组织工程

REVIEW PAPER
DESIGN AND DEVELOPMENT OF
THREE-DIMENSIONAL SCAFFOLDS
FOR TISSUE ENGINEERING
C.Liu1,ÃZ.Xia2and J.T.Czernuszka1
1Department of Materials,University of Oxford,Oxford,UK.
2Botnar Research Centre,Nuffield Department of Orthopaedic Surgery,University of Oxford,Nuffield Orthopaedic Centre,Headington,Oxford,UK.
Abstract:Tissue engineering is a concept whereby cells are taken from a patient,their number expanded and seeded on a scaffold.The appropriate stimuli(chemical,biological, mechanical and electrical)are applied and over a relatively short time new tissue is formed. This new tissue is implanted to help restore function in the patient.The scaffold is a three-dimensional substrate and it serves as a template for tissue regeneration.The ideal scaffolds should have an appropriate surface chemistry and microstructures to facilitate cellular attach-ment,proliferation and differentiation.In addition,the scaffolds should possess adequate mechanical strength and biodegradation rate without any undesirable by-products.Research in this area has been intense over the past10years or so on biopolymer formulation and on scaffold fabrication.This paper summarized some important issues related to scaffold design and development from biodegradable polymers.The mechanical properties and bio-compatibility of commonly used biopolymers are reviewed.The scaffold design and fabrication techniques are overviewed,their advantages and manufacturing feasibility are compared.The scaffold architecture,including pore size and size distributions,and its effects on the cells’growth are discussed.The scaffold should offer a hierarchical structure that varies over length scales of0.1 1mm.Conventional processing techniques can not yet fabricate a scaf-fold with control over both architecture and surface chemistry.There is,however,an emerging scaffold fabricating technique using solid free form fabrication(SFF).It has shown to be highly effective in integrating structural architecture with changes in surface chemistry of the scaffolds, and integration of growth factors.
Keywords:scaffold;tissue engineering;solid freeform fabrication;collagen.
INTRODUCTION
Tissue engineering is an interdisciplinaryfield that combines the knowledge and technology of cells,engineering materials,and suitable biochemical factors to create artificial organs and tissues,or to regenerate damaged tissues (Langer and Vacanti,1993).It involves the seeding of cells onto a scaffold,this whole is cultured in vitro andfinally implanted into the body as a prosthesis when matured(Rabkin and Schoen,2002).The natural tissue regen-eration processes then take place,blood vessels infiltrate the structure and the scaffold eventually degrades leading to newly formed tissue in place.The general process of tissue engineering is illustrated in Figure1.
The scaffold provides a framework and initial support for the cells to attach,proliferate and differentiate,and form an extracellular matrix(ECM)(Agrawal and Ray,2001;
Sachols et al.,2003a).It is this ECM which provides the structural integrity of tissue.
The scaffold also serves as a carrier for cells,growth factors or other biomolecular sig-nals.It is vital for the scaffold to mimic the structure and properties of human tissue to direct the macroscopic process of tissue for-mation.An ideal scaffold should have the following characteristics:(1)an extensive net-work of interconnecting pores so that cells can migrate,multiply and attach deep within the scaffolds;(2)channels through which oxygen and nutrients are provided to cells 1051Vol85(A7)1051–1064
ÃCorrespondence to:
Dr C.Liu,Healthcare Engineering Group,Wolfson School of Mechanical& Manufacturing Engineering, Loughborough University, Loughborough LE113TU, UK.
E-mail:DES6CL@
DOI:10.1205/cherd06196 0263–8762/07/
$30.00þ0.00
Chemical Engineering Research and Design Trans IChemE,
Part A,July2007
#2007Institution
of Chemical Engineers
deep inside the scaffold,and the waste products can be easily carried away;(3)biocompatibility with a high affinity for cells to attach and proliferate;(4)right shape,however complex as desired by the surgeon;and (5)appropriate mechanical strength and biodegradation profile.Tissue engineering would greatly benefit from such scaffolds.
