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. Author manuscript; available in PMC: 2010 Mar 1.
Published in final edited form as: Wiley Interdiscip Rev Nanomed Nanobiotechnol. 2009 Mar;1(2):226–236. doi: 10.1002/wnan.26

Nano-structured polymer scaffolds for tissue engineering and regenerative medicine

IO Smith, XH Liu, LA Smith, PX Ma
PMCID: PMC2800311  NIHMSID: NIHMS129535  PMID: 20049793

Abstract

The structural features of tissue engineering scaffolds affect cell response and must be engineered to support cell adhesion, proliferation and differentiation. The scaffold acts as an interim synthetic extracellular matrix (ECM) that cells interact prior to forming new tissue. In this review, bone tissue engineering is used as the primary example because of our group’s focus and for the sake of brevity. We focus on nano-fibrous scaffolds and the incorporation of other components including other nanofeatures into the scaffold structure. Since the ECM is comprised in large part of collagen fibers, between 50–500 nm in diameter, well-designed nano-fibrous scaffolds mimic this structure. Our group has developed a novel thermally-induced phase separation (TIPS) process in which a solution of biodegradable polymer is cast into a porous scaffold, resulting in a nano-fibrous pore-wall structure. These nano-scale fibers have a diameter (50–500 nm) comparable to those collagen fibers found in the ECM. This process can then be combined with a porogen leaching technique, also developed by our group, to engineer an interconnected pore structure that promotes cell migration and tissue ingrowth in three dimensions. To improve upon efforts to incorporate a ceramic component into polymer scaffolds by mixing, our group has also developed a technique where apatite crystals are grown onto biodegradable polymer scaffolds by soaking them in simulated body fluid (SBF). By changing the polymer used, the concentration of ions in the SBF and by varying the treatment time, the size and distribution of these crystals is varied. Work is currently being done to improve the distribution of these crystals throughout three-dimensional scaffolds and to create nano-scale apatite deposits that better mimic those found in the ECM. In both nano-fibrous and composite scaffolds, cell adhesion, proliferation and differentiation improved when compared to control scaffolds. Additionally, composite scaffolds showed a decrease in incidence of apoptosis when compared to polymer control in bone tissue engineering. Nanoparticles have been integrated into the nano-structured scaffolds to deliver biologically active molecules such as growth and differentiation factors to regulate cell behavior for optimal tissue regeneration.

Keywords: Biomimetic Materials, Cells, Nanofibers, Nanoparticles, Tissue Scaffolding

The need for readily available replacement tissue is greater than ever. As of 2004, over 1.5 million tissue graft procedures are performed each year in the United States alone [1], of which it is estimated that 500,000 are bone tissue grafts [2]. The current gold standard for bone tissue grafts is the autograft, which involves some inherent issues. The retrieval of autograft tissue requires surgery at a second site, presenting the possibility of complications, including infection and donor site morbidity. Allograft tissue is another option, but has its own inherent risk for complications, including its limited supply, as well as the possibility of disease transmission and tissue rejection. The growth of replacement tissue using tissue engineering and regenerative medicine is one method of using technology to create an effective replacement for these types of graft tissue.

Tissue engineering is a field in which our knowledge in the life sciences and medical fields is applied using engineering principles, in order to design therapeutic treatments that positively affect tissue function [3]. While tissue engineering encompasses three approaches: Cells and cell substitutes, factors for tissue induction and the seeding of cells onto matrices, the most common tissue engineering approach is the third one, to place cells onto a biomaterial matrix [3]. This combination is designed to grow tissue in vitro, prior to implantation within the subject.

