the scaffold, proliferate, differentiate, and get organized, forming a normal, healthy tissue while the scaffold itself degrades. At the end, healthy tissue !lls the defect site and no arti!cial matrix remains. Scaffolds, cells, and signaling biomolecules are the main components of tissue engineering devices. All of these components play very important roles during tissue regeneration. The extracel-lular matrix (ECM) is a microenvironment composed of parenchymal cells (functional cells), mesenchymal cells (support cells), and structural materials. Every tissue and organ in our body is formed in these microenvironments. The body serves as a bioreactor, which applies biomechanical forces and provides biochemical signals to the cells and the ECM to develop and maintain tissue and organs (Barnes et al., 2007). In the tissue engineering approach, the biological way of tissue formation in the body is mimicked by taking place in a porous matrix.
A scaffold is one of the essential components of tissue engineering since it provides 3D support for the cells, as ECM does. In vitro, cells do not grow in a way to !ll a volume if there is no 3D support. Porous scaffolds provide some place for cell attachment and viability through transport of nutrients and metabolites, and allow cell expansion and tis-sue organization. There are some requirements for proper scaffolds. First, they should be biocompatible, and therefore should not cause any adverse responses, such as allergic and carcinogenic reactions in the body. They must have an interconnected, highly porous struc-ture with a high surface area for cell adhesion and reorganization, for transport of nutri-ents and removal of metabolic waste, and for homogeneous cell distribution throughout the scaffold. They should have adequate mechanical strength to withstand the pressures and mechanical forces applied by the biological system. The degradation rate of the scaf-fold should be in accordance with the tissue growth; when the tissue is totally regenerated, the scaffold should be almost fully degraded. A suitable surface chemistry is extremely important for cell attachment, proliferation, and differentiation Ma, 2004a; Stevens et al., 2008. The material of the scaffold should be easily processed to achieve the desired shapes and sizes. Finally, the scaffold should have some speci!c properties for speci!c tissue engi-neering applications, such as osteoconductivity for bone, electrical conductivity for nerve, and contraction for myocardial tissue engineering.
Various materials have been studied as potential scaffolds. Some metals are good choices for load-bearing implants since they have superior mechanical properties (Catledge et al., 2004). Recently, porous titanium (Ti) and titanium alloys were used as scaffold biomaterials for bone tissue engineering, but they are not degradable in the body. Inorganic materials are also used in bone tissue engineering studies (Hench and Polak, 2002). Ceramics such as hydroxyapatite (HAp), Bioglass, A-W glass ceramics, and
-tricalcium phosphate ( -TCP) closely mimic bone tissues. They are osteoconductive or osteoinductive, and enhance the absorption of bone ECM proteins, resulting in improved osteoblast adhesion (Ducheyne and Qiu, 1999; Dieudonne et al., 2002; Smith et al., 2004).
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However, ceramics have some drawbacks, like fragility, brittleness, low tensile strength, and being mostly not biodegradable. Biological and synthetic polymeric materials have been widely used in various tissue engineering applications due to their degradability, biocompatibility, mechanical properties suitable for many applications, and ease of pro-cessability. Some examples of natural biodegradable polymers are collagen, !brinogen, gelatin, chitosan, cellulose, alginate, starch, silk, and hyaluronic acid. The main advan-tages of these materials are their low immunogenicity, bioactivity, capability of inter-acting with the host tissue, chemical versatility, and availability from various sources, as in starch and chitosan. Of these, collagen, !brin, hyaluronic acid, agarose, chitosan, and alginate are materials especially used in the production of scaffolds for engineering of bone and cartilage (Sabir et al., 2009). The most widely used synthetic polymers are a family of linear aliphatic polyesters, poly( -hydroxy acid) (PHA), such as poly(lactic acid) (PLA), poly(glycolic acid) (PGA), and their copolymer poly(lactic acid-co-glycolic acid) (PLGA) (Mooney et al., 1996; Seal et al., 2001). For example, PGA has been used in the engineering of cartilage, tendon, ureter, intestine, blood vessel, heart valve, and other tissues. Poly( -caprolactone) (PCL), poly(propylene fumarate) (PPF), poly(carbonate), poly(phosphazenes), poly(phosphoesters) and poly(anhydrides), tyrosine-derived poly-mers, and biodegradable urethane-based polymers are other important synthetic, bio-degradable polymers used in tissue engineering applications. A problem with synthetic polymers is their degradation products, which may reduce the local pH, induce an in"ammatory response, or create some adverse reactions and being nonbiocompatible.
In the past few years, polymer-ceramic composites have gained increased importance in the engineering of several types of tissues, such as bone, cartilage, tendon, and ligament (Marra et al., 1999; Zhang and Ma, 1999; Ma et al., 2001; Zhao et al., 2002; Liu and Ma, 2004). The composite is expected to have improved compressive mechanical properties compared to the polymer, and better structural integrity and "exibility than ceramics, and thus the combination of ceramic and polymer created reinforced porous scaffolds with enhanced bioactivity and controlled resorption rates (Zhang and Ma, 1999; Ma et al., 2001).
During the selection of a scaffold material, an important step is to decide on the fab-rication technique. The processing technique is expected not to change certain material properties, like biocompatibility and surface chemistry. The technique should be repro-ducible in terms of scaffold porosity, pore size, pore distribution, and interconnectivity (Leong et al., 2003). There are many techniques that can be used to process materials into scaffolds, such as solvent casting, phase inversion, !ber bonding, melt-based technologies, high-pressure-based methods, freeze drying, and rapid prototyping (Ma and Langer, 1999; Kinikoglu et al., 2009; Sangsanoh et al., 2009; Yilgor et al., 2008; Vrana et al., 2007; Zorlutuna et al., 2007; Mooney 1996; Thomson et al., 1995; Mikos et al., 1993). Each production tech-nique has some disadvantages and advantages over the others and may not be applicable to some polymers. Therefore, appropriate processing techniques should be chosen to pro-duce a scaffold with the desired properties for the speci!c application.
To date, scaffolds have been produced in our group in various forms, like foam (Zorlutuna et al., 2008; Kinikoglu et al., 2009, Dogan et al., 2009, Ulubayram et al., 2001), !lm (Vrana et al., 2007; Zorlutuna et al., 2007; Kenar et al., 2008; Zorlutuna et al., 2009), and !bers (Ndreu et al., 2008; Kenar et al., 2009; Yucel et al., 2010), each having different chemistries, and a number of studies have been carried out with them either in vitro or in vivo. In the last decades, foam and !lm-based scaffolds were popular for 3D and 2D applications; however, in recent years there has been an increase in the use of !brous scaffolds, especially made by electrospinning.
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10.3 Importance of Nano/Microfibrous Scaffolds
Many extracellular proteins have a !brous structure with diameters on the low or submi-crometer scales. For example, collagen is the most abundant ECM protein in the human body, and it has a !brous structure with !ber bundle diameters varying from 50 to 500 nm, depending on the site and species (Hay, 1991). In the native tissues, the structural ECM proteins are one to two orders of magnitude smaller than the cells, and therefore, the cells are in direct contact with many ECM !bers. It is now well known that many biologi-cally active molecules, extracellular matrix components, and cells interact with each other using functional groups on the nanoscale. It is therefore a useful approach to mimic the !brous nature of the extracellular matrix when fabricating scaffolds (Barnes et al., 2007). Nano!bers have a high surface-to-volume ratio, which enhances the number of cells that adhere to the surface (Ma and Langer, 1999). Cell migration, proliferation, and differentia-tion follow adhesion (Palecek et al., 1997), and it is expected that these reactions will be much higher on nano!brous scaffolds than on foams or !lms. As a result, nano!bers have attracted the attention of many researchers who considered them as potential scaffolds for applications such as tissue engineered vasculature (Xu et al., 2004; Lee et al., 2007), bone (Sui et al., 2007; Cui et al., 2007), nerve (Yang et al., 2005; Schnell et al., 2007), and tendon and ligament (Lee et al., 2005; Sahoo et al., 2006).
10.4 Fabrication of Nano/Microfibrous Scaffolds
Since !brous scaffolds with nanometric !ber diameters have been found to be satisfac-tory for various tissue engineering applications, extensive research toward developing processes for the fabrications of these !brous structures is being conducted. The main methods used in the fabrication of !brous scaffolds are self-assembly, template synthe-sis, drawing and extrusion, phase separation, wet spinning, and electrospinning. Each of these procedures leads to !bers with different dimensions and organizations.
In the case of self-assembly, atoms and molecules arrange themselves through weak and noncovalent interactions, and it is known as a bottom-up method since the built-up nanoscale !bers are formed starting from smaller molecules. It yields !bers with small dimensions (less than 100 nm wide and up to a few micrometers long), and it offers novel properties and functionalities that cannot be achieved by conventional organic synthesis (Hasseinkhan et al., 2006). The main disadvantage of this method is its complexity, in addition to being a long and extremely elaborate process with low productivity (Ma et al., 2005).
Another fabrication method is template synthesis. As the name implies, a nanoporous membrane is used as a mold or template in order to obtain the material of interest in the form of solid nano!brous or hollow-shaped tubules. This method uses a polymer solution to be extruded through a porous membrane by application of pressure. Fibers are formed when the extruded polymer comes into contact with a precipitating solution (Feng et al., 2002). Fiber diameter depends on the pore size of the template used, and it varies from a few to a hundred nanometers. Many materials, like metals, polymers, and carbon, can be utilized as templates for !ber fabrication; however, this method is limited in that the length of nano!bers can only be a few micrometers.
