ken's paper

REVIEW PAPER

Biodegradable Polyphosphazene-Based Blendsfor Regenerative Engineering

Kenneth S. Ogueri1,2,3 & Jorge L. Escobar Ivirico2,3,4 & Lakshmi S. Nair1,2,3,4,5 &

Harry R. Allcock6& Cato T. Laurencin1,2,3,4,5,7

Received: 23 September 2016 /Accepted: 22 December 2016# The Regenerative Engineering Society 2017

AbstractThe occurrence of musculoskeletal tissue injury or disease andthe subsequent functional impairment is at an alarming rate. Itcontinues to be one of the most challenging problems in thehuman health care. Regenerative engineering offers a promis-ing transdisciplinary strategy for tissues regeneration based onthe convergence of tissue engineering, advanced materialsscience, stem cell science, developmental biology, and clinicaltranslation. Biomaterials are emerging as extracellular-mimicking matrices designed to provide instructive cues tocontrol cell behavior and ultimately be applied as therapiesto regenerate damaged tissues. Biodegradable polymers con-stitute an attractive class of biomaterials for the developmentof scaffolds due to their flexibility in chemistry and the abilityto be excreted or resorbed by the body. Herein, the focus will

be on biodegradable polyphosphazene-based blend systems.The synthetic flexibility of polyphosphazene, combined withthe unique inorganic backbone, has provided a springboardfor more research and subsequent development of numerousnovel materials that are capable of forming miscible blendswith poly(lactide-co-glycolide) (PLAGA). Laurencin and co-workers have demonstrated the exploitation of the syntheticflexibility of polyphosphazene that will allow the design ofnovel polymers, which can form miscible blends withPLAGA for biomedical applications. These novel blends,due to their well-tuned biodegradability, and mechanical andbiological properties coupled with the buffering capacity ofthe degradation products, constitute ideal materials for regen-eration of various musculoskeletal tissues.

Lay SummaryRegenerative engineering aims to regenerate complex tissuesto address the clinical challenge of organ damage. Tissue en-gineering has largely focused on the restoration and repair ofindividual tissues and organs, but over the past 25 years, sci-entific, engineering, and medical advances have led to theintroduction of this new approach which involves the regen-eration of complex tissues and biological systems such as aknee or a whole limb. While a number of excellent advancedbiomaterials have been developed, the choice of biomaterials,however, has increased over the past years to include poly-mers that can be designed with a range of mechanical proper-ties, degradation rates, and chemical functionality. Thepolyphosphazenes are one good example. Their chemical ver-satility and hydrogen bonding capability encourages blendingwith other biologically relevant polymers. The further devel-opment of polyphosphazene-based blends will present a widespectrum of advanced biomaterials that can be used as scaf-folds for regenerative engineering as well as other biomedicalapplications.

* Cato T. [email protected]

1 Department of Materials Science and Engineering, University ofConnecticut, Storrs, CT 06269, USA

2 Institute for Regenerative Engineering, University of ConnecticutHealth Center, Farmington, CT 06030, USA

3 Raymond and Beverly Sackler Center for Biomedical, Biological,Physical and Engineering Sciences, University of Connecticut HealthCenter, Farmington, CT 06030, USA

4 Department of Orthopaedic Surgery, University of ConnecticutHealth Center, Farmington, CT 06030, USA

5 Department of Biomedical Engineering, University of Connecticut,Storrs, CT 06269, USA

6 Department of Chemistry, The Pennsylvania State University,University Park, State College, PA 16802, USA

7 Department of Chemical and Biomolecular Engineering, Universityof Connecticut, Storrs, CT 06269, USA

Regen. Eng. Transl. Med.DOI 10.1007/s40883-016-0022-7

http://crossmark.crossref.org/dialog/?doi=10.1007/s40883-016-0022-7&domain=pdf

Keywords Regenerative engineering .Musculoskeletal .

Biodegradablepolymers .Dipeptide-basedpolyphosphazene .

Polyphosphazene blends

Introduction

Regenerative engineering has been defined as the conver-gence of advanced materials sciences, stem cell sciences,physics, developmental biology, and clinical translation forthe regeneration of complex tissues and organ systems[1–3]. One of the most important aspects involved in regener-ative engineering is the way cells, especially stem cells, reactwhen in contact with biomaterials. Precisely, the use of bio-materials as a supportive template for tissue development isvital to this approach.