The design of the scaffold prior to exposure to cells is of vital importance. The scaffold must present a surface that promotes cell attachment, growth and differentiation, while providing a porous network for tissue growth [4,5]. The material chosen is of great importance when designing a scaffold. It must degrade at a rate matching that of the new tissue formation, must be biocompatible and the products of its degradation must be biocompatible. Once implanted, the scaffold must have the mechanical properties necessary to temporarily offer structural support until the new tissue has formed. In addition to consisting of an effective biomaterial, the scaffold must also possess key morphological characteristics. It must be highly porous and offer a suitable path for nutrient transmission and tissue ingrowth. To achieve these requirements, tissue engineering scaffolds are often designed to mimic the structure of the naturally occurring extracellular matrix (ECM) [6,7].

During bone tissue formation, collagen fibers between 50 – 500 nm in diameter are deposited as part of the ECM [8]. These fibers act as a construct, onto which apatite crystals are deposited, forming the earliest phase of bone. This fibrous collagen may play an important role in regulating cell attachment, proliferation and differentiation and trends in scaffold design have aimed to better mimic this structure [6,7].

This review is focused on the forefront of nano-scale aspects of scaffold design, including the design of novel techniques for fabricating nano-fibrous polymer scaffolds, nano-scale polymer/ceramic composite scaffolds and nano-particle integrated factor delivery scaffolds, as well as how these features affect cell response. While these techniques are applicable to many types of tissue, this review focuses on bone tissue engineering.

Nano-structured polymer scaffold fabrication

Methods of nanofiber fabrication

Three different methods have been employed in the fabrication of nano-fibrous scaffolds for tissue engineering: electrospinning, self-assembly, and phase separation. The electrospinning process was first patented in the 1900s [9]. The principle of electrospinning is to use an electric field to draw a polymer solution from an orifice to a collector, producing polymer fibers with diameters in the range of nanometers to micrometers [9,10]. Due to the simplicity of this method, electrospinning has received considerable attention for use in tissue engineering. A variety of synthetic and natural biomaterials, including poly(lactic-co-glycolic acid) (PLGA), poly(L-lactic acid) (PLLA), poly(caprolactone) (PCL), poly(ethylene oxide) (PEO), poly(vinyl alcohol) (PVA), gelatin, collagen, silk protein and fibrinogen have been used to form nano-fibrous scaffolds for tissue engineering [11–21]. In general, the diameters of electrospun fibers are at least one or two orders of magnitude smaller than those fabricated by conventional extrusion methods. However, electrospinning typically is used to produce thin two-dimensional (2D) sheets. While three-dimensional (3D) nanofibrous scaffolds have been fabricated by layering these 2D sheets [22], it is inherently difficult to create 3D scaffolds with a well-defined pore architecture and complex geometries, including interconnected macropore networks. Initial attempts at combining electrospinning with 3D printing have yielded some success [23], but further assessment of this process is needed.

Molecular self-assembly is a useful approach for fabricating supramolecular architectures [24]. Molecular self-assembly is mediated by non-covalent bonds such as hydrogen bonds, van der Waals interactions, electrostatic interactions, and hydrophobic interactions. Self-assembled molecules are ubiquitous in nature. Biomolecules, such as peptides and proteins, interact and self-organize to form well-defined architectures that are associated with functionality. Under certain conditions, nano-structured fibers can be formed by molecular self-assembly [25,26]. The fiber diameter created by molecular self-assembly usually is much smaller than those produced using electrospinning [27]. While molecular self-assembly is a fairly new technique for the formation of nano-scale scaffolds, it has limited ability to form macropores, which are important for cell accommodation and mass transport. The mechanical properties of self-assembled scaffolds also have to be improved before they can be used in tissue engineering applications.