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Drawing is a method to produce single nano!bers. It requires a minimum amount of equipment and is a discontinuous process. A micropipette is dipped into a droplet near the solution-solid surface contact line via a micromanipulator, and then the micropipette is withdrawn from the liquid at a certain speed, yielding a nano!ber. These steps are repeated many times, each time with a different droplet. The solution viscosity, however, increases with solvent evaporation, and some !ber breaking occurs due to instabilities that occur during the process (Ondarcuhu and Joachim, 1998). The main limitation of this method is that it can only use solutions of viscoelastic materials, which can undergo strong deformations that are cohesive enough to withstand the stresses developed during the pull. Again, the !ber diameter is dependent on the ori!ce size used. It is dif!cult to obtain !ber diameters of less than 100 nm.
Phase separation is another process utilized in nano!ber production. The main principle of this method is that a polymer dissolved in an appropriate solvent is stirred at a certain gelation temperature for a period of time until a homogeneous solution is obtained. Then, the formed gel is immersed in water several times for solvent exchange to occur. The !nal gel is lyophilized. Phase separation occurs due to chemical incompatibility and results in nano!bers, but the problem is that a long time is needed to convert the polymer into a nanoporous foam. The !ber diameter ranges from 50 to 500 nm with a length of a few micrometers (Zhang et al., 2005), which is a disadvantage for some applications. Moreover, only polymers that have gelation capability can be used to obtain nano!brous structures.
Wet spinning is a process similar to drawing, with the difference being that a polymeric solution is precipitated or coagulated by dilution, adding into a nonsolvent or a chemical reaction. Fibers produced by this method are continuous and in microscale (from 10 to 100 µm). Polymer solution concentration and the spinneret diameter are the two crucial parameters that in"uence the resulting !ber diameter.
The last, and one of the most preferred, methods used to prepare !bers in nano- and low microscale is the electrospinning process. High electrostatic forces are applied to draw continuous !bers from a polymeric solution through a syringe. Basically, four components are required to accomplish the process: a capillary tube ending in a needle of small diam-eter, a high-voltage supplier, a syringe pump, and a metallic component. One of the elec-trodes is connected to the needle of the syringe containing the polymer, and the other one is connected to the collector. Leakage of the polymer solution is prevented by surface tension; however, as the applied potential increases, the surface tension contribution starts to decrease as a result of charge repulsion, and the "uid shape at the needle tip becomes like a cone, what is called the Taylor cone. The voltage is increased until the repulsive elec-trostatic forces overcome the in"uence of solution surface tension. Meanwhile, the solvent evaporates and the charged jet of the solution is ejected from the needle tip in the shape of a long, continuous, thin !ber. Therefore, !bers with various diameters are collected on either a stationary or rotating collector. Rotating collectors are used to produce aligned nano!bers, which are more preferable in certain applications. A variety of polymers have been used until now to produce !bers from tens of nanometers to a few micrometers (Jin et al., 2004; Li et al., 2005; Ma et al., 2005; Ndreu et al., 2008; Erisken et al., 2008; Jeong et al., 2009; Kenar et al., 2009; Yucel et al., 2010).
Certain properties are expected from a polymer that is proven to be spinnable. The !bers should be consistent, defect-free, and have controllable diameters. The main parameters to control are process conditions such as polymer type, its concentration, solvents, applied potential, distance between the needle tip and the collector, and the collector type. These parameters are modi!ed to control !ber thickness, porosity and pore size (or rather the spacing between the !bers or !ber density), surface chemistry, and surface topography
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Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering 201
(Deitzel et al., 2001; Huang et al., 2003; Katti et al., 2004; Barhate et al., 2006; Ndreu et al., 2008; Wang J et al., 2009). Therefore, optimization of the process conditions in order to get the best results is challenging with so many parameters.
Finally, it can be said that compared to the other nano/micro!ber-production methods, electrospinning is more economical, simpler, yields continuous !bers, and is versatile enough to be used for the production of nano!bers from a variety of materials. Therefore, electrospinning represents an attractive technique to be used for producing nano!ber-based scaffolds for tissue engineering.
10.5 Nano/Microfibers in Tissue Engineering Applications
The use of polymeric nano/micro!bers in tissue engineering applications is rapidly grow-ing. The main areas include the engineering of skin (Venugopal and Ramakrishna, 2005; Noh et al., 2006; Yang et al., 2009; Jeong et al., 2009; Torres et al., 2009), ligament and tendon (Petrigliano et al., 2006; Teh et al., 2007, 2008; Sahoo et al., 2009; Fan et al., 2009; Yin et al., 2009), skeletal muscle (Riboldi et al., 2005; Choi et al., 2008), cartilage (Li WJ et al., 2005; Li et al., 2006; Janjanin et al., 2008; Lu et al., 2008; Hu et al., 2009), bone (Santos et al., 2008; Zhang et al., 2008; Reed et al., 2009; Wang J et al., 2011; Prabhakaran et al., 2009), cardio-vascular (Kenar et al., 2009; Rockwood et al., 2008; Li WJ et al., 2006), and nerve (Yucel et al., 2010; Neal et al., 2009; Nisbet et al., 2008; Corey et al., 2007) tissues. The increase in the number of review articles (Ashammakhi et al., 2007a, 2007b, 2008; Agarwal et al., 2009; Jang et al., 2009) and publications related to the application of nano/micro!bers in these !elds shows the interest in electrospinning for tissue engineering. In this review we will concentrate more on the studies carried out by our research group, and also by some of the leading researchers in the !elds of bone, cardiovascular, and nerve tissue engineering.
10.5.1 Bone Tissue Engineering
Prior to designing a bone substitute or a tissue-engineered bone, it is important to know well the structure and function of the natural bone so that the material type chosen is the most suitable to regenerate the function of the damaged bone. Bone is a complex, physically hard, rigid, and highly organized tissue. This tissue is composed of an intracellular matrix in a !brous form made mainly of collagen types I and V, and some inorganic materials. Apart from the major collagen matrix, some other noncollageneous proteins, like osteo-calcin and osteopontin, are also present. There are three cell types present in the bone structure: osteoblasts, osteocytes, and osteoclasts. Osteoblasts, derived from mesenchymal precursor cells in the marrow, are the bone cells responsible for formation and structural organization of the bone matrix, and also bone mineralization. When osteoblasts become trapped in the matrix, they secrete and become osteocytes. Osteocytes are the most abun-dant cells found in compact bone. They are networked to each other via long cytoplasmic extensions that occupy tiny canals called canaliculi, which are used for exchange of nutri-ents and waste. The space that an osteocyte occupies is called a lacuna. Unlike osteoblasts, these cells have reduced synthetic activity and are not capable of mitotic division. They are actively involved in the turnover of bony matrix, through various mechanosensory mech-anisms. They destroy bone through a mechanism called osteocytic osteolysis. The third bone cell type, osteoclasts, are polarized cells and have a crucial role in bone resorption,
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202 Bionanotechnology II: Global Prospects
which includes an initial mineral dissolution and then an organic phase degradation (Jang et al., 2009).
As mentioned above, the main approach in the tissue engineering process is mimicking the ECM. Therefore, for bone tissue engineering it is crucial to make a design whose prop-erties would mimic best the structure and function of bone ECM. It is important to choose a material that interacts well with bone cells in the bone formation processes. Moreover, it is favorable that materials with good mechanical properties and can withstand various external stresses applied on bone are incorporated into the material design. Since the orga-nization of all bone compartments is in nanoscale or at the molecular level, a nanoscale-based design would be optimal to mimic the natural bone. Various strategies have been tried in designing a tissue engineered bone.
Both synthetic and natural materials have been utilized in this !eld. Among all poly-meric materials, a group of polyesters, such as PLA, PGA, PCL, poly(3-hydroxybutyric acid) (PHB), and their copolymers, has been the most extensively studied polymers to produce the nano!brous system for the regeneration of bone tissues (Burg et al., 2000). A summary of nano/micro!brous scaffolds used in bone tissue engineering studies is given in Table 10.1. Yoshimoto et al. (2003) used electrospun PCL nano!bers, and microscopic analysis of mes-enchymal stem cell-seeded scaffolds revealed a good cell penetration in addition to a high level of mineralization and collagen type I production after 4 weeks of culture (Yoshimoto et al., 2003). Sombatmankhong et al. produced mats from PHB, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and their 50/50 w/w blends and studied the biocompatibility of these mats by a human osteosarcoma cell line (Saos-2) and mouse !broblasts (L929). Both cells adhered well on all types of !brous scaffolds; however, the highest alkaline phos-phatase (ALP) activity was seen on the PHB/PHBV blend (Sombatmankhong et al., 2007).
In another study, PLA and poly(ethylene glycol) (PEG)-PLA diblock polymers with con-trolled topographies and chemistries were prepared by electrospinning in order to study the in"uence of surface topography and !ber diameter on cell behavior. In the presence of osteogenic factors the cell numbers on !brous scaffolds were reported to be equal to or higher than those on smooth surfaces. Furthermore, the authors indicated an increase in cell density with an increase in !ber diameter and no in"uence of surface substratum on ALP activity (Badami et al., 2006). The in"uence of !ber diameter on cell behavior was also studied by other researchers. Ndreu et al. (2008) studied the in"uence of different types of nano/micro!bers (diameter range: 300 nm–1.5 µm) on Saos-2 cells. Good cell adherence, proliferation, and in!ltration were observed on all types of scaffolds (PHBV, PHBV-PLGA, PHBV-P(L,DL)LA, and PHBV-PLLA); however, the highest number of cell attachments was observed on the !bers with the lowest diameter (PHBV-PLLA !ber diameter: ~350 nm) when the scaffolds were examined with a scanning electron microscope (SEM) and a con-focal laser scanning microscope (CLSM) (Figure 10.1).