In the list of existing biomaterials, biodegradable polymersis top and keeps attracting the attention of many researchers dueto their design flexibility in physical and biological propertiesto meet the complex requirements for the target applications[4–7]. It is possible to confer specific hydrophilic/hydrophobicentities, biodegradable repeating units, or multifunctionalgroups [8–10]. In tissue-engineering applications, biodegrad-able polymers such as poly(lactide), poly(glycolide), and theircopolymers (PLAGA) are the most commonly used [11–17].However, the bulk erosion of these materials results in theaccumulation of acid in the region of implants. This acidicaccumulation has been reported to cause catastrophic failureof structural integrity and adversely affects biocompatibilityboth in vitro and in vivo [18–23]. It usually results in inflam-matory responses and foreign body reactions [24]. In addition,these acidic degradation products potentially diminish the bio-activity of growth factors [25]. One encouraging way to bypassthese limitations is to blend PLAGA with other macromole-cules that can buffer the acidic degradation products with acontrolled degradation rate [15, 18, 19, 22, 26–28].

Due to their unique buffering degradation products, biode-gradable poly(organophosphazenes) (PPHOS) have generatedenormous interest and have proved to provide good candidatematerials to achieve this objective [18, 19, 29, 30]. These areinorganic–organic hybrid polymers with a backbone of alter-nating phosphorus and nitrogen atoms and with each phos-phorus atom bearing two organic side groups [4, 8, 10,30–35]. Specific characteristics are deliberately fashioned byintroduction of controlled amounts of particular organic sidegroups [36–38]. This has made available a wide array ofpolyphosphazenes with diverse properties. Chemical andphysical properties are largely dependent upon the type of sidegroups attached to the polyphosphazene backbone.Incorporation of different side groups can alter the degradationrate and mechanical properties of the polymer [4, 9, 28, 31,38–40]. For example, amino acid ester side groups will insti-gate hydrolysis within the polymer backbone [5, 9, 31, 39, 41]

while there is a retardation of hydrolysis in the presence ofhydrophobic phenylphenoxy side groups [4, 8, 15, 19, 42].Unlike the polyester family, amino acid ethyl ester substitutedpolyphosphazenes undergo hydrolysis generating non-toxicand buffering degradation products composedmainly of phos-phate, ammonia, and corresponding side groups [10, 23, 38,43–46]. Therefore, the substitution of both amino acid estergroups and hydrophobic groups into the polyphosphazenebackbone provides a tremendous influence on the requireddegradation pattern for a specific application [4, 7, 15, 18,19, 26, 28, 31, 47, 48]. Furthermore, Laurencin and co-workers have demonstrated that the mechanical properties ofthe PPHOS can be improved by substituting in bulkier sidegroups such as phenylphenoxy into the backbone [10, 23, 28,38, 40, 46, 49].

Despite polyphosphazenes having unique tunability throughtheir side group chemistry, occasionally this is not enough tomeet properties requirements for specific biomedical applica-tions and thus polyphosphazene-based blends have also beenexplored as potential biomaterials [12, 14, 15, 18, 19, 22, 23,26, 28, 29, 36, 38, 40, 42, 47, 50]. Blending technique is a verycost-effective way of producing new polymeric materials withunique properties and without having to go through the rigor-ous synthetic protocol. It involves the synergistic combinationof blend components which offers versatility in optimizing andtailoring properties of the final products, for example, in theblend system of polyetheretherketone/polyetherimide(PEEK/PEI) where PEEK is known for its excellent mechani-cal properties but suffers from a poor modulus at temperatureabove its Tg (145 °C). PEI, on the contrary, possesses higher Tg(215 °C) with lower chemical resistance and mechanical prop-erties than PEEK. Blending PEEK and PEI combines the com-plementary properties of both polymers [51]. Generally, poly-mer blends can be divided into three types depending on thefree energy of mixing: (1) immiscible polymer blends, whichare usually made up of two phases and two glass transitiontemperatures due to the presence of two polymers; and (2)compatible polymer blends which by physical observationhave a single uniform phase due to sufficiently strong interac-tions between the component polymers. However, two differ-ent glass transition temperatures from the individual polymersare observed: (3) miscible blends, which have a single-phasestructure with a single glass transition temperature that corre-sponds to the average of the two Tg’s of the component poly-mers. Depending on applications, properties of interest arefound either in compatible or miscible blends. Both of theseblends exhibit a single phase due to strong interaction betweencomponent polymers [52, 53]. Blending to obtain a compatibleor miscible polymer blend is a complex process that involvesthe act of mixing two high molecular weight polymers withdifferent interfacial energies. Therefore, there is a need forstrong molecular interactions between the polymer chains toallow the fabrication of miscible polymer blends.