A novel thermally induced phase separation (TIPS) technique was developed recently to fabricate nano-fibers to mimic natural collagen fibers [6]. The TIPS process for nanofiber formation typically includes five steps: polymer dissolution, phase separation and gelation, solvent extraction, freezing, and freeze-drying under vacuum. The fiber network formation depends on the solvent of the polymer solution and the gelation temperature. The fibers formed in this manner have diameters ranging from 50–500 nm, and have a porosity in excess of 98% (Figure 1). This nano-fibrous matrix has a much higher surface-to-volume ratio than those of fibrous non-woven fabrics fabricated with the textile technology or foam fabricated with other techniques [6,28]. A distinct advantage of the TIPS technique is that it can be combined with other processing techniques (such as particulate leaching or 3D printing) to design complex 3D structures with well-defined pore morphologies [28–30]. In the next section, we present a few examples to illustrate how the TIPS technique is combined with other technologies to create 3D nano-fibrous scaffolds with predesigned macropores.

Figure 1.

Figure 1

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SEM micrographs of a PLLA fibrous matrix prepared from 2.5% (wt/v) PLLA/THF solution at a gelation temperature of 8°C: (a) ×50; (b) ×20K. (From Ma and Zhang [6], copyright 1999 by John Wiley & Sons, Inc. Reprinted with permission.)

Three dimensional nano-fibrous scaffolds with predesigned macropores

To engineer functional tissues and organs successfully, the scaffolds have to be designed to facilitate cell distribution and guide tissue regeneration in three dimensions. Macroscopic pores (>100 μm) in a scaffold play an important role in cell seeding distribution, cell migration throughout the 3D space, and neo-vascularization after implantation of the scaffold in vivo [5].

To create a nano-fibrous matrix with a macroscopic pore network, a combination of TIPS and particulate leaching can be used [7]. In one example, PLLA-THF polymer solution was dripped slowly onto sugar particles in a mold and then cooled to a preset gelation temperature. After phase separation, the gel-sugar composite was immersed in distilled water to simultaneously extract the solvent and leach the sugar from the composite. The sample was freeze-dried, resulting in a 3D nano-fibrous matrix with macropores left from the leached sugar particles [7].

In addition to pore size, pores interconnectivity in a scaffold is also important for cell migration, cell signaling, and mass transport. While particle leaching techniques can create well formed macropores, it is difficult to control the degree of interconnectivity. To better control the interconnectivity of the macropores in a scaffold, our group developed several novel molding techniques [4,28,31]. By combining these techniques with TIPS technique, 3D nano-fibrous scaffolds with well-controlled interconnectivity are created (Figure 2).

Figure 2.

Figure 2

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SEM images of NF-PLLA scaffolds. (a) x100, (b) x2000. (From Liu et.al [38], copyright 2005 by American Scientific Publishers. Reprinted with permission.)

Surface modification of nano-fibrous scaffolds

In tissue engineering, it is important to consider the interactions of cells with the scaffold materials. The nature of the scaffold surface can directly affect cellular response, ultimately influencing the rate and quality of new tissue formation [32,33]. Although a variety of synthetic biodegradable polymers have been used as tissue engineering scaffold materials, one disadvantage of these scaffolds is a lack of biological recognition on their surface. Therefore, the scaffold surface has to be modified to obtain more desirable characteristics to promote cell-scaffold interactions [5].

Several approaches have been developed to modify the scaffold surface [34–37]. However, most of this work has been focusing on 2D film surfaces or very thin 3D constructs. True 3D scaffolding surface modification is still a challenge. In our lab, several effective techniques have been developed for 3D surface modification of complex shaped scaffolds [38–40]. For example, an electrostatic layer-by-layer self-assembly technique has been used to modify nano-fibrous PLLA (NF-PLLA) scaffolds with gelatin [38]. One advantage of this technique is that the layer-by-layer self-assembly method can be used for any complex 3D geometry as long as the pores are interconnected. Cell proliferation was more effectively facilitated on the surface-modified NF-PLLA scaffolds than on the control scaffolds. Histological slides showed that cells were distributed evenly throughout the scaffolds, and cell density on the surface-modified NF-PLLA scaffold was higher than that on the NF-PLLA control scaffold.