Reed and his co-workers (2009) carried out a study on the reconstruction of a compound tissue based on the fact that healing does not occur as a single tissue, but as a compound one, such as bone-periosteum-skin. In order to perform this study, they seeded human foreskin !broblasts, murine keratinocytes, and periosteal cells on PCL nano!bers and showed that these scaffolds supported the growth of all these cell types. Even though keratinocytes grew randomly throughout the scaffolds, they reported improved longevity of the cocultured cells and a strong degree of osteoinduction. As a consequence, construc-tion of a composite tissue has been reported by the authors (Reed et al., 2009).
Compared to the synthetic materials, natural materials have better biocompatibility, and more importantly, they possess the main cues that are necessary for a cell to attach and grow. Natural polymers are also advantageous in that they can be degraded by naturally
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Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering 203
TAB
LE 1
0.1
Fibr
ous
Scaf
fold
s Fa
bric
ated
by
Elec
tros
pinn
ing
for
Bone
Tis
sue
Engi
neer
ing
Poly
mer
Typ
ePo
lym
er C
once
ntra
tion
/Com
posi
tion
/Sol
vent
Fibe
r D
iam
eter
In V
itro
Stu
dies
: C
ell T
ype
Ref
eren
ce
PLA
/BG
nan
o!lle
r3.
5%, w
/v; 9
0:10
, w:w
CH
LH
undr
eds
of n
anom
eter
sM
C3T
3-E1
Noh
et a
l., 2
010
Gel
atin
/silo
xane
15–2
5% w
/wG
elat
in:G
PMS:
CaN
O3 fo
rmic
aci
dN
anom
eter
sca
leBM
SCs
Ren
et a
l., 2
010
PVA
/CO
L/n-
HA
p7%
w/w
PVA
:CO
L (5
5:45
)n-
HA
p: 5
–10%
w/w
of P
VA d
eion
ized
wat
er, H
Ac
~320
nm
—A
sran
et a
l., 2
010
PAN
/-T
CP
10%
w/w
DM
F~3
00 n
mM
G-6
3Li
u et
al.,
201
0
CH
I7%
w/w
TFA
/DC
M 7
0:30
v/v
126
20
nmM
C3T
3-E1
Sang
sano
h et
al.,
200
9
PCL/
nano
-apa
tite
(nA
p)80
:20%
w/w
80%
TFE
in H
2O32
0–43
0 nm
RBM
Yang
F. e
t al.,
200
9
CO
L-H
Ap
80:2
0% w
/wSo
l con
cent
ratio
n (0
.023
–0.0
40 g
/ml)
HFP
75–1
60 n
mM
C3T
3-E1
Song
et a
l., 2
008
CH
I/PV
A a
nd C
ECS/
PVA
CH
I (7%
w/w
): PV
A (1
0% w
/w) (
80:2
0 v/
v)A
q. H
Ac
100–
700
nmL9
29Ya
ng e
t al.,
200
8
CH
I-H
Ap
HA
p/C
HI (
30:7
0)
10%
w/w
UH
MW
PEO
HA
c:D
MSO
(10:
1, w
/w)
214
25
nmhF
OB
Zha
ng e
t al.,
200
8
PCL-
-TC
P12
% w
/wD
CM
0–15
% w
/w
-TC
P
200–
2,00
0 nm
MC
3T3-
E1Er
iske
n et
al.,
200
8
PLG
A/C
OL
8 g/
ml (
50:5
0 (w
:w))
HFP
382
125
nm
BMH
SCs
Ma
et a
l., 2
008
PLA
/HA
p6%
w/w
DC
M a
nd 1
,4-d
ioxa
ne31
3 nm
MG
63Su
i et a
l., 2
007 (con
tinue
d )
K10254_C010.indd 203 5/12/11 7:18:07 PM
204 Bionanotechnology II: Global ProspectsTA
BLE
10.
1 (C
ON
TIN
UED
)
Fibr
ous
Scaf
fold
s Fa
bric
ated
by
Elec
tros
pinn
ing
for
Bone
Tis
sue
Engi
neer
ing
Poly
mer
Typ
ePo
lym
er C
once
ntra
tion
/Com
posi
tion
/Sol
vent
Fibe
r D
iam
eter
In V
itro
Stu
dies
: C
ell T
ype
Ref
eren
ce
PHB,
PH
BV, P
HB
- PH
BV
(50:
50 w
/w)
14%
w/v
CH
L2,
300–
4,00
0 nm
SaO
S-2
and
L929
Som
batm
ankh
ong
et a
l., 2
007
Silk
!br
oin
(Nan
g-La
i and
D
OA
E-7
type
)10
–40%
(w/v
)85
% F
A21
7–61
0 nm
(Nan
g-La
i)18
3–81
0 nm
(DO
AE-
7)M
C3T
3-E1
Mee
chai
sue
et a
l., 2
007
PLA
(L- a
nd D
L-ty
pe)/
PEG
8.2%
w/w
(L-t
ype)
, 5%
w/w
(DL-
type
), 26
% w
/w (P
EG-P
LLA
), 26
% w
/w (P
EG-P
DLL
A)
HFP
246
79
nm (L
-typ
e)14
1 3
9 nm
(DL-
type
)17
1 6
6 nm
(PEG
-PLL
A)
889
446
nm
(PEG
-PD
LLA
)
MC
3T3-
E1Ba
dam
i et a
l., 2
006
Silk
/PEO
/nH
Ap/
BMP-
27.
86%
(w/v
),si
lk:P
EO (8
2:18
w/w
)5
g nH
AP/
100
g si
lk3
mg
BMP-
2/m
g si
lk !
broi
n
520
55
nmhM
SCs
Li e
t al.,
200
6
Gel
atin
/HA
p80
:20
(% w
/w)
HFP
200–
400
nmM
G63
Kim
et a
l., 2
005
PCL-
CaC
O3
PCL:
CaC
O3
75:
25 a
nd 2
5:75
% w
/wC
HL:
MeO
H (7
5:25
% w
/w)
760–
900
nmhF
OB
Fujih
ara
et a
l., 2
005
Silk
!br
oin/
PEO
7.5%
w/w
(80/
20 w
/w) i
n H
2O70
0 5
0 nm
Bone
mar
row
str
omal
ce
lls (B
MSC
s)Jin
et a
l., 2
004
PCL,
gel
atin
, PC
L/ge
latin
Gel
atin
: 2.5
–12.
5% w
/vPC
L: 1
0% w
/v50
:50
blen
d: 1
0% w
/vTF
E
Tens
of n
m–1
#m
BMSC
Zha
ng e
t al.,
200
5
PCL
10%
w/w
CH
L40
0 2
00 n
mBM
SCYo
shim
oto
et a
l., 2
003
Abb
revi
atio
ns: B
G, b
ioac
tive
glas
s; B
MSC
s, b
one
mar
row
-der
ived
mes
ench
ymal
ste
m c
ells
; CEC
S, N
-car
boxy
ethy
l chi
tosa
n; C
HI,
chito
san;
CH
L, c
hlor
ofor
m; C
OL,
col
-la
gen;
DC
M, d
ichl
orom
etha
ne; D
MF,
N,N
-dim
ethy
l fo
rmam
ide;
DM
SO, d
imet
hyl
sulfo
xide
; FA
, for
mic
aci
d; G
PMS,
3-(
glyc
idox
ypro
pyl)
trim
etho
xysi
lane
; H
Ac,
ace
tic a
cid;
HA
p, h
ydro
xyap
atite
; hFO
B, h
uman
feta
l ost
eobl
ast;
HFP
, 1,1
,1,3
,3,3
-hex
a"uo
ro-2
-pro
pano
l; hM
SC, h
uman
bon
e m
arro
w-d
eriv
ed m
esen
chy-
mal
ste
m c
ell;
L929
, mou
se !
brob
last
s; M
eOH
, met
hano
l; M
C3T
3-E1
, mou
se c
alva
ria-
deri
ved
mur
ine
preo
steo
blas
t cel
ls; M
G63
, hum
an o
steo
blas
ts c
ells
; PA
N,
poly
(acr
ylon
itrile
); PC
L, p
oly(
-cap
rola
cton
e); P
EG, p
oly(
ethy
lene
gly
col);
PEO
, pol
y(et
hyle
ne o
xide
); PH
B, p
oly(
hydr
oxyb
utyr
ic a
cid)
; PH
BV, p
oly(
3-hy
drox
y-bu
tyri
c-co
-3-h
ydro
xyva
leri
c ac
id);
PLA
, pol
y(la
ctic
aci
d); P
LGA
, pol
y(la
ctic
aci
d-co
-gly
colic
aci
d); R
BM, r
at b
one
mar
row
cel
ls; T
FA, t
ri"u
oroa
cetic
aci
d; T
FE,
2,2,
2-tr
i"uo
roet
hano
l; -T
CP,
-t
rica
lciu
m p
hosp
hate
.