Regen. Eng. Transl. Med.

Previous work has been successful in developing misciblePPHOS-PLAGA blends through hydrogen bond interactions[18]. In vitro and in vivo studies on these misciblepolyphosphazene-based blends showed promising prospectsas they were able to support cell adhesion and proliferationand show elevated phenotype expression compared to poly-esters. The pH profiles of the media in which these blendsdegraded were much higher than that of the PLAGA’s as thepolyphosphazene degradation products were able to neutralizethe acidic degradation products from the PLAGA. However,majority of these blend systems do not possess requisite me-chanical properties needed for bone tissue engineering [5, 9,18, 19, 23, 38, 40, 42, 45, 46, 49, 54]. It is still a lingering issueto fabricate completely miscible PPHOS-PLAGA blends withappropriate mechanical properties for bone tissue engineeringapplications. For instance, mechanically endowedphenylphenoxy side group hinders hydrogen bonding neededin forming miscible blends due to its steric bulk [55]. Work onthe development and optimization of biodegradablepolyphosphazenes with high strength and hydrogen bondingability is ongoing. The current work aims to developdipeptide-based PPHOS-PLAGA blends with superior me-chanical strength as PEEK and yet degradable.

In this article, we highlight the importance ofpolyphosphazene-based blends in regenerative engineering,thereby providing fundamental insights into synthetic unique-ness of polyphosphazene. Advances made so far on the devel-opment of these unique biomaterials will be discussed with afocus on dipeptide-based polyphosphazenes and PPHOS-PLAGA blend systems.

Synthesis of Polyphosphazene—Mechanismsof Polymerization

Thermal Ring Opening Polymerization—Bulk Phase

The most widely used route to poly(organo)phosphazenes is viathe precursor poly(dichlorophosphazene) (PDCP) developed byAllcock et al. [8, 33, 43, 56]. Polyphosphazenes are commonlysynthesized via a two-step reaction process starting from the com-mercially available cyclic trimer hexachlorocyclotriphosphazene(HCCTP). The first step involves the synthesis of linear PDCPwhich can be achieved via the controlled thermal ring openingpolymerization of cyclic trimer at 250 °Cunder vacuum [4, 10, 15,21, 31, 36, 38, 39, 41, 43, 48, 55, 57–59]. This reaction is usuallycarried out under vacuum in a sealed glass tube at high tempera-tures, typically 250 °C for several hours. Around this temperature,chlorine atoms in HCCTP can be cleaved to give a cationicphosphazenium species which initiates the ring opening of a sec-ond ring, thus propagating the polymerization (Fig. 1). Efficientcontrol of temperature is important for complete polymerizationsince Cl cleavage is minimal at temperatures far below 250 °C [5,

10, 31, 38, 39, 43, 60]. Significant crosslinking may occur when250 °C is exceeded [10]. Similar to conventional polymerizationreactions, the reaction temperature can be lowered to around200 °C by the addition of a Lewis acid catalyst, most commonlyanhydrous AlCl3. A solution state method that makes use oftrichlorobenzene as a solvent, at 214 °C, with CaSO4·2H2O as apromoter and HSO3 (NH2) as a catalyst, has been reported [5, 8,10, 39, 56, 61, 62]. Another route to poly(dichlorophospazene) isby direct reaction of phosphorus pentachloride (PCl5) and ammo-nium chloride. This method circumvents the use of thehexachlorophosphazene trimer; however, the sublimation ofPCl5 can pose a problem at the high temperatures required forthe ring opening [5, 8, 10, 39, 43, 56, 61].

Ring opening route exhibits high molecular weight andbroad polydispersity due to the uncontrollable Cl cleavage inHCCTP [5, 8, 10, 31, 36, 38, 39, 41, 43, 48, 56, 57, 60–65].Uncontrollable branching is also common in this type of po-lymerization [10, 38, 66].