Nanospheres

Nanoparticles are defined as being on the scale of less-than 100 nm in diameter. They possess a high surface to volume ratio, which makes them an effective vehicle for the release of biological factors such as growth factors, drugs and genes. By incorporating PLGA nanospheres, a type of nanoparticle, into nano-fibrous PLLA scaffolds, recombinant human bone morphogenetic protein (rhBMP-7) was delivered to the region of cell culture, and ectopic bone formation was positively affected [41]. An example of this configuration is shown in Figure 3.

Figure 3.

Figure 3

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SEM micrographs of PLLA nano-fibrous scaffolds with incorporated PLGA50–64K nanospheres. (From Wei and Ma [41], copyright 2007 by Elsevier. Reprinted with permission.)

Polymer/Ceramic Nano-composite Scaffolds

Ceramic materials have long been studied as potential candidates for bone tissue engineering applications [42]. In particular, calcium phosphate (CaP) ceramics, including hydroxyapatite (HA), have drawn interest due to their similarity to bone mineral, as well as their inherent biocompatibility and osteoconductivity [43,44]. HA exhibits a strong propensity for attracting osteoblasts but possesses a low resorption rate in vivo and is brittle, especially in highly porous forms.

In order to alleviate some of these inherent issues, while still maintaining its benefits, HA has been combined with several natural and synthetic polymers, including poly (lactic acid) (PLA) [45–50], poly(lactic-co-glycolic acid) (PLGA) [51,52], polyamide [53], polycaprolactone [54], polyethylene [55], gelatin [56], chitosan [57] and collagen [49,58,59] to produce composite scaffolds. Advantages of pairing HA with natural and/or synthetic polymers include improved control of design parameters such as porosity, degradability and mechanical properties when compared to pure HA scaffolds. Porosities greater than 90% are possible, where pure HA scaffold porosity is usually less than 70% [60].

The addition of nano-crystalline HA to natural polymer scaffolds has been shown to improve mechanical properties compared to polymer control scaffolds [50] and the presence of HA may potentially reduce adverse effects associated with the degradation of some synthetic polymers [61].

The most common methods for fabricating polymer/HA composite scaffolds fall into two categories: incorporation during processing and biomimetically growing the apatite onto a prefabricated polymer scaffold.

Incorporation of nano-scale ceramic into polymer matrix

One method of fabricating composite scaffolds is by incorporating a ceramic component directly into the polymer solution prior to solidification. One technique used to combine hydroxyapatite (HA) with PLLA is thermally induced phase separation (TIPS). Our group has used this technique to incorporate nano-crystalline HA into PLLA, yielding composite scaffolds with porosities between 89 – 95% [50]. An example of this type of scaffold is shown in Figure 4. Porosity and pore morphology were controlled in two ways: by using different solvents, including pure dioxane, pure benzene and various mixtures of dioxane/water and by varying the size of the HA particles used. Using nano-scale HA powder in place of micron-scale powder resulted in a more regular network of pores [50].

Figure 4.

Figure 4

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SEM micrographs of nano-HA/PLLA 50:50 scaffold, x100, x1000. (From Wei and Ma [50], copyright 2004 by Elsevier. Reprinted with permission.)

While these techniques have proved to be effective for low volume fractions of HA in PLLA, the incorporation of larger amounts of ceramic was prohibitive [50] and a percentage of the incorporated ceramic remained locked inside of the scaffold structure. To address these limitations, a biomimetic method has been used to effectively “grow” the ceramic component onto the pore walls.

Biomimetic Polymer/Apatite Composite Scaffolds

The methods described above create composite scaffold structures with high porosity, good interconnectivity and varying degrees of control over pore size and shape. However, these methods all involve a ceramic content that is largely contained within the bulk of the scaffold material, rather than at the surface. Since all interactions with biological components occur at the pore surface, the non-exposed ceramic is in effect wasted. In order to better utilize the ceramic component, polymer/ceramic composite scaffolds have also been fabricated using a biomimetic approach, where a prefabricated polymer scaffold is soaked in simulated body fluid (SBF) in order to allow apatite crystals to grow onto its pore surfaces. Figure 5 is an SEM micrograph showing apatite crystals on the surface of a PDLLA scaffold. This mechanism is described in detail elsewhere [62,63]. This technique is simple, requires no special equipment and can be used to grow apatite onto complex scaffold structures, unlike many surface modification techniques, which require a flat surface.