K10254_C010.indd 204 5/12/11 7:18:07 PM
Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering 205
occurring enzymes, and therefore, they can be used as biodegradable scaffolds. The most commonly used natural polymers are proteins (collagen, gelatin, !brinogen, silk, casein, etc.) and polysaccharides (chitosan, cellulose, hyaluronic acid, etc.). Sangsanoh et al. (2009) studied biological evaluation of electrospun chitosan nano!bers and !lms in vitro with four different cell types: Schwann cells, osteoblast-like cells, keratinocytes, and !broblasts. Both substrate forms were seen to support cell attachment and especially promoted pro-liferation of keratinocytes. While Schwann cells attached well on nano!brous structures, they did not adhere on !lms. On the other hand, even though osteoblast-like cells and !bro-blasts showed good attachment on both nano!brous and !lm surfaces they did not prolifer-ate well (Sangsanoh et al., 2009). Jin et al. (2004) produced silk !broin/poly(ethylene oxide) (PEO) nano!brous matrices (700–750 nm), examined the behavior of human bone marrow stromal cells (BMSCs), and reported that the matrices supported BMSC attachment and proliferation during 14 days of incubation with extensive matrix coverage (Jin et al., 2004).
A blend of natural and synthetic polymers is advantageous in that they combine the better mechanical properties of synthetic polymers with the biofunctionality of natural ones. In this approach the properties of the scaffold can be changed easily and can be arranged as desired by changing the composition and the ratio of the blend. Ngiam et al. (2009) prepared nano-hydroxyapatite (n-HAp) containing electrospun PLGA and PLGA/collagen (COL) blend nano!bers. Biomineralization of n-HAp on electrospun nano!bers was achieved by using an alternate calcium and phosphate solution dipping method. n-HAp crystallite deposition was predominant on PLGA/COL nano!bers compared to on PLGA, which shows that collagen is a good template for n-HAp nucleation. It was shown that the amount of n-HAp present had a greater in"uence on early cell behavior, such as attachment and proliferation, than in the intermediate stages because their ALP and total protein expression results for mineralized PLGA and PLGA/COL were comparable dur-ing the culture period (Ngiam et al., 2009).
An important requirement in bone tissue engineering is that the scaffold should be highly porous so that cells can be homogeneously distributed throughout the structure. Poor cell in!ltration into the structure is reported when electrospun scaffolds are used; the cells are observed more on the surface than the inside of the scaffold. Therefore, it is of great importance to !nd an approach to produce nano!brous scaffolds with a higher degree of cell penetration. Ekaputra et al. (2008) used three strategies to solve this problem: selective leaching of a water-soluble !ber phase (PEO or gelatin), the use of micron-sized
A B C
D
FIGURE 10.1SEM micrographs of (A) unseeded and (B) Saos-2 cell-seeded PHBV-P(L,DL-LA) (70:30) scaffolds 14 days after incubation. (C) CLSM images of Saos-2-seeded PHBV10 scaffolds 7 days after incubation (top view showing cell-matrix interaction). (D) CLSM of cell in!ltration within the scaffold as viewed in cross section (Z-axis direction). (From Ndreu et al., Nanomed, 3, 45–60, 2008.)
K10254_C010.indd 205 5/12/11 7:18:07 PM
206 Bionanotechnology II: Global Prospects
!bers as the scaffold, and a combination of micron-sized !bers with codeposition of a hyaluronic acid (HA)-derived hydrogel, Heprasil. In the !rst procedure, PCL/COL solu-tion was electrospun in the presence of PEO, which was then removed selectively in order to increase the scaffold porosity. In the second approach, PCL/COL was electrospun in micrometer-sized PCL/COL (µmPCL/COL). The third method involved the cospraying of µmPCL/COL matrix with Heprasil. All the scaffold types supported attachment and pro-liferation of human fetal osteoblasts (hFOB). While selective leaching improved cellular in!ltration to a low degree compared to meshes obtained by conventional electrospinning, µmPCL/COL micro!bers allowed better penetration. This effect was more pronounced when Heprasil regions were present in the structure.
A fabrication method that can be used to solve the low penetration problem is the use of electrospun nano/micro!bers in combination with 3D macro!ber deposition. In this design, macro!bers are expected to play a role in the structural integrity and mechani-cal properties of the scaffolds, whereas the nano/micro!bers can act as a cell entrapment system and a cell adhesion promoter. The in"uence of this nano/micro!brous network on the behavior of endothelial cells (EC) seeded onto a scaffold based on a starch and PCL blend was studied by Santos et al. (2008). They have shown that the nano!bers in"uenced cell morphology and the maintenance of the physiological expression pattern. They also showed these EC-seeded scaffolds to be sensitive to pro-in"ammatory stimulus; that is, the cells could migrate and form capillary-like structures within a 3D gel of collagen in response to some angiogenesis-stimulating factors (Santos et al., 2008).
A procedure that has been recently reported is on-site layer-by-layer (LBL) tissue regen-eration, which involves electrospinning one layer of the polymeric solution, then seeding the cells onto this layer and repeating these steps to create a 3D scaffold with the cells already inside. This method is advantageous in that a highly speci!c 3D environment can be created by varying the composition of the !brous layers, !ber thickness, diameter, and orientation as desired, and the neotissue formation can be unambiguously reduced (Agarwal et al., 2009).
Another way to improve cell carrier properties is surface modi!cation of the !bers. A higher degree of spreading and growth of Saos-2 on PHBV and some of its blends has been shown by our research group by treating with oxygen plasma in order to increase hydro-philicity by adding some functional groups on the scaffold surface (Ndreu et al., 2008).
An appropriate biomechanical property is another challenge in bone tissue engineer-ing, since the engineered tissue must be able to retain its shape and withstand the forces applied after implantation. It is well known that a signi!cant part of the bone is constituted of calcium phosphate mineral phase. The presence of bone-inducing inorganic components within bone substitutes enhances calcium phosphate deposition after the osteogenic dif-ferentiation process. The apatite minerals and proteins/peptides, as well as drugs encap-sulated within the nano!bers, are a promising strategy used for nano!brous materials to achieve adequate mechanical properties and bone-speci!c bioactivity (Jang et al., 2009).
HAp plays a crucial role in the biomechanics of bone tissue, especially in increasing the compressive strength. Therefore, an increase in the number of papers reporting the addi-tion of this material to nano!brous scaffolds is being seen. Gelatin-HAp electrospun nano-!brous scaffolds were produced by Kim et al. (2005) to mimic the human bone matrix, and higher cellular activity was reported with these scaffolds, compared to those prepared with only gelatin. Zhang et al. (2008) have reported the use of electrospinning in combina-tion with an in situ coprecipitation synthesis in the preparation of nanocomposite !bers of HAp/chitosan (CHI). Production of continuous nano!bers was possible after addition of a !ber-forming polymer, ultrahigh molecular weight PEO. Continuous HAp/CHI nano!bers
K10254_C010.indd 206 5/12/11 7:18:07 PM
Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering 207
with a diameter of 214 25 nm were produced successfully, and the HAp nanoparticles were incorporated into the electrospun nano!bers. In vitro cell culture with hFOB showed that the incorporation of HAp nanoparticles into CHI nano!brous scaffolds led to signi!-cantly more bone formation than that with electrospun pure CHI scaffolds (Zhang et al., 2008). In another study, COL and HAp were combined in PLLA, forming three different scaffold types: PLLA, PLLA/HA, and PLLA/COL/HAp. In these scaffolds COL and HAp were expected to provide cell recognition sites and introduce osteoconductivity, an impor-tant issue in bone mineralization, respectively. Fiber diameter was found to decrease upon the addition of HAp and COL to PLLA. Human fetal osteoblasts were found to adhere and grow actively on PLLA/COL/HAp nano!bers, with mineral deposition increased by 57% in comparison to PLLA/HAp nano!bers (Prabhakaran et al., 2009).
Some other ingredients introduced to polymers to improve the biomechanical properties and material-cell interaction for bone tissue engineering applications are calcium carbon-ate (CaCO3) and -TCP nanoparticles. Fujihara and his co-workers (2005) used a guided bone regeneration (GBR) approach with the aim of producing a calcium-rich and mechani-cally stable membrane. This was achieved by the preparation of PCL nano!bers contain-ing CaCO3 nanoparticles. A PCL-based membrane was compared with that based on PCL/CaCO3, and a higher cell adherence was observed in the latter. The tensile strength of PCL nano!brous membranes decreased when the amount of calcium carbonate nanoparticles was increased; that is, an increase in the amount of CaCO3 resulted in more brittle mem-branes. However, it was stated that these membranes have suf!cient tensile strength for guided bone regeneration (Fujihara et al., 2005).
Functionally graded, nonwoven mats of PCL with -TCP nanoparticles were produced by using a hybrid, twin-screw extrusion/electrospinning (TSEE) process (Erisken et al., 2008). This procedure allows feeding of various liquid and solid materials, achieving melt-ing, dispersion, pressurization, and electrospinning within a single process, with the com-position changing in a time-dependent manner. The potential of these scaffolds for tissue engineering and bone-cartilage interface formation was demonstrated by using mouse calvaria-derived murine preosteoblast (MC3T3-E1) cells. The ability to control the incorpo-ration of -TCP resulted in a better mimicking of the compositional and structural charac-teristics of bone tissue (Erisken et al., 2008).