Biomaterials used in tissue engineering scaffold fabrication can be divided into broad categories of synthetic or naturally derived,with a middle ground of semisynthetic materials rapidly emerging (Griffith,2002).Most materials commonly in use in tissue engineering are adapted from other surgical uses,such as sutures,hemostatic agents and wound dres-sings (Bacon,2002;Lee and Ansell,2003).These include synthetic biodegradable materials,such as aliphatic polyesters (polyglycolic acid,polylactic acid and their co-polymers)(Chen et al.,2001;Lu et al.,2000a,b;Miko et al.,1994;Yang et al.,2004),hydroxyapatite (HA)(Kikuchi et al.,2004;Rodrigues et al.,2003;Zhang and Ma,1999a),and naturally derived materials such as collagen and chitin (Chen et al.,2001;Chung et al.,2002;Sachols et al.,2003b;Sachols et al.,2003c;Shen et al.,2000;Sukhodub et al.,2004;Yang et al.,2004).The techniques to make tissue engineering scaffolds from these biomaterials include solvent casting particulate leaching,phase separation (Liu and Ma,2004;Ma,2004;Zhao et al.,2002),gas forming,emulsion freeze drying (Shen et al.,2000;Whang et al.,1995)and fibre-meshes (Freed et al.,2006).Tissue engin-eering products developed to date have only succeed in either thin structure (skin)or tissues without a blood supply (cartilage)(Cao et al.,2006a;Mikos et al.,2006).A number of other techniques are being pursued for the creation of scaf-folds with an aim to overcome the abovementioned short-comings.The SSF technique is being studied by a number of groups around the world (Chu et al.,2002;Hollister et al.,2002;Hutmacher et al.,2004;Lin et al.,2004;Taboas et al.,2003;Xiong et al.,2002).The main advantage of this technique is in being able to precisely control the archi-tecture of the scaffolds.It is also possible to integrate cell seeding within the scaffold fabrication process,thus avoiding the problems with poor cell infiltration into the scaffold that other techniques may encounter (Salusbury,2005).
The aim of this paper is to review the development of the tissue engineering scaffold,in particular to address the
materials selection and emerging scaffold fabrication tech-niques.The authors’work on the design and fabrication of collagen based scaffolds using inkjet printing techniques,and preliminary in vitro results on the resultant scaffolds are also presented.
BIOMATERIALS FOR SCAFFOLD FABRICATION
The basic requirements for biomaterials used for scaffolds are their biocompatibility and appropriate surface properties to favour cellular attachment,proliferation and differentiation.Synthetic biomaterials (bioceramics and biopolymers)are the primary materials use for scaffold fabrication in various tissue engineering applications.
Scaffold materials can be either natural or synthetic.How-ever,synthetic biopolymers offer an advantage over natural materials in that they can be tailored to give a wide range of properties and which are more predictable.In particular,many investigations have concentrated on synthetic biode-gradable polymers that are already approved by the food and drug administration (FDA).The most common biode-gradable polymers being used or studied include polylactic acid (PLA),polyglycolic acid (PGA),polyanhydrides,polyfu-marates (PF),polyorthoesters,polycaprolactones (PCL)and polycarbontes (Vail et al.,1999).
PLA,PGA and their co-polymer,poly(DL-lactic acid-co-gly-colic acid)(PLGA)are widely used in the fabrication of scaf-folds.Scaffolds fabricated by electrospinning PLGA nanofibres on to the surfaces of a knitted PLGA demon-strated a good mechanical strength and internal hierarchical structure.The nanofibres within the scaffold facilitated cell attachment and new ECM deposition.Plasmid DNA was suc-cessfully incorporated into a non-woven,nano-fibred scaffold,fabricated from PLGA and PLA-PEG block copolymer,and it was used for therapeutic application in gene delivery for tissue engineering (Luu et al.,2003).
Poly(e -caprolactone)(PCL)is a semicrystalline,bioresorb-able polymer belonging to the aliphatic polyester family.It is regarded as a soft and hard tissue-compatible bioresorbable material and has been used as scaffold for tissue engineering (Burkersroda et al.,2002).It has similar biocompatibility to PLA and PGA,but a much lower degradation rate (Lowry et al.,1997).The slow degradation makes it less attractive for general tissue engineering,but it does make it an appro-priate candidate as a long-term drug delivery carrier.It is often combined with other materials,such as bioceramics,to increase its Young’s modulus and adjust its biodegradation rate.On biodegradation,PCL /HA scaffold do not show a local decreased pH value as commonly observed in PLA.Other important synthetic biodegradable polymers include poly(ortho esters)and polyanhydrides (from nonphysiological monomers),and they possess biocompatible,well-defined degradation characteristics (Muggli et al.,1999).They are pri-marily designed for controlled drug delivery (Burkoth et al.,2000;Hanes et al.,1998;Ibim et al.,1998),however they have also been explored for use in tissue engineering.One particular study on the degradation of porous poly(anhy-dride-co-imide)microspheres demonstrated that water pen-etration and anhydride bond cleavage occurred rapidly (,5days)(Hanes et al.,1998).Ibim et al .(1998)reported that the biocompatibility of poly(anhydride-co-imide)is equal to PLGA,and supported cortical bone regeneration.It
has
Figure 1.General process of tissue engineering.It involves seed cells on scaffold,culturing in vitro and implant into the patient (Liu and Czernuszka,2006).