Figure 5.

Figure 5

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SEM micrographs of PDLAA scaffolds incubated in 1.5x SBF for 30 days. (From Zhang and Ma [52], copyright 2004 by John Wiley & Sons, Inc. Reprinted with permission.)

The methods used to fabricate polymer scaffolds to be soaked in SBF vary. Thin scaffolds on which apatite has been successfully grown have been created using electrospinning techniques [34]. However, this method normally yields a roughly two-dimensional product that is not practical for use in large bone defects. TIPS has been used repeatedly and with much success [46,52]. By varying the solvents used and the polymer concentration, porosity and pore morphology are optimized to allow apatite deposition throughout the three-dimensional scaffold construct. Porogens have also been used to control pore shape, size and interconnectivity, including the inclusion and leaching of salt particles and sugar spheres [30,52].

Work is currently underway to study the effect of various material properties inherent to different polymers on the ability of scaffolds to more effectively induce and control apatite deposition. For example, it is likely that polymer functional groups play a role in the nucleation and deposition of apatite. Our group tested the effect of the carboxylic acid group on apatite nucleation and growth on PMMA and PMMA-MAA scaffolds. Smaller apatite particles appeared in larger numbers after 30 days on PMMA vs. PMMA-MMA, and the size of the particles increased while the frequency of particles decreased with increased amounts of MMA [52]. While this example showed that the presence of functional groups led to fewer deposits, other functional groups such as COOH and OH have led to increased nucleation rates [46]. Other properties that may affect apatite nucleation and growth are polymer crystallinity, hydrophobicity/hydrophilicity of the surface, pore size, shape and interconnectivity [52]. The optimal combination of scaffold parameters to induce nano-scale apatite deposition is yet to be discovered, and this is an area of ongoing research.

Biological Effects

A. The Biological Effects of Nanofibers

Nano-fibrous scaffolds enhance cellular functions compared to more traditional solid-walled scaffolds, enabling tissue engineers to fabricate more organized tissues and move closer to the functional replacement of ailing tissue. In this section, we will discuss the effects of nano-fibrous materials on cellular attachment, proliferation, and differentiation, all of which are vital to functional tissue formation. It is important to note that the use of nano-fibrous scaffolds in tissue engineering is in its early stages and there is limited data on the biological affects of nano-fibers on cellular function and tissue formation.

Attachment

To proliferate, migrate and differentiate, most cells require anchorage [64]. Therefore, cellular attachment is the first step toward the development of tissue. From an early stage in development, type I collagen provides the base attachment structure for cells within many tissues [65]. As development progresses, other proteins and bio-molecules are either adsorbed from the serum or secreted from the cells and join with type I collagen to form the various native ECM of differentiating tissues. The mature ECM then mediates cellular attachment, migration, proliferation and differentiation. The artificial ECMs used in tissue engineering attempt to mimic this native structure.

Several key attachment proteins (fibronectin, vitronectin and laminin) have been found to adsorb to the nano-fibrous scaffolds at levels 2.6 to 3.9 times higher than solid-walled scaffolds [66]. Additionally, this adsorption was found to be selective and could not be explained by the surface area alone of the nano-fibrous scaffold [66]. This may provide cells attached to the nano-fibrous scaffolds an ECM which more closely resembles in vivo conditions compared to solid-walled scaffolds.