10.5.2 Nerve Tissue Engineering
The structure of the nervous system is important for its functionality; therefore, the archi-tecture of the nerve tissue should be well known in order to be able to design the most suit-able nerve substitute for nerve regeneration. In nervous tissue the axons are bundled into cables that provide long-distance communication between the brain and spinal cord and the rest of the body. The nerves in the peripheral nervous system (PNS) are composed of bundled motor and sensory axons in parallel arrays in connective tissue and are called the nerve trunk (Schmidt and Leach, 2003). The individual axons and their Schwann cell sheaths are surrounded by endoneurium, which is composed predominantly of oriented collagen !bers. Groups of axons are wrapped by the perinerium, which is composed of many layers of "attened cells (i.e., !broblasts) and collagen to form fascicles. An outer sheath of loose !brocollagenous tissue, the epineurium, assembles individual nerve fas-cicles into a nerve trunk. The vascularization in these nerves is provided by capillaries within the support tissue of the nerve trunk, or by vessels penetrating into the nerve from surrounding arteries and veins. Within the central nervous system (CNS), the brain and spinal cord, such bundled together axons are called nerve tracts. Typically, the neuron
K10254_C010.indd 207 5/12/11 7:18:07 PM
208 Bionanotechnology II: Global Prospects
consists of dendrites, an axon, and a cell body (Schmidt and Leach, 2003). A gray matter localized in the center of the spinal cord is composed of the cell bodies of excitatory neu-rons, glial cells, and blood vessels. The gray matter is surrounded by white matter, which provides protection and insulation of the spinal cord. White matter consists of axons and glial cells, such as oligodendrocytes, astrocytes, and microglia. Oligodendrocytes myeli-nate the axons in the CNS, while astrocytes are active in the blood-nerve barrier, separat-ing the CNS from blood proteins and cells. Fascicles, axons in bundles, come out from the white matter and then exit the encasing bone of the spinal column. Following travel through the PNS-CNS transition zone, nerves enter the PNS.
Nervous system injuries may result in permanent functional disabilities, and thus have a signi!cant impact on the patient’s quality of life. In severe damage of the peripheral nervous system, self-regeneration of the tissue is dif!cult, and it is almost impossible in the central nervous system (Meek et al., 2002; Schmidt and Leach, 2003). For that reason, the repair of the damaged tissue in the nervous system is complicated. Surgical inter-ventions (end-to-end anastomosis) are used for small nerve gaps (Sanghvi et al., 2004); however, for long gaps (longer than 5 mm) the common treatment is to use autografts. The inherent drawbacks of autografts, such as limited supply, permanent loss of the nerve function at the donor site, and the requirement of a second surgery, are great challenges for researchers and compel them to investigate alternative treatments (Millesi, 1991; Terzis et al., 1997).
The rapidly developing alternative, nerve tissue engineering, aims to restore the function of damaged neural tissue through use of native cells and cell carriers. The cells commonly used in nerve tissue engineering are mature neurons, stem cells, and supportive cells, such as Schwann cells, astrocytes, and oligodendrocytes. The construct developed by nerve tis-sue engineering should imitate the anatomic structure of the nervous system for proper performance. When the native architecture of the nerve tissue is considered, it becomes apparent that a directional cell growth is a prerequisite for functional nerve regeneration. The desired physical properties of a cell carrier are biodegradability, high porosity, and also a cell guiding tool, such as an internal oriented matrix with appropriate chemical structure (ECM proteins, especially laminin, for nerve tissue). Topographical cues for cell migration could serve to mimic the native nerve fascicles (Hudson et al., 1999). The guid-ance needed for the tissue-engineered nerve construct may be achieved by the use of !lms with patterned surfaces, or longitudinally oriented microchannels and !brous scaffolds.
Fibrous scaffolds to be used in nerve regeneration could be produced by electrospinning and self-assembly (Table 10.2). Since they are inherently porous and have a high surface area-to-volume ratio, these !brous scaffolds promote attachment, survival, and growth of cells (Chiu et al., 2005). An architecture similar to ECM makes !brous scaffolds a promis-ing tool for nerve tissue engineering. The ideal scaffold should be biocompatible to avoid in"ammatory response, be biodegradable, and have appropriate mechanical properties to provide the support required for proper healing (Hudson et al., 1999). The material of the cell carrier is a very important determinant in achieving a scaffold suitable for nerve tissue engineering. The degradation rate and mechanical properties of synthetic poly-mers such PLLA, PLGA, PHBV, and PCL can be controlled by chosing the appropriate compound, and as such, they are very suitable for tissue engineering. Natural materi-als such as PHBV, collagen, gelatin, chitosan, and especially laminin have speci!c physi-cal properties and biomolecular recognition sites as in native tissue and, therefore, they have been increasingly investigated as !brous scaffolds for nerve tissue regeneration (Cao et al., 2009). Blends of polymers have also been extensively used in nerve tissue engineer-ing to achieve the desired scaffold properties. For instance, natural polymers lack proper
K10254_C010.indd 208 5/12/11 7:18:07 PM
Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering 209TA
BLE
10.
2N
ano/
Mic
ro!b
rous
Tex
ture
s U
sed
for
Ner
ve T
issu
e En
gine
erin
g
Poly
mer
Typ
e
Poly
mer
C
once
ntra
tion
/C
ompo
siti
on/S
olve
ntFi
ber
Dia
met
erIn
Vit
ro S
tudi
es:
Cel
l Typ
eIn
Viv
o St
udie
s:N
erve
Inj
ury
Mod
elR
efer
ence
Fibe
rs F
abri
cate
d by
El
ectr
ospi
nnin
g
Lam
inin
3–8%
w/v
HFP
90–3
00 n
m (R
F)H
uman
adi
pose
ste
m
cells
and
PC
12 c
ells
—N
eal e
t al.,
200
9
Lam
inin
-PLL
A b
lend
10%
w/v
; 1:2
50, w
:wH
FP10
0–50
0 nm
(RF)
Rat
PC
12 c
ells
—K
oh e
t al.,
200
8
PCL
10%
w/v
CH
L:M
eOH
3:1,
v:v
750
100
nm
(RF)
Brai
n-de
rive
d ne
ural
st
em c
ells
—N
isbe
t et a
l., 2
008
PCL-
pp*
PCL-
PPPC
L-PP
16%
w/w
DC
M:M
eOH
8:2
, v:v
2.26
0
.08
µm (R
F)1.
03
0.0
3 µm
(AF)
Hum
an S
chw
ann
cells
—C
hew
et a
l., 2
008
CH
L:M
eOH
3:1,
v:v
100–
500
nm (R
F)M
ouse
(129
str
ain)
embr
yoni
c st
em c
ells
—Th
ouas
et a
l., 2
008
20%
w/v
DC
M: D
MF
80:2
0, v
:v25
0 nm
(RF/
AF)
Mou
se e
mbr
yoni
c st
em
cells
CE3
and
RW
4—
Xie
et a
l., 2
009b
10–2
0% w
/vD
CM
:DM
F22
0 3
6 nm
(50
3 n
m s
hell
thic
knes
s)C
hick
em
bryo
nic
(E8)
do
rsal
root
gan
glia
ex
plan
t
—X
ie e
t al.,
200
9a
PCL/
GEL
10%
w/v
; 1:1
, w:w
TFE
232
194
nm
(RF)
160
86
nm (A
F)R
at S
chw
ann
cell
line
(RT4
-D6P
2T)
—G
upta
et a
l., 2
009
PCL/
CH
I6%
w/w
; 70:
30, w
:wH
FP18
9 5
6 nm
(RF/
AF)
Neo
nata
l mou
se
cere
bellu
m C
17.2
ste
m
cells
—G
hase
mi-M
obar
akeh
et
al.,
200
8
PCL:
CH
I, 75
:25,
w:w
PCL
in H
FIP,
CH
I in
TFA
/DC
M
190
26
nm (R
F)R
at S
chw
ann
cell
line
RT4-
D6P
2T—
Prab
haka
ran
et a
l.,
2008
* PC
L-PP
: PC
L co
ated
with
pol
ypyr
role
(con
tinue
d )
K10254_C010.indd 209 5/12/11 7:18:07 PM
210 Bionanotechnology II: Global ProspectsTA
BLE
10.
2 (C
ON
TIN
UED
)
Nan
o/M
icro
!bro
us T
extu
res
Use
d fo
r N
erve
Tis
sue
Engi
neer
ing
Poly
mer
Typ
e
Poly
mer
C
once
ntra
tion
/C
ompo
siti
on/S
olve
ntFi
ber
Dia
met
erIn
Vit
ro S
tudi
es:
Cel
l Typ
eIn
Viv
o St
udie
s:N
erve
Inj
ury
Mod
elR
efer
ence
PLLA
PLLA
3 w
/w%
CH
L52
4 3
05 n
m (R
F/A
F)D
orsa
l roo
t gan
glia
ex
plan
t—
Cor
ey e
t al.,
200
7
1% w
/w3%
w/w
2% w
/w5%
w/w
DC
M:D
MF
70:3
0, v
:v
250
nm (R
F)1.