Living Cationic Polymerization

An alternative approach for the synthesis of macromolecularintermediate is by living cationic polymerization ofphosphoranimines at ambient temperature. This renders con-trol over the molecular weight of the PDCP and narrow poly-dispersities are usually obtained. The reaction mechanism in-volves catalyzed condensation of monomer trichloro(trimethylsilyl)phosphoranimine, (CH3)3Si–N=PCl3 with loss of (CH3)3SiCl(Fig. 2) [10, 31, 39, 41, 60, 62].

Functionalization of the Poly(dichlorophosphazene)Precursor

In the second step, the reactive chlorine atoms on thepoly(dichlorophosphazene) are replaced via macromolecularsubstitution reaction with various organic nucleophiles asshown in Fig. 3 [5, 8, 10, 31, 36, 38, 39, 41, 43, 48, 56, 57,60–65]. A few specialized polyphosphazenes, particularlythose with hydrolytically sensitive groups such as amino acidester side groups, have been developed extensively as matri-ces for bone regeneration.

Macromolecular substitution reactions allow the introduc-tion of different side groups that detect the polymer propertiesin a controlled manner [4, 5, 9, 10, 15, 31, 38, 39, 57, 61, 62,64, 67, 68]. In addition to single side group substitution, mul-tiple substituents can be covalently linked to the polymerbackbone (Fig. 4). For example, different and dissimilar typesof side groups can be substituted on the reactive intermediatepolymer via sequential substitution, allowing the bulkiergroup to react first followed by the smaller reactive group insolution phase during substitution [10, 27, 31–34, 56, 60, 61,69]. The modification of type and ratios of the side chains ofthe polymer affords the ability to fine-tune degradation rates


and physical properties based on these substituents, which isimportant to the synthesis of a biomaterial suitable for tissueengineering and therapeutic delivery [5, 9, 10, 31, 36, 38, 46,70–72]. By macromolecular substitution, over 250 differentside groups have been covalently linked to the backbone viareplacement of the labile chlorine atoms [5, 31, 60].

Biodegradable Polyphosphazene(Bio-functionalized) and its Blends

Most synthetic polymers are stable in water. However, poly-mers that degrade when exposed to aqueous media may beuseful for medical devices as in tissue regeneration matrices orcontrolled drug release. Numerous biodegradablepolyphosphazenes have been synthesized by substitution ofchlorine atoms of the poly(dichlorophospazene) with hydro-lytically labile groups such as amino acid esters, glycolate orlactate ester, imidazole, steroidal residue, and glyceryl [5, 9,10, 12, 30, 31, 38, 44–46, 60, 62, 64, 65, 69, 70, 73–75].Table 1 summarizes the physiochemical properties of someof the representative polyphosphazenes used in biomedicalapplications [5–7, 9, 10, 15, 22, 26–28, 31, 32, 36, 38, 40,45, 48, 50, 64, 67, 74, 76–80]. Bulkier side groups result in ahigher Tg [10, 27, 34, 36, 45, 60, 76, 77].

Amino-based polyphosphazenes constitute the largest andmost researched category of biodegradable polyphosphazenesand were first reported by Allcock and Kugel in 1966 [76].Hydrolytic sensitivity and bio-erosion to non-toxic products

can be observed in polyphosphazenes that contain amino acidester units covalently bonded to the backbone through theiramino terminus [5, 7, 31, 34, 44, 74, 76]. The rate of hydro-lysis generally depends on the hydrophobicity and steric hin-drance of the units located at the α-carbon of the amino acidresidues [9, 18, 27, 31, 38, 42, 45, 64, 76].

The terminal nitrogen in the amino acid ester unit possesseshydrogen-bonding proton. This is available for interactionswith other polymers in polymer blending (Fig. 5). A previousstudy has capitalized on these hydrogen-bonding capabilitiesto blend of polyphosphazenes with poly(lactide-co-glycolide)(PLAGA) [5, 18, 19, 22, 23, 40, 42, 47, 68, 81]. In that work,it was discovered that miscible blends can be formed betweenpolyphosphazenes that contain glycine ethyl ester side groupsand PLGA with a lactic to glycolic acid ratio of 50:50 [81].Miscible blends can also be obtained blending alanine ethylester phosphazene polymers with PLAGA (50:50). However,the presence of the steric methyl group at the α-carbon placessome difficulties on the formation of hydrogen bonding withPLAGA [22, 27, 42, 47, 68, 81].