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1052LIU et al.
been reported that an implantable scaffold made from poly(anhydride-co-imide)could be used in orthopaedic sur-gery,even in weight-bearing applications(Burkoth et al., 2000).Tyrosine-derived polycarbonates based on natural metabolites(the amino acid tyrosine)are another synthetic biopolymer currently under investigation as a tissue engineering scaffold.An in vivo experiment carried out in a canine bone chamber model revealed that tyrosine-derived polycarbonates,are comparable,if not superior,to PLA in terms of biocompatibility(Choueka et al.,1996).Poly(propy-lene fumarate)(PPF)is a linear polyester that contains mul-tiple unsaturated double bonds that are available for covalent crosslinking of the polymer in the presence of free-radical initiators(Hedberg et al.,2005a),and can degrade through hydrolysis of the ester bonds(Peter et al.,1998). An advantage of PPF over many other biodegradable syn-thetic polymers is that it can be utilized as an injectable system,allowing for direct application into a defect site and cross-linking in situ.Although crosslinked PPF is biocompati-ble and osteoconductive,PPF alone is not osteoinductive.It is often used incoorporation with another osteoinductive com-ponent,such as b-tricalciumphosphate(b-TCP)(Peter et al., 2000),osteogenic peptides or proteins through the use of PLGA based microparticles(Hedberg et al.,2005a,b; Kempen et al.,2006;Schek et al.,2006).Peter et al. (1998)reported that the mechanical properties of PPF/TCP composite scaffold are dependent on the crosslinking density of the network and the percentage of TCP inclusion,and could exhibit initial mechanical properties similar to human trabecular bone and maintained these properties over sev-eral weeks of degradation.
Synthetic polymers(such as PLA,PGA,PLLA,PLGA,PFF and so on)undergo bulk degradation once implanted into human body.The molecular weight of the polymer starts to decrease upon placement in an aqueous media.However, the mass loss does not start until the molecular chains are reduced to a size which allows them to freely diffuse out of the polymer matrix.The mass loss is accompanied by a release gradient of acidic by-products.If the capacity of the surrounding tissue to eliminate the by-products is low,due to the poor vascularization or low metabolic activity,in vivo, massive release of acidic degradation and resorption by-products may lead to local temporary disturbances,or may results in inflammatory reactions(Anderson,2001; Hutmacher,2000;Peter et al.,1998).The foreign body reac-tion associated with inflammation would limit the performance or even led to the failure of the tissue engineered products.To limit this side effect,it is important for the scaffolds/cell con-struct to be exposed at all times to sufficient quantities of neu-tral media/environment,especially during the period where the mass loss of the scaffold occurs(Hutmacher,2000).A common route to alleviate the local inflammatory is by incor-poration of ceramics such as TCP,HA powders into a synthetic polymer matrix produces a composite scaffold. Apart from the benefit of improve biocompatibility,the basic resorption products of HA or TCP would buffer the acidic resorption by products of synthetic polymers and therefore help to avoid the formation of an unfavorable environment for the cells due to a decreased pH(Kempen et al.,2006). Collagen is afibrous protein and a major natural extracellu-lar matrix component.It is the most abundant protein in mam-mals and is the main structural element in skin,bone,tendon, cartilage and blood vessels and heart valve(Creighton,1993;Kose et al.,2005;Lee et al.,2000;Taylor et al.,2006).There are25types of collagen differing in their chemical compo-sition and molecular structure have been identified.Among them,type I collagen offers a suitable environment for the induction of osteoblastic differentiation in vitro and osteogen-esis in vivo.As natural polymers,type I collagen have also been used for tissue engineering applications,especially for soft tissue repair such as skin(Kose et al.,2005).Collagen has a more native surface(relative to the synthetic polymers) which favours cellular attachment as well as being chemotac-tic to cells.Thus it has useful biological properties desirable for tissue engineering applications.Therefore,collagen is widely used on its own,or as a component of a composite, for tissue engineering applications(Kikuchi et al.,2004;Ma et al.,2004a,b;Rodrigues et al.,2003;Rothamel et al., 2005).Even denatured collagen(gelatin)has been pro-cessed into porous materials for tissue repair(Choi et al., 1999).