25 µ
m (R
F)30
0 nm
(AF)
1.5
µm (A
F)
Neo
nata
l mou
se
cere
bellu
m C
17.2
ste
m
cells
—Ya
ng e
t al.,
200
5
PLLA
8% w
/wC
HL
1.2–
1.6
#m (A
F)Em
bryo
nic
(E9)
chi
ck
dors
al ro
ot g
angl
ia
expl
ants
and
rat
Sc
hwan
n ce
lls
—W
ang
et a
l., 2
009
PLLA
/ PL
GA
10%
w/v
THF
and
DM
F51
0 3
80 n
m (P
LLA
, RF)
760
300
nm
(PLG
A, R
F)M
ouse
em
bryo
nic
cort
ical
neu
rons
—N
isbe
t et a
l., 2
007
PLG
A, 7
5/25
(coa
ted
with
pol
ypyr
role
)6.
5% w
/w (R
F)7.
0% w
/w (A
F)H
FIP
520
150
nm
(RF)
430
180
nm
(AF)
(85
41
nm s
hell
thic
knes
s)
PC12
cel
ls a
nd r
at
embr
yoni
c hi
ppoc
ampa
l ne
uron
s
—Le
e et
al.,
200
9
Poly
(-c
apro
lact
one-
co-e
thyl
eth
ylen
e ph
osph
ate)
(PC
LEEP
)
12%
w/w
DC
M5.
08
0.0
5 #m
(AF)
—15
mm
ner
ve le
sion
gap
in
rat s
ciat
ic n
erve
(cel
l-fre
e)C
hew
et a
l., 2
007
Blen
d of
PLG
A 7
5:25
/PC
L5.
5% w
/w P
CL:
4% w
/w P
LGA
CH
L:M
eOH
3:1
279
87
nm (R
F)—
10 m
m n
erve
gap
in r
at
scia
tic n
erve
(cel
l-fre
e)Pa
nser
i et a
l., 2
008
Fibe
rs F
abri
cate
d by
Se
lf-A
ssem
bly
RA
D16
-I*
(Arg
-Ala
-A
sp) a
nd R
AD
16-I
I**
Cel
l cul
ture
med
ia10
–20
nmR
at P
C12
cel
ls
Mou
se h
ippo
cam
pal a
nd
cere
bella
r gr
anul
e ne
uron
sR
at h
ippo
cam
pal
neur
ons
Inje
ctio
n of
sel
f-as
sem
bled
(2
0 µl
) in
rat r
ight
leg
(cel
l-fre
e)
Hol
mes
et a
l., 2
000
K10254_C010.indd 210 5/12/11 7:18:07 PM
Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering 211IK
VAV
(I
le–L
ys–V
al–A
la–V
al)
Cel
l cul
ture
med
ia5–
8 nm
Mou
se (E
13) n
eura
l pr
ogen
itor
cells
Spin
al c
ord
inje
ctio
n (T
10)
at a
dep
th o
f 1.5
mm
in
rats
(cel
l fre
e)
Silv
a et
al.,
200
4
RA
DA
16-
I (A
rg–A
la–A
sp–A
la)
Mill
i-Q w
ater
10 n
m—
1.5–
2 m
m d
eep
cut w
ound
in
the
optic
trac
t, su
peri
or
colli
culu
s in
ham
ster
bra
in
(cel
l fre
e)
Ellis
-Beh
nke
et a
l.,
2006
RA
DA
16-I
pep
tide
Cel
l cul
ture
med
ia10
nm
Rat
neu
ral p
roge
nito
r ce
lls a
nd S
chw
ann
cells
1 m
m ti
ssue
rem
oval
by
a tr
anse
ctio
n in
spi
nal c
ord
dors
al c
olum
n (b
etw
een
C6
and
C7)
in r
ats
Guo
et a
l., 2
007
Abb
revi
atio
ns: R
F, r
ando
m !
ber;
AF,
alig
ned
!ber
; HFP
, 1,1
,1,3
,3,3
-hex
a"uo
ro-2
-pro
pano
l; PC
L, p
oly(
-cap
rola
cton
e); C
HL,
chl
orof
orm
; MeO
H, m
etha
nol;
DC
M,
dich
loro
met
hane
; DM
F, d
imet
hylf
orm
amid
e; G
EL, g
elat
in; T
FE, 2
,2,2
-tri
"uor
oeth
anol
; CH
I, ch
itos
an; H
FIP,
hex
a"uo
ro-2
-pro
pano
l; TH
F, te
trah
ydro
fura
n;
TFA
, tri
"uor
oace
tic
acid
; PLL
A, p
oly(
L-la
ctic
aci
d); P
LGA
, pol
y(la
ctic
aci
d-co
-gly
colic
aci
d).
K10254_C010.indd 211 5/12/11 7:18:08 PM
212 Bionanotechnology II: Global Prospects
mechanical properties and also lose mechanical strength very quickly due to degradation, while synthetic polymers alone are generally too hydrophobic and lack binding sites for cell adhesion (Gupta et al., 2009). Consequently, a blend or a graft of natural and synthetic polymers is an effective way to obtain scaffolds with desirable properties. A summary of nano/micro!brous scaffolds used in vitro and in vivo in nerve regeneration studies is given in Table 10.2.
Electrospinning is a straightforward technique to obtain random and aligned nano/micro!bers. Even though the randomly oriented !bers are easier to obtain and are more frequently used in nerve tissue engineering, the use of the aligned !bers is a challenging but rewarding approach in mimicking the unidirectional orientation to maintain func-tion of the nerve tissue (Table 10.2). Fibers aligned longitudinally (parallel to nerve axis) may provide a suitable environment that encourages contact guidance and the directional neuronal/axonal growth desired in nerve tissue engineering. Yang et al. (2005) observed the positive effect of contact guidance by aligned !brous scaffolds (diameters of aligned micro- and nano!bers: 1.5 µm and 300 nm, respectively) on neural stem cells that had been induced to elongate neurites parallel to the !ber axis. The cell elongation and the neurite outgrowth were random on the randomly oriented mats (diameters of random micro- and nano!bers: 1.25 µm and 250 nm, respectively), proving the signi!cance of guidance. This guidance was observed on aligned nano/micro!bers regardless of !ber diameter. The effect of !ber arrangement was also observed in our studies carried out with neural stem cells seeded on !brous mats. Electrospun PHBV-PLGA !bers were produced under opti-mized conditions (PHBV-PLGA solution: 1:1 w/w, 15% w/v, in chloroform: dimethylforma-mide (CHL:DMF)) with a diameter of ca. 1.5 µm (Yucel et al., 2010). The stem cells seeded on the aligned !bers were well oriented along the axis of the !bers, while the cells on the random !bers appeared to be in clusters distributed in all directions (Figure 10.2). Similar results were also obtained with dorsal root ganglion (DRG) explants cultured on PLLA nano!bers (Patel et al., 2007). Signi!cant extension of neurites was observed on aligned !bers, while there was no neurite outgrowth on randomly oriented !bers. Like other cells, human Schwann cells responded to the !ber orientation with elongation along the !bers
A B
20 µmCentral Laboratory
20 µmM
etu Central Laboratory
FIGURE 10.2CLSM images of neural stem cells on !bronectin-coated PHBV-PLGA scaffolds. (A) Randomly oriented and (B) aligned !bers (100 ; scale bar, 250 µm; two-sided arrow, direction of !ber axis; Acridine orange stain). Insets show the SEM images of the !brous mats.
K10254_C010.indd 212 5/12/11 7:18:08 PM
Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering 213
(Chew et al., 2008). In addition, aligned electrospun !bers appeared to enhance Schwann cell maturation more than the randomly oriented !bers did, as was shown by the upregu-lation of the myelin-speci!c gene. In vitro results were con!rmed by the in vivo studies in which anatomical and functional measurements showed that constructs with aligned polymer !bers signi!cantly enhanced nerve regeneration in 17 mm peripheral nerve gaps in rats over those of randomly oriented !bers (Kim et al., 2008).
Micro- and nanoscale topography was shown to play an important role in neural stem cell differentiation. Yang et al. (2005) observed that the mouse neural stem cell differentia-tion rate might depend on the !ber dimension. The differentiation rate on nano!bers 300 nm in diameter was twice as high as on micro!bers with a 1.5 µm diameter. However, these results on the effect of !ber diameter on neural cell differentiation are controversial. Christopherson and his colleagues (2009) found that under differentiation conditions, rat neural stem cells stretched multidirectionally and preferentially differentiated into oligo-dendrocytes on !bers with a diameter of 283 nm, but they elongated along a single !ber axis and preferentially differentiated into neuronal lineage on !bers 749 nm and 1,452 nm thick. Despite these con"icting results, topographical and biological cues seem to be important in the control of stem cell behavior.
Electrospinning approaches involving blending, coaxial extrusion, and emulsion spin-ning are used in the incorporation of bioactive agents like proteins and growth factors to enhance cell attachment, growth, differentiation, and survival of cells in nerve tissue engineering applications. Incorporation of laminin, a neurite-promoting basement mem-brane protein, to a synthetic polymer by blending was reported as a more ef!cient method to produce biomimetic scaffolds than covalent immobilization and physical adsorption of the bioactive agents (Koh et al., 2008). In a recent study, nerve growth factor (NGF), a common neurotrophic factor involved in peripheral nerve regeneration, was incorporated in poly(L-lactide-co- -caprolactone) !bers by emulsion electrospinning (Li et al., 2010). Pheochromocytoma 12 (PC12) cells, which differentiate into a neuronal phenotype in the presence of NGF, were used in a study where NGF was released from !brous mats for a sustained period, revealing its bioactivity. A glial cell-derived neurotrophic factor (GDNF), important in motor neuron survival, was entrapped in aligned biodegradable polymeric !bers (a copolymer of -caprolactone and ethyl ethylene phosphate (PCLEEP)) (Chew et al., 2007). An in vitro release pro!le of GDNF showed that after an initial burst release of about 30%, the remaining GDNF was released in a fairly sustained manner for almost 2 months.