Furthermore, the presence of both hydrolysis-sensitivegroups and hydrophobic groups in a mixed-substituent poly-mer offers control over the degradation rate (Fig. 6). Previousstudies by Dr. Laurencin and co-workers have reported thatthe combination of a hydrolytically active group and a hydro-phobic group in a 50:50 ratio results in a complete degradationwithin 12–24 weeks for bone tissue engineering [18].

Degradation Mechanisms

During polymer degradation, the organic side groups on thepolyphosphazene are usually the first target. They are attackedby water molecules. After the removal of the side groups fromthe polymer backbone, P–OH units are formed followed by themigration of the proton from oxygen to nitrogen, which sensi-tizes the polymer backbone to hydrolysis. The polymer

P

ClCl

N N

P P

N

Cl

Cl

Cl

Cl

P N **

Cl

Cln

P

ClCl

N N

P P

N Cl

Cl

Cl

P

ClCl

N N

P P

N

ClCl

Cl

Cl

P

ClCl

N N

P P

N Cl

Cl

ClN

P

Cl Cl

NP

N

ClCl

P

Cl Cl

Cl

250oC

Vacuum

250oC Vacuum

Cl

Fig. 1 Synthesis ofpoly(dichlorophospazene):synthetic schematics of ringopening polymerization [10]

Si

CH3

CH3

H3C N P

Cl

Cl

Cl P N **

Cl

Cln

25oC

PCl5

Fig. 2 Alternative route to the synthesis of poly(dichlorophospazene):living cationic polymerization of a phosphoranamine monomer [31]


ultimately degrades into non-toxic degradation products com-prising mainly of ammonia, phosphoric acid, and the corre-sponding side groups [5, 18, 26, 29, 44, 60, 65, 82, 83]. Agenera l ized degradat ion scheme for degradablepolyphosphazenes is presented in Fig. 7. For a mixed-substituent polymer, the first target is usually the most hydrolyt-ically sensitive group. It is broken down first from thepolyphosphazene backbone and is followed by bulkier sidegroups such as p-methylphenoxy, p-phenylphenoxy, tyrosine,or pyrrolidone [9, 34, 44, 60, 77]. The degradation products suchas ammonium phosphate constitute a natural buffer system in thelocal microenvironment both in vitro and in vivo [44, 60, 84].

Allcock and co-workers investigated the effects of estergroups and α-substituents on the hydrolytic sensitivity of ami-no acid ester-substituted polyphosphazenes [76]. Methyl, ethyl,tert-butyl, and benzyl esters of glycine-, alanine-, valine-, andphenylalanine-substituted polyphosphazenes were incubated indeionized water, and the molecular weight changes were mon-itored by gel permeation chromatography analysis. An inverserelationship was observed between the size of the ester groupand hydrolytic sensitivity. The hydrolytic sensitivity of thepolymer increased when the size of the ester groups decreased[34, 38, 44, 45, 60, 76, 77, 82, 85]. Laurencin et al. [27] dem-onstrated the possibility of efficiently modifying the degrada-tion rate of ethyl glycinato-substituted polyphosphazene by

incorporating a hydrophobic co-substituent, such as amethylphenoxy group. Various studies have shown that thechemistry of polyphosphazene synthesis can be fine-tuned tocontrol degradation rate to meet the requirements of a specificbiomedical application [27, 76, 77]. Ambrosio et al. have de-signed a novel biodegradable poly[(50% ethyl glycinato) (50%p-methylphenoxy) phosphazene] (PNEGmPh) blend whosedegradation products were studied and compared withPLAGA. The media in which the degradation of the polymerblends occurred was monitored for pH changes over time. Theblank which consists of medium without any matrix was usedas control. No significant differences were observed in pHchange of the media in which PNEGmPh degradation occurredover a period of 40weeks as compared to the control. However,pH of 2.9 was reached at 6 weeks for the media with PLAGAwhile media from the blend had a pH of 6 at the same time.These data indicated the extent to which the blend controlledthe drop in pH of the matrix degradation media [70].