There are concerns with the use of collagen regarding the antigenicity and immunogenicity where applied to the biome-dical devices(Lynn et al.,2004),potential pathogen trans-mission,immune reactions,poor handling and mechanical properties,and less controlled biodegradability(Ma et al., 2004).Various efforts such as cross-linking of collagen and hybridization with other biomaterials are being made to overcome these potential drawbacks.Ma et al.(2004a,b) have reported using a water soluble cross-linking agent carbodiimide,1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide(EDAC)and N-hydroxysuccinimide(NHS)in the presence of amino acids(glycie,glutamic acid or lysine),which function as a cross-linking bridge between col-lagen molecular chains.In vitro assessment revealed that the collagenase biodegradation degree was greatly decreased when lysine was added,resulting in a more biological stable scaffold.Collagen based composites(either with bio-ceramics or biopolymers)exhibited the advantages of both the hybrid biomaterial and collagen(Chang and Tanaka 2002a,b;Chen et al.,2001;Kikuchi et al.,2004;Rodrigues et al.,2003;Sukhodub et al.,2004).These include increased mechanical strength with enhanced cell seeding and promoted cell interactions.
Chitosan is another important natural biopolymer compris-ing glucosamine and N-acetylglucosamine,obtained by dea-cetylation of chitin(Chung et al.,2002;Shen et al.,2000; Zhao et al.,2002).It has been reported to be safe,heamo-static and osteoconductive and to promote wound healing. HA/chitosan-gelatin scaffolds have been fabricated,in vitro examination demonstrated that extracellular matrices could be synthesized and osteoid and bone like tissue could be formed(Zhao et al.,2002).Starch is another natural material that has also been studied as tissue engineering scaffold,the varied properties of starch make it,potentially,a suitable natural material for use in a wide biomedical application ran-ging from bone replacement to engineering of tissue scaffold and drug delivery systems(Lam et al.,2002;Levy et al., 2004).In practice,starch is often used in combination with other biomaterials,such as with cellulose acetate(Levy et al.,2004;Salgado et al.,2002),hydroxyapatite(Marques and Reis,2005),poly(ethylene-vinyl-alcohol)and poly(lactic acid)(Neves et al.,2005)to confer the scaffold with different properties to be used in different applications.Although the physico-chemical properties of starch-based composite can have an influence on cells adhesion,proliferation and
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DESIGN AND DEVELOPMENT OF THREE-DIMENSIONAL SCAFFOLDS1053
morphology,in vitro study showed that osteoblasts adhered and proliferated on the starch scaffold.Also cells mainitained the osteogenic phenotype,and mineralized extracellular matrix could be detected after3weeks culturing(Marques and Reis,2005).
In addition to the use of relatively pure natural macromol-ecules extracted from an animal or plant tissue source, processed extracellular matrix(decellularized)materials with multiple natural macromolecules are also used as scaf-folds for tissue engineering or repair applications.One such example is small intestinal submucosa(SIS),which contains type I collagen,glycosaminoglycans(GAGs),and some growth factors.A preliminary study has suggested that SIS patches can be used for small bowel regeneration(Chen and Badylak,2001).
As we have shown,there is a wide variety of biocompatible materials which includes bioceramics,synthetic and natural biopolymers available for tissue engineering.Each material has its own characteristics,and within each family of materials there is a range of properties and characteristics. There seems to be less emphasis on new materials,but rather on the combination,processing or other treatment of established systems in novel ways.The selection of materials therefore depends on the specific requirement dic-tated by the application and suitable fabrication technique. TCP and HA(Ca10(PO4)6(OH)2),and their combinations are the most frequently used bioceramics in scaffold manu-facturing(Daculsi,1996;Meenen et al.,1992).These two bioceramics have excellent biocompatibility with hard tissues, and high osteoconductivity and bioactivity.They have neither antigenicity nor cytotoxicity and can be processed into porous form for use as bone substitutes or scaffolds(Kikuchi et al.,2004;Liu,1997;Miko and Temenoff,2000;Sukhodub et al.,2004;Vail et al.,1999).However,their usage is limited because of their brittle nature and the difficulty in processing into highly porous structures with controlled porosity(Liu, 1997;Meenan et al.,2000).To overcome these disadvan-tages and to enhance their biocompatibility and cell attachment,these bioceramics(HA and TCP)are usually combined with collagen to make HA/collagen composite scaffolds(Clarke et al.,1993;Rodrigues et al.,2003).This can be fabricated by dissolving and thoroughly mixingfine HA powder and collagen in an acidic solution.The composite is harvested by centrifugation and then freeze dried.Kikuchi and co-workers(Kikuchi et al.,2004;Zhang and Ma,1999b) have fabricated a bone-like HA/collagen composite by using a biomimetic co-precipitation method.The bone tissue reac-tions demonstrated excellent osteoclastic resorption and bone formation which is very similar to the reaction of a trans-planted autogenous bone.The in vitro study using human osteoblasts revealed that the cells adhered and spread on both the HA particle surface and the collagenfibres,and was proposed as an ideal scaffold for osteoconduction.