Electrical stimulation is another tool that affects neurite extension. Electrical stimula-tion using an external electrical !eld and an electroconductive polypyrrole (PPy) !lm was shown to enhance neurite extension in vitro (Schmidt et al., 1997). Xie et al. (2009a) cul-tured DRG explants on conductive (PPy) core-sheath nano!bers electrospun with PCL, and results showed that upon electrical stimulation, the rate of neurite extension was enhanced on both random and aligned nano!bers without any change in the direction of the neurites’ extension. In another study, PPy was deposited on electrospun PLGA !bers to investigate the response of PC12 cells (Lee et al., 2009). Upon application of 10 mV/cm on PPy-PLGA scaffolds, PC12 cells exhibited 40–50% longer neurites and 40–90% more neurite formation than unstimulated cells on the same scaffolds.
In conclusion, the topography of nano/micro!brous scaffolds closely resembles that of the natural ECM, and this is an important factor in tissue engineering. Considering the architecture of longitudinally oriented nerve tissue, uniaxially aligned !bers could create an appropriate environment for guided nerve regeneration. The use of the aforementioned tools with aligned !brous scaffolds could be a promising approach in the design of a bio-mimetic substitute for nerve regeneration.
K10254_C010.indd 213 5/12/11 7:18:08 PM
214 Bionanotechnology II: Global Prospects
10.5.3 Cardiac Muscle Tissue Engineering
The heart is the center of the circulation system and distributes blood throughout the body. The function of the heart is vital to supply oxygen and nutrients to, and remove waste products from, the body via the blood in order to maintain the balance that is nec-essary to sustain life (Vander et al., 1994). Strong muscular contractions in the ventricles pump blood out of the heart and into the circulatory system. These muscular contrac-tions are produced by the muscle tissue that makes up the walls of the ventricles. Healthy heart muscle wall is composed of three layers: a muscular sheet, the myocardium, lined on either side by two collagenous membranes (containing type I and type III collagen and elastin); the endocardium, which is populated by endothelial cells; and the epicardium, also called visceral pericardium (van de Graaff, 1998). The myocardium is the layer of functional beating muscle that consists of !broblasts and highly oriented cardiomyocytes (muscle !bers) in a matrix of collagen. The cardiomyocytes are connected end to end in the longitudinal direction and side by side in the transverse direction (Parker and Ingber, 2007). Collagen types I and III are the predominant interstitial collagens in the myocar-dium that generate structural integrity for the adjoining cardiomyocytes, providing the means by which myocyte shortening is translated into overall ventricular pump function. Basement membrane components include laminin, entactin, !bronectin, collagen type IV, and !brillin, and proteoglycans include chondroitin sulfate, dermatan sulfate, and hepa-ran sulfate (Kassiri and Khokha, 2005). The cardiomyocytes facilitate conduction of the electrical signals needed to initiate contractile movement in order to pump blood out of the ventricles (Vander et al., 1994). The elementary myocardial functional units are not lone cardiomyocytes, but rather are multicellular assemblies of these highly oriented cells. The cardiomyocytes are connected with intercalated discs that integrate individual electrical activation and contraction into a pumping action. Gap junctions in these intercalated discs allow the action potential to travel through the membranes of the myocytes, thus facilitat-ing signal propagation and a synchronized contractile pulse (Vander et al., 1994).
Heart failure, stemming from cardiovascular diseases, is the number one cause of death in industrialized countries. About 5 million people in the United States have heart failure, and the number is growing; heart failure contributes to or causes about 300,000 deaths each year. The inability of the heart to deliver suf!cient blood to meet the body’s meta-bolic requirements leads to heart failure. The most common causes are coronary artery disease and hypertension (high blood pressure), which damage the myocardium. Damage to any part of the intricate structure of the heart, though, can impair cardiac performance and result in heart failure; examples include diseases of the heart valves, the electrical conduction system, or external pressure around the heart, due to constriction of the peri-cardial sac in which the heart is located (Jawad et al., 2007). Congenital heart disease is also a considerable problem worldwide, affecting approximately 1% of infants (Wu et al., 2006). Congenital heart defects such as atrial septal defect, ventricular septal defect, double outlet ventricles, and hypoplastic left heart syndrome are associated with aplastic, defec-tive, or necrotic myocardial structures, where patch closure, correction of the defect, or revascularization is usually required (Ko!dis et al., 2002). Although the currently available prostheses are adequate for restoring ventricular geometry and maintaining ventricular pressure, and thus may be life saving, they do not actively adapt to the physiological envi-ronment and mechanical demands, as they are nonliving materials. When implanted into the immature heart of a child, these materials do not grow with the pediatric patient. Therefore, these prostheses have limited durability and are prone to infection, immuno-logic reactivity, and thrombosis, which often requires repeated operations (Mirensky and
K10254_C010.indd 214 5/12/11 7:18:08 PM
Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering 215
Breuer, 2008). These constructs are also subject to obstructive tissue ingrowths and !brotic responses with shrinkage and calci!cation leading to graft failure (Endo et al., 2001). At that point, tissue engineering provides a new experimental approach aimed at vital, semi-autologous replacement structures with the capacity of growth and adaptability.
The negligible ability of adult cardiomyocytes to proliferate has triggered an intense search for progenitor cells that can be used in repairing damaged myocardium. It is clear that natural cardiomyocyte regeneration, including differentiation of progenitors residing in the myocardium or the recruitment of stem cells from outside (e.g., from endothelial cells or from the bone marrow), is insuf!cient to overcome cardiomyocyte death in the acutely or chronically damaged heart (Nakamura and Schneider, 2003). The potential of cardiac stem cells resident in the heart, embryonic stem cells, bone marrow stem cells, and fetal stem cells (from tissues of umbilical cord) to differentiate into cardiomyogenic lineage either in vivo or in vitro in the presence of differentiation factors, conditioned media, or cocultures has been investigated, but there is still a need for elaboration and improvement in the procedures to be able to obtain high numbers of fully differentiated cardiomyocytes. Since the main concern in myocardial tissue engineering is the functionality of the !nal construct, the majority of in vitro studies use rat neonatal cardiomyocytes or cardiomyo-cytes derived from embryonic stem cells that are already able to contract.
Fibrous scaffolds whose !ber diameter ranges from several microns to submicron sizes (Table 10.3) are increasingly gaining importance over spongy scaffolds with random pores used to obtain tissue-engineered cardiac muscle. The main obstacle in overcoming the issues involved in the engineering of cardiovascular grafts is production of substitutes that can withstand the pulsation (i.e., have adequate shape recovery), the high pressure, and the "ow rate of the blood (Nisbet et al., 2009). An additional challenge in cardiovascu-lar tissue engineering is the necessity for mechanical and biological compliance. Polymers naturally found in the cardiac tissue, like collagen and elastin, are good choices as scaf-fold materials, since they carry attachment sites for cells, and especially elastin has elastic properties, but they have poor mechanical properties. On the other hand, synthetic poly-mers have higher mechanical strength, but have problems, such as being too hydrophobic and rigid, lacking cell binding functional groups, or having inaproppriate degradation rates (rapidly degrading or nondegrading). Thus, as mentioned earlier, blends or copoly-mers of synthetic or natural polymers are employed often in the production of scaffolds with the desired properties (Zong et al., 2005, Li et al., 2006a, 2006b). Currently cell survival in 3D cardiac grafts of spongy design is a critical issue; generally the cells are concentrated in the outer regions of the grafts and are scarce in the inside due to restricted transporta-tion of nutrients and waste in and out of the scaffold, resulting in an undesired core effect (Bursac et al., 1999). Electrospinning enables us to fabricate highly porous scaffolds with extensively interconnected pores that allow cell-to-cell contact and migration in all direc-tions, facilitate the transportation of nutrients and metabolites, and encourage blood vessel formation. All of these combined help cell survival throughout the scaffold, including the central portion. Electrospun mats of PCL with random, unaligned !bers were seeded with cardiomyocytes by Ishii et al. (2005), and the mats were stacked to generate a 3D cardiac graft. Five layers of these mats were successfully stacked without any indication of core ischemia. The individual layers adhered intimately, morphological and electrical commu-nication between the layers was established, and synchronized beating was also observed. This study demonstrated the feasibility of obtaining a functional 3D cardiac graft from cell sheets supported by nano!bers mimicking the native structure of cardiac ECM. Fiber density on the electrospun mats could be tailored depending on the intended use. For example, electrospun mats with tight !ber spacing may act as a sieve to prevent blood cell
K10254_C010.indd 215 5/12/11 7:18:08 PM
216 Bionanotechnology II: Global ProspectsTA
BLE
10.