Structure–Property Relationship of Polyphosphazene and itsBlends

The properties of any polymer are dependent on the structuresof the backbone and the side groups [27, 31–34, 38, 56, 64].High torsional freedom within the phosphorus-nitrogen

Fig. 3 Macromolecularsubstitution of PDCP with a widearray of organic substituentgroups forming single-substituentpolymers (simultaneousreplacement of chlorine atoms) ormixed-substituent polymers(sequential substitution ofchlorine atoms) [8, 31]

Fig. 4 Schematic illustrations ofa mixed-substituentbiodegradable polyphosphazenepoly[(glycine ethylglycinato)1(phenylphenoxy)1phosphazene](PNGEGPhPh) [18]


backbone and a corresponding chain flexibility offer a differentkind of polymer properties that range from elastomers, or filmor fiber-forming polymers, to rigid solids depending on the sidegroups. The glass transition temperature of the polymer isgreatly affected by the restriction on the rotation of the back-bone. Factors such as bulky side group, ionic forces, hydrogenbonding, or coordination interactions decrease molecular mo-bility within the backbone [5, 6, 10, 22, 34, 38, 61]. As a resultof this restriction on rotation, the glass transition temperatureincreases. Thus, side groups can be selected to tune the glasstransition temperature over the range of −100 °C (alkoxy sidegroups) to 200 °C or higher with adamantylamino groups. Thestability of the polymer backbone depends on the side groups[34, 38, 64]. Hydrolytically active side groups attached to thepolyphosphazene backbone can hydrolyze. This tendency,which is useful in tissue regeneration and other biomedicalprocesses, can provide a calculated means of causing the poly-mers to break down into phosphate, ammonia, amino acid, andethanol [31, 38, 44, 45, 62, 64, 76, 85]. On the other hand,

groups such as aryloxy, fluoroalkoxy, and higher alkoxy pos-sess the tendency to shed water which prevents the hydrolysisof the polymer backbone. Therefore, changes in ratios of thetwo side groups of a mixed-substituted PPHOS with bothhydrolysis-sensitizing groups and hydrolysis-retarding groupsoffer considerable opportunity for having control over the rateof degradation [34]. Furthermore, the hydrogen bonding capa-bility of the PPHOS side groups and the compositional changesof the blend in polyphosphazene-polyester system can have aprofound influence on the properties of the polyphosphazene-based blend system. Laurencin and co-workers demonstratedthe high miscibility of polyphosphazenes with glycine sidegroups [22, 80, 86]. In that study, a series of glycyl-glycineethyl ester dipeptide-based PPHOS were synthesized andblended with both PLAGA (50:50) and PLAGA (85:15).There was formation of completely miscible blends with allcompositions of PLAGA due to the increased number of hy-drogen sites in dipeptide-based PPHOS. It was also found thatthe Tg for each blend was lower than that of the parent poly-mers, implying both PPHOS and PLAGA acted as plasticizersfor each other in the blend.

Suitability of Polyphosphazene Blendsas Biomaterials

Biocompatibility

Scaffold for regenerative engineering must be biocompatible;cells must adhere, function normally, and migrate onto the sur-face and eventually through the scaffold and begin to prolifer-ate before laying down new matrix. After implantation, thescaffold or tissue-engineered construct must induce a negligibleimmune reaction in order to prevent it from causing a severeinflammatory response that might reduce healing or cause re-jection by the body [5, 7, 9, 13, 28, 34, 47, 60, 68, 72, 74].

In Vitro Biocompatibility

The biocompatibility of the amino acid ester-substitutedpolyphosphazenes for tissue-engineering applications wasfirst examined by Laurencin and co-workers who comparedrat primary osteoblast adhesion to poly[(ethyl glycinato)phosphazene] (PNEG) with well-known poly(lactic acid-co-glycolic acid) (PLAGA) and poly(anhydrides) [6, 87]. Datafrom this study showed that the osteoblast cells adhered to thePNEGmaterial to the same extent as the control materials for aperiod of 8 h. The results from this study also suggested thatcells responded favorably to polyphosphazene materials, es-pecially those with a high ratio of ethyl glycinato substituentsand that cell adhesion and proliferation characteristics werenot diminished in comparison to tissue culture plate andPLAGA controls. The polymers with 50% and greater of ethyl

Table 1 Illustrations of the effects of side groups on glass transitiontemperatures (Tg’s) and degradation rates of single substituentpolyphosphazenes