TECHNIQUES IN SCAFFOLD FABRICATION There are many techniques/methods to process biomater-ials into various scaffolds.These include conventional tech-niques such as impregnatation and sintering ceramic scaffolds processing,solvent casting and particulate leach-ing,gas forming,non-wovenfibre,fibre knitting,phase separ-ation/emulsion freeze drying for manufacturing scaffold. Emerging fabrication techniques are under development particularly solid free form fabrication techniques including 3D printing technique and fused deposition techniques. Table1summarized the key features of each techniques commonly used in scaffold fabrication.Characteristics differ-entiating the various techniques include the use of solvents, heat,pressure,or pore creating agents.Scaffold fabrication techniques could be categorized into four methods,via solvent casting in combination with particulate leaching,fibre networking,phase separation in combination with freeze drying/critical point drying,and solid free form fabrica-tion.From a scaffold design and function view point,each technique has its pros and cons,we shall attempt to analyse some of the major categories of technique.
Solvent casting,in combination with particulate leaching involves casting a polymer solution with water soluble parti-culates into a mould.After the evaporation of the solvent, the particulates are leached away using water to form the pores of the scaffold.The process is easy to carry out,but it works only for thin membranes or very thin3D specimens. In thicker sample preparation,it is very difficult to remove all the soluble particulates from the polymer matrix(Miko and Temenoff,2000;Miko et al.,1994).A modified method using laminated thin sheets overcomes the particulate leach-ing problem(Miko et al.,1993),but layering of thin porous sheets is time consuming and allows for only a limited number of connected pore networks.The extensive use of solvents(some of which are toxic)in this method also pre-sents a difficulty,as any residuals of the solvent would hinder the cell attachment and proliferation onto the scaffold (Chen et al.,2001;Mooney et al.,1996).A recent publication which has shown that low toxicity solvents can be used in this technique and residues brought down to acceptable levels for application(Cao et al.,2006b).
Fibre networking technique uses biodegradablefibers to fabricate scaffolds via a textile method,such as non-woven, knitting andfibre bonding routes(Cooper et al.,2005/2000; Freed et al.,2006).Electrospinning method is reported capable to fabricate polymerfibres range from a few nan-ometer to hundreds of microns(Xu et al.,2004).The ability to co-spin polymers with various additives offers the possi-bility of functionalfibres.PGA and PLAfibres are two com-monly used biopolymers used with this technique(Ouyang et al.,2003).They have been used either alone or combined with other biomaterials,such as collagen,to enhance their biocompatibility.The technique of non-woven and knitting are quite straightforward,and non-toxic chemicals are involved.However there are several limitations such as low rigidity and difficulty in controlling pore size and pore shape.Infibre bonding,twofibre materials can be attached to each other via‘heat fuse’or‘embed’methods,then one material is dissolved in a selective solvent to obtain the fibre scaffold.Althoughfibre bonding technique can produ-cing highly porous scaffolds with interconnected pores that are suitable for tissue regeneration,this method involves the use of solvents that could be toxic to cells if not comple-tely removed(Miko and Temenoff,2000),which could reduce the ability of cells to form new tissue in vivo.