As a result, several studies have noted differences in the expression of integrins, a family of transmembrane receptors that govern cellular interactions with the ECM [67,68]. Neonatal mouse osteoblasts cultured on nano-fibrous scaffolds exhibited increased expression integrins associated with collagen (α2 β1), fibronectin (αV β3) and vitronectin (αV β3) binding compared to solid-walled scaffolds. Notably, the expression of α2 integrin remained up-regulated on the nano-fibrous scaffolds compared to solid-walled scaffolds even when collagen fibril formation was inhibited [68], potentially indicating a direct interaction between the cell and the nano-fibrous scaffold via a α2 integrin-mediated dimer.

Changes in cellular morphology, more closely proximating that observed in vivo, have been observed on nano-fibrous material compared to control. Neonatal mouse osteoblasts cultured on nano-fibrous scaffolds exhibited long slender processes connecting to neighboring cells while cells on the solid-walled scaffolds were flat and smooth (Figure 6) [68]. Similarly, processes were observed in human mesenchymal stem cells on nano-fibrous thin films [69].

Figure 6.

Figure 6

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Scanning electron microscopic views of neonatal mouse osteoblasts cultured for 3 days: (A) on the solid-walled scaffolds and (B–D) on the nano-fibrous scaffolds. Original magnification; A and B 1000X, C and D 8000X. (From Woo et al. [68], copyright 2004 by Elsevier. Reprinted with permission.)

The extracellular environment formed on nanofibers compared to that on solid-walled surfaces has led to the report of increased cellular attachment with several cells lines including osteoblastic cells [66,68], fibroblasts [70], normal rat kidney cells [70], smooth muscle cells [71], neural stem cells [72] and embryonic stem cells [73]. This increased attachment across various cells types provides tissue engineers with a potential tool to generate functional tissues in shorter time frames than would be possible on more traditional scaffolds.

Proliferation

Most tissue engineering strategies require the migration and proliferation of cells to fully populate the scaffold to enable new tissue development. More rapid proliferation of the desired cell type in vivo would lead to a reduction in scar tissue formation.

Several cell types have been reported to exhibit increased proliferative ability on nano-fibrous materials compared to controls [28,67,69–71,74]. Specifically, pre-osteoblasts have shown a significant increase in proliferation from 7 to 12 days of culture on nano-fibrous scaffolds compared to solid-walled scaffolds [28], while chondrocytes have shown increased proliferation on nanofibers compared to control over 3 weeks of culture [67].

Differentiation

The progression of cells from immature phenotypes to the highly specialized phenotypes present in tissues is a complex process governed by many factors. Nanofibers have enhanced the differentiation and function of several cell types including neural progenitors [25,75], hepatocytes [76,77], chondrocytes [78], and osteoblasts [28,68]. Nano-fibrous scaffolds have also been attributed with rescuing cells from regression to a more immature phenotype during expansion culture [67]. However, this falls outside the scope of this review and will not be discussed here in detail. Bone is a tissue typically identified by its ECM. As immature cells differentiate toward the osteoblastic lineage, they begin to express a sequential series of markers which culminate in the secretion of proteins like osteocalcin and bone sialoprotein and the mineralization of the ECM. Alkaline phosphatase, an early marker of osteogenic differentiation, has been shown to be up-regulated on nano-fibrous materials compared to controls in culture periods of 3 days or more in vitro [68,79] while late osteogenic markers, such as osteocalcin and bone sialoprotein, have been shown to be up-regulated on nanofibers compared to control over culture periods of two or more weeks [28,68,79]. Specifically, after 2 weeks of culture on nano-fibrous scaffolds pre-ectoblastic cells expressed roughly 7.5 times more osteocalcin and 1.8 times more bone sialoprotein compared to solid-walled scaffolds [28]. The increased expression of these markers on nano-fibrous scaffolds compared to solid-walled scaffolds continued throughout the 6 week study.

Type I collagen is the only osteogenic marker down regulated on nano-fibrous scaffolds compared to solid-walled control scaffolds [28]. However, several studies have found type I collagen distributed throughout the nano-fibrous materials [16,80]. The down-regulation of type I collagen could be due to accelerated maturation of the ECM on the nano-fibrous material compared to control, or the ability of the cells to interact with the nanofibers in a manner similar to type I collagen therefore requiring less secreted type I collagen to complete the ECM.