Polymer Side group Tg (°C) Degradability

PNEG Glycine −40 Months

PNEA Alanine −10 Months

PNEL Leucine 15.4 Months

PNMV Valine 24.8 Months

PNEPhA Phenylalanine 41.6 Months

PNGEG Glycine dipeptide 56.4 Weeks

PNMEEP Methoxy ethoxy ethoxy −84 Water soluble

PNmPh Methyl phenoxy 2.1 Non-degradable

Table shows the great tunability of physico-chemical properties viamodification of side group chemistry [55]

PNEG poly[bis(glycine ethyl ester)phosphazene], PNEA poly[(ethylalanato)polyphosphazene, PNEL poly[(ethyl leucine)phosphazene,PNMV p o l y ( m e t h o x y v a l i n e ) p h o s p h a z e n e , PNEP h Ap o l y ( p h e n y l a l a n i n e e t h y l e s t e r ) p h o s p h a z e n e , PNGEGp o l y [ ( g l y c i n e e t h y l g l y c i n a t o ) p h o s p h a z e n e , PNMEEPpo ly [b i s (me thoxye t hoxye t hoxy )phosphazen e ] , PNmPhpoly[bis(methylphenoxy)phosphazene]

Fig. 5 Illustration of intermolecular hydrogen bonding network betweenPLAGA and PNGEGPhPh through the glycylglycine dipeptides [18]


glycinato substituents demonstrated improved cell growth incomparison to the tissue culture plate, and the polymer with25% ethyl glycinato substitution was only slightly less effec-tive than the tissue culture plate. However, all of these werebetter than the PLAGA control, which has been widely ac-cepted as a biocompatible material. Studies by Laurencin and

co-workers also demonstrated the tissue biocompatibility ofbiodegradable PPHOS-PLAGA blends [18–20, 29, 47, 75,83, 84]. Nair et al. [36] have developed biocompatible blendsof PNEA and PLAGA (85:15) at two weight ratios of 25:75and 50:50. The two blends were characterized for in vitroosteocompatibility. The blend matrices showed significantlyhigher osteoblast growth rates than pristine PLAGA after14 days. By contrast, the significant decrease of cell numberson PLAGA matrices after 14 days has been attributed to thefast polymer degradation and the subsequent accumulation ofacidic degradation products. Increase in the polyphosphazenecontent in the blend matrix improved cell growth. Also, theALP activity of blend was found to be significantly higherthan PLAGA after 7 days.

In Vivo Biocompatibility

Deng et al. [18] evaluated the in vivo biocompatibility of twoweight ratios of dipeptide-based PPHOS to PLAGA blends(25:75 and 50:50) by using a rat subcutaneous model withPLAGA as a control. The tissue responses for both blendimplants were minimal, which was characterized by the pres-ence of few neutrophils, erythrocytes, and lymphocytes.PLAGA implants were found to be completely absorbed

HNO

HN

O

O

PN* *

HN

O

O

1

1

nHN O

OPN* *O

CH31

1

n

HNO

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OO

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n

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R=Organic groups

PN **

R

R

n

PNEAPhPhPNEGET

PNEGMEEP

PNLGEG

PNEGmPh

Fig. 6 Structures of mixed-substituent PPHOS for coordination with PLAGA. The primary selection criterion for side groups was based on its capacityto form miscible blends with PLAGA through intermolecular hydrogen bonding interactions [15]

Fig. 7 Pathway to the degradation of biodegradable polyphosphazenes [60]


within 7 weeks. Blend tissue compatibility was characterizedby the extent of inflammation and fibrous capsule formationvia hematoxylin and eosin (H&E) and Masson’s trichromestain (TRI). Both blend matrices were found to possess betterhistological responses with thinner fibrous capsules thanPLAGA. Therefore, biodegradable polyphosphazene blendsare attractive candidate biomaterials for biomedical applica-tions such as regenerative engineering, tissue engineering, anddrug delivery (Fig. 8) [9, 18, 74, 84].

Biodegradability

Scaffolds and constructs are not intended as permanent im-plants. The scaffold must therefore be biodegradable so as toallow cells to produce their own extracellular matrix [72]. Thedegradation should result in non-toxic products that can bemetabolized and excreted by the body, with controllable deg-radation kinetics to match the rate of bone healing process(12–24 weeks) [18] so that the newly formed biological sys-tem compensates the mechanical and mass loss of the degrad-ed matrices.