Phase separation technique uses the fact that a homo-geneous multicomponent system such as a polymer–water emulsion,can become thermodynamically unstable and sep-arates in order to lower the free energy(Whang et al.,1995). In respect to scaffold fabrication,a thermally induced phase separation can be used to result in a polymer-rich phase
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and a polymer-lean phase.After the solvent is removed (either via lyophilization or solvent extraction)(Chen et al., 2001;Zhang and Ma,1999b),the space originally taken by the solvent becomes the pores,thus a porous scaffold is formed.The architecture of the scaffold could be controlled by phase separation conditions.Ma and Zhang(2001)have reported a highly porous scaffold fabricated with an oriented array of open microtubules by using a modified phase separ-ation method.This technique has also been reported to fab-ricate afibrous scaffold(Ma and Zhang,1999).As this technique uses organic solvents to create pores within the scaffolds,the removal of the solvent remains a problem and forms a potential source of toxicity for cells.High pressure CO2gas techniques(Mooney et al.,1996)can avoid the use of toxic solvents in scaffold fabrication.How-ever,the resultant scaffolds contain a nonporous surface film with mixed open and closed cell structures,not suitable for application as tissue engineering scaffolds.
In vitro evaluation,commonly reveals that the cells are only able to survive close to the surface to within a critical depth which is cell dependent.In order to support the growth of a large volume of tissue(typically more than1mm),it is necessary to promote cell growth within the scaffold.This can only be achieved when nutrients are delivered to the cells and waste products removed.Vascularization of the scaffold is key to the success of this strategy.The growth of large organized cell communities requires optimization of scaffolds that can maximize cell utilization,minimize the time in suspension for anchorage-dependent and shear-sensitive cells and permit spatially uniform tissue regeneration(Freed and Vunjak-Novakovic,1998).These requirements can be met using a highly porous scaffold with suitable interior features.
Solid freeform fabrication(SFF)is a developing technology that enables the fabrication of custom made devices directly from computer data such as computer aided design(CAD), computed tomography(CT)and magnetic resonance ima-ging(MRI)data(Das et al.,2003).The digital information is then converted to a machine specific cross-sectional format,expressing the model as a series of layers.Thefile is then implemented on the SFF machine,which builds cus-tomer designed3D objects by layered manufacturing strat-egy(Calvert et al.,1998).Each layer represents the shape of the cross-section of the model at a specific level.Over the past two decades,more than20SFF systems have been developed and commercialized,these include stereo-lithography(SLA),laminated object manufacturing(LOM), selective laser sintering(SLS),fused deposition modelling (FDM)and ink jet printing(IJP).Detailed information on the various SSF technologies are widely available in the literature (Fang et al.,2005;Hutmacher,2001;Hutmacher et al.,2004; Khalil et al.,2005;Lam et al.,2002;Sun and Lal,2002; Taboas et al.,2003;Wilson et al.,2004;Xiong et al.,2002) and will only be discussed here,briefly.SFF allows the
Table1.Methods used to process biomaterials into tissue engineering scaffolds.
Fabrication technique Requirement for
materials Reproducibility
Scaffold
architecture Biomaterials Problems Reference
Impregnate sintering Withstand high
temperature
Sensitive to
sintering
Pore size:
200 1000m m;
porosity:.50%
foam dependent
HA,TCP Brittle(Lee and Kim,1996;
Liu,1997;Meenan
et al.,2000;
Meenen et al.,
1992;Wells et al.,
1996),
Solvent casting and
particulate
leaching Soluble in cell
non-toxic
solvent
User and
materials
dependent
Pore size:
50 1000m m;
porosity:
30 90%
PLA,PLGA,
collagen and so
on
Solvent toxicity
Particulate
remanet
(Chen et al.,2001;
Miko et al.,1994)
Phase
separation/
emulsion in
combination
with
freezing
drying/critical
point drying Soluble in cell
non-toxic
solvent
Emulsion
formation
sensitive to
stirring
Pore size,200m m;
Porosity:
70 95%
PLGA,PLA,PLLA
and collagen
Solvent toxicity
Pore size
difficult to
control
(Whang et al.,1995;
Zhao et al.,2002)
Fiber knitting/ non-woven/
bonding Fiber Machine control
Solvent
sensitive
Interconnected
channels,
20 100m m in
diameter
PVA,PLA,PLGA Lack of rigidity(Cooper et al.,2005;
Cooper et al.,2005
2000;Ouyang
et al.,2003)
Solid free form Low melting
point and
thermoplastic Computer
control
Interconnected
channels
Complex shape
and structure
.150m m
Customer based
PEG,PLA,PLGA
Collagen,starch,
HA,TCP
Costly(Calvert et al.,1998;
Chu et al.,2002;
Das et al.,2003;
Hollister et al.,
2002;Hoque et al.,
2005;Hutmacher
et al.,2004;Khalil
et al.,2005;Lin
et al.,2004;
Sachlos and
Czernuszka2003;
Taboas et al.,
2003;Woodfield
et al.,2004)
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