3
Nan
o/M
icro
!bro
us T
extu
res
Use
d in
Car
diac
Mus
cle
Tiss
ue E
ngin
eeri
ng
Poly
mer
Typ
ePo
lym
er C
once
ntra
tion
/C
ompo
siti
on/S
olve
ntFi
ber
Dia
met
erIn
Vit
ro S
tudi
es: C
ell T
ype
Ref
eren
ce
Extr
usio
nPG
A—
13 #
mN
eona
tal r
at c
ardi
omyo
cyte
sBu
rsac
et a
l., 1
999
Papa
daki
et a
l., 2
001
Col
lage
n ty
pe I
25 m
g/m
lD
I wat
er-H
Cl
(pH
2.5
–3.0
)
~ 25
0 nm
(AF)
Rat
em
bryo
nic
card
iac
myo
cyte
sEv
ans
et a
l., 2
003
Yost
et a
l., 2
004
Elec
tros
pinn
ing
PCL
10%
(w/w
)C
HL:
MeO
H (1
:1)
100
nm–5
#m
(RF)
Neo
nata
l rat
car
diom
yocy
tes
Shin
et a
l., 2
004
Ishi
i et a
l., 2
005
PLLA
Blen
d of
PG
LA (9
0:10
)/PL
LABl
end
of P
LGA
(75:
25)/
PEG
-PD
LLA
30–5
0% (w
/w)
DM
F0.
9–1
#m (R
F/A
F)N
eona
tal r
at c
ardi
omyo
cyte
sZ
ong
et a
l., 2
005
Blen
d of
PLG
A/g
elat
in/
-ela
stin
10%
PLG
A:8
% g
elat
in: 2
0%
elas
tin (w
/v) H
FP38
0 8
0 nm
(RF)
H9c
2 ra
t car
diac
myo
blas
tsN
eona
tal r
at b
one
mar
row
str
omal
cel
lsLi
et a
l., 2
006a
Blen
ds o
f PA
Ni/
gela
tin3%
(w/v
) CPS
A-P
AN
i8%
(w/v
) gel
atin
H
FP
From
803
1
21 n
m to
61
1
3nm
(RF)
H9c
2 ra
t car
diac
myo
blas
tsLi
et a
l., 2
006b
Biod
egra
dabl
e po
lyur
etha
ne15
% (w
/v)
DC
M2–
10 #
m (R
F)M
ouse
ESC
-der
ived
car
diom
yocy
tes
C2C
12 s
kele
tal m
yobl
ast l
ine
HL-
1 ca
rdio
myo
cyte
cel
l lin
e
From
stei
n et
al.,
200
8
Biod
egra
dabl
e po
lyur
etha
ne18
% (w
/v)
DC
M1.
8 1
.26
#m (A
F)2.
0 1
.92
#m (R
F)N
eona
tal r
at c
ardi
omyo
cyte
sR
ockw
ood
et a
l., 2
008
Blen
d of
PH
BV/P
(L-D
,L)L
A/P
GS
5% (w
/v)
PHBV
:P(L
-D,L
)LA
:PG
S 49
:49:
2 (w
/w)
CH
L:D
MF
(10:
1)
1.10
–1.2
5 #m
(AF)
Hum
an W
hart
on’s
jelly
MSC
sK
enar
et a
l., 2
009
Abb
revi
atio
ns: P
GA
, pol
y(gl
ycol
ic a
cid)
; PC
L, p
oly(
-cap
rola
cton
e); P
LLA
, pol
y(L-
lact
ide)
; PG
LA, p
oly(
glyc
olid
e-co
-lac
tide
); PL
GA
, pol
y(la
ctid
e-co
-gly
colid
e);
PEG
, pol
y(et
hyle
ne g
lyco
l); P
DLL
A, p
oly(
D,L
-lac
tide
); PH
BV, p
oly(
3-hy
drox
ybut
yrat
e-co
-3-h
ydro
xyva
lera
te);
P(L-
D,L
)LA
, pol
y(L-
D,L
-lac
tic
acid
); PG
S,
poly
(gly
cero
l se
baca
te);
CH
L, c
hlor
ofor
m;
MeO
H,
met
hano
l; D
MF,
N,N
-dim
ethy
l fo
rmam
ide;
HFP
, 1,
1,1,
3,3,
3-he
xa"u
oro-
2-pr
opan
ol;
DC
M,
dich
lo-
rom
etha
ne; P
AN
i, po
lyan
iline
(con
duct
ive
poly
mer
).
K10254_C010.indd 216 5/12/11 7:18:08 PM
Nanotechnology in Biomaterials: Nanofibers in Tissue Engineering 217
leakage or provide a homogeneous surface to obtain intact endothelial cell sheets, or act as a supporting outer surface in cardiovascular scaffolds (Hong et al., 2009; Soletti et al., 2010). Conversely, !ber spacing and density may also be adjusted and loosened to promote cell in!ltration in the myocardial patches.
Anisotropy in the architecture of the constructs plays an important role in achieving compliance and functionality in cardiovascular tissue engineering. Parallel submicron !bers may be produced to have circumferential orientations similar to those of the cells and !brils of the medial layer of the native artery (Xu et al., 2004). It was demonstrated that anisotropic poly(ester urethane)urea scaffolds exhibit mechanical properties closely resem-bling those of the native pulmonary heart valve lea"ets (Courtney et al., 2006). Cardiac muscle contracts like a syncytium owing to the multicellular assembly of myo!bers ori-ented parallel to each other, connected end to end via intercalated discs in the longitudinal direction and connected side to side in the transverse direction. It is this cellular arrange-ment that integrates individual contraction into a synchronous pumping action, whereas myo!ber disarrays are known to cause arrhythmia. It was shown by Black et al. (2009) that cardiomyocyte alignment augments the twitch force. Prior to emergence of techniques to produce aligned nano/micro!brous scaffolds, anisotropy had been introduced to the car-diomyocytes on the polymer scaffolds in three different ways: (1) by micropatterning (e.g., using biodegradable, elastomeric polyurethane !lms patterned by microcontact printing of laminin (McDevitt et al., 2003)), (2) by cyclic stretching (e.g., stretching of cardiomyo-cyte-seeded scaffolds (Akhyari et al., 2002; Zimmermann et al., 2002, 2004; Zimmermann and Eschenhagen, 2003; Fedak et al., 2003)), and (3) by applying electrical signals designed to mimic those in the native heart to the tissue engineered constructs (e.g., rat cardiomyo-ctes resuspended in Matrigel were seeded onto collagen sponges (Radisic et al., 2004)). Electrospinning, on the other hand, offers a much simpler way to achieve the anisotropy and allows greater control over composition, mechanical properties, and structure of a graft, thus making it easier to match the properties of the scaffold and the native tissue. It was shown by Zong et al. (2005) that cardiomyocytes can align parallel to micron-size !bers. Their electrospun mats with oriented !bers of PLLA and PLGA were used to assess the in"uence of scaffolds on primary cardiomyocyte attachment, structure, and function. The primary cardiomyocytes showed a preference for relatively hydrophobic surfaces (PLLA), where they aligned along the direction of !bers and developed mature contractile machinery (sarcomers). The only drawback of these aligned !ber scaffolds was the inabil-ity of the cardiomyocytes to penetrate into the scaffold due to the inadequate space among the !bers. A very similar phenomenon was reported by Evans et al. (2003), who observed that rat embryonic cardiac myocytes were unable to penetrate the 3D aligned collagen !ber scaffold with a pore size of 1–10 microns on the outer surface of a tubular scaffold. Aligned !ber scaffolds with a !ber density suitable for cell penetration were produced by our group by using a frame collector to obtain aligned !bers (Kenar et al., 2010). At least eight or nine Wharton’s jelly (WJ) MSC layers could be obtained in a single mat of average thickness of 12 3 #m. Both cytoskeletal and nuclear alignments of the cells were observed (Figure 10.3). A wrapping method was used to obtain a thicker homogeneous construct, which maintained its original anisotropic property. Another technique, microintegra-tion, was developed by Stankus et al. (2006), where simultaneous electrospraying of cells and electrospinning of poly(ester urethane)urea (PEUU) was utilized in order to fabricate thicker constructs with more uniform cell incorporation for use in cardiovascular tissue engineering applications. Smooth muscle cell-microintegrated PEUU was strong, "exible, and anisotropic, and Trypan blue staining revealed no signi!cant decrease in cell viability as a result of the fabrication process.
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Studies carried out with aligned !ber mats with !ber diameters ranging from 0.25 to 3.00 #m show that cells grown on them align equally well along the direction of the !bers (Evans et al., 2003; Yost et al., 2004; Zong et al., 2005; Rockwood et al., 2008; Kenar et al., 2010). The single study carried out on cardiomyocyte function on electrospun random and aligned polymer !bers showed that cellular alignment enhances cardiomyocyte matura-tion; cells grown on aligned electrospun polyurethanes had signi!cantly lower steady-state levels of atrial natriuretic peptide (ANP) and, as a result, released less ANP over time than cardiomyocytes on random !bers (Rockwood et al., 2008).
In conclusion, nano!bers are novel scaffold materials for tissue engineering, and the accumulating data show them to be superior to most other designs due to the versatility of the approach. They are especially suited to the highly organized nature of biological tissues. With developments in the processing systems, they will become the major scaffold type.
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FIGURE 10.3(See color insert.) Cross-sectional (X-axis ~ 60 tilted) CLSM images of FITC-Phalloidin and propidium iodide-stained WJ MSCs on a single, electrospun, aligned !ber mat of PHBV:P(L-D,L)LA:PGS (poly(glycerol sebacate)) (100 ) showing (A) F-actin and (B) nuclear alignment (two-sided arrow, direction of aligned !ber axis). Top images are the magni!ed versions of insets.
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