Polyphosphazenes are attractive because they have beenshown to degrade into non-toxic byproducts that are easilymetabolized by the body. In the case of amino acid esterphosphazenes, these hydrolytic degradation products includethe amino acid, the corresponding alcohol of the ester, ammo-nia, and phosphates [34, 38, 44, 45, 76]. Unlike the acidicproducts produced from the hydrolysis of other polymers,the ammonia and phosphates act as a buffering system andprevent fluctuations in pH, which could otherwise be detri-mental to the tissue. Their degradation pattern can also be

modified using the side chain chemistry [20, 22, 38, 44, 60,64].

In Vitro Biodegradation

The self-neutralizing ability with controllable degradation rateis one of the major benefits of the blend system over PLAGA.Deng et al. [50] carried out a study on the degradation ofPPHOS-PLAGA in PBS. In that study, the self-neutralizingability of the blend system was examined by monitoring thepH profile of degradation in PBS. The media was set atpH 7.4, similar to the physiological environment. The blendsystem showed a higher pH as compared to the pristinePLAGA. This suggests that phosphates and ammonia pro-duced from the polyphosphazene degradation can buffer theacidity that stems from the bulk degradation of PLAGA. Thisbuffering effect can further reduce the acidic catalysis of hy-drolysis of PLAGA. Furthermore, degradation over 12 weekswas much slower for blends than the pristine PLAGA (Figs. 9and 10). It was also observed that the increase in thepolyphosphazene composition in the blend resulted in aslower degradation rate. It reaffirms that the polyphosphazenedegradation products were able to neutralize the acidic prod-ucts from PLAGA and, as a result, retard the degradation ofboth PLAGA and polyphosphazene components in the blend.A 12-week in vitro degradation led to 70–80% mass loss[18–20, 23, 40, 50, 70, 86, 88, 89]. Furthermore, by appropri-ately changing the polymer chemistry and adjusting the com-position of the blends, the buffering capacity and the resultingpH environment can be well tuned. The most unique factabout the degradation of the dipeptide PPHOS-PLAGA

Fig. 8 Structures of bioerodiblepolyphosphazene [5]


blends lies in the morphology of the matrix as a function oftime. Figure 9 shows the representative SEM images of blendsduring in vitro degradation as a function of time. The figurespresent the unique three-dimensional porous structuresformed during degradation with pore sizes in the micronrange. It has been found that the size of the microspheresformed as well as the pore diameter can be varied by varyingthe blend composition. These unique in situ created porousstructures, which are structurally similar to sintered micro-sphere matrix, have significant potential for cell in-growthand tissue regeneration [18, 50].

In Vivo Biodegradation

In the in vivo degradation study, there were observed similar-ities in the way molecular weight decreased for both PLAGAand polyphosphazene components in the blend throughout thepost-implantation period, which agrees with the in vitro find-ings. Moreover, 82–87% porosity, which is structurally simi-lar to cancellous bone, was seen after a 12-week in vivo deg-radation [18]. A direct correlation between sphere size and theamount of polyphosphazene in the blend was observed, i.e.,the sphere size increased with PPHOS composition in theblend. On the other hand, compositional changes inpolyphosphazene component of the blend did not seem to

have an effect on the porosity. This is believed to offer anadvantage in maintaining a highly porous structure whilefine-tuning the pore system for better cell infiltration and tis-sue in-growth [90]. For example, image analysis has revealedthat the average pore size increases with the sphere diameter[11, 90, 91]. Additional surface area and space useful for en-hancing cell-material interaction are provided by the intricateporous structure within the polymer spheres. Faster degrada-tion of PLAGA in the blend matrix can facilitate pore forma-tion while the polyphosphazene microspheres maintained thematrix structural integrity during the tissue in-growth. It canbe seen in Fig. 11 that the porous structures start formingalong the edge of the implants. The micrographs (H&E andTRI staining) also revealed significant collagen tissue in-growth in the porous structure indicating the capability ofthe structures to serve as an in situ forming three-dimensional tissue engineering scaffold.

Mechanical Properties

Ideally, the scaffold should be mechanically competent(e.g., compressive modulus in the range of 20–900 MPa)in order to tolerate the local forces [92]. Producing scaf-folds with adequate mechanical properties is one of thegreat challenges in attempting to engineer bone or

Fig. 9 Time-dependent topographical imaging of the polymeric blendsimmersed in aqueous media