As cells begin to express markers of late stage bone differentiation, the ECM begins to mineralize. Several studies have reported increased mineralization of the nano-fibrous scaffolds compared to control [28,68,81]. One study found 13 times more physiologically deposited calcium on the nano-fibrous scaffold than on the solid-walled control scaffolds [68]. The mineral on the nano-fibrous scaffold was found to be more evenly distributed throughout the scaffolds than the mineral on the solid-walled scaffolds [28], as seen in Figure 7.

Figure 7.

Figure 7

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In vitro response of NF and SW PLLA scaffolds after seeding with MC3T3-E1 osteoblasts and cultured under differentiation conditions for 6 weeks. Shown are histological sections of representative areas within the scaffold. H&E staining showing (a) overview of a NF scaffold, (b) overview of a SW scaffold, (c) center region of a NF scaffold, and (d) center region of a SW scaffold. Von Kossa’s silver nitrate staining showing (e) center region of a NF scaffold, and (f) center region of a SW scaffold. Scale bars of (a), (b), (e), and (f); 500 μm. Scale bars of (c) and (d); 100 μm. * denotes the PLLA scaffold, # a scaffold pore. Arrows in von Kossa stained sections denote mineralization. NF scaffolds retained a small amount of the histological dyes and therefore are more visible than the SW scaffolds in the pictures. (From Chen et al. [28], copyright 1999 by Wiley & Sons, Inc. Reprinted with permission.)

Additionally, a more even cellular distribution and tissue-like cellular organization was seen on the nano-fibrous scaffolds compared to solid-walled control (Figure 7). With a combination of in vitro and in vivo culture, nano-fibrous scaffolds have produced bone tissue containing vascularization, mineralization and embedded osteocyte-like cells [80]. Overall, this indicates that nano-fibrous scaffolds possess the potential to generate functional bone tissue. However, this will only be achieved with further enhancement of culture conditions and scaffold design.

B. Biological Effects of Nano-composite Scaffolds

By decreasing the grain size of HA down to the nano-scale in dense substrates, osteoblast behavior is positively affected. Webster et al. showed an increase in osteoblast attachment, proliferation and differentiation on nano-scale HA (67 nm) vs. micro-scale HA (179 nm) [82]. While this phenomenon is not fully understood, it may be due in part to improved adsorption of key ECM proteins, including vitronectin and collagen, on the substrate [82]. While the dense substrates used in the work of that group would not be useful tissue engineering scaffolds, this phenomenon of improved protein adsorption is observed in HA/PLLA composite scaffolds and is tied to a decrease in osteoblast apoptosis [83].

Conclusion

Nano-structure in tissue engineering is of increasing importance. By mimicking the ECM and including nano-scale structural elements in tissue engineering scaffolds, cell response is affected at the cell-scaffold interface. Nano-fibrous scaffolds are a synthetic replica of the naturally occurring ECM which promotes cell distribution and have the potential to promote new tissue formation. The addition of nano-scale apatite crystals to a polymer scaffold mimics the deposition of apatite crystals onto the ECM, as well as the multi-component composite structure of bone. Integration of nano-particles into scaffolds enhances biological regulation of cell behavior for regeneration. Further work is underway to study the effect of these components on mechanical properties, biological effects and the mechanisms by which these properties are changed.

Acknowledgments

This research was supported in kind by the NIH (DE015384, GM075840, DE017689, P60DK020572-29S1: PXM), an NIH Stem Cell Core Pilot Grant (P20GM069985: PXM), an NIH Tissue Engineering and Training Grant (T32DE07057: LAS), an NIH Regenerative Science Training Grant (T90DK070071: IOS) and the NSF Graduate Research Fellowship Program (LAS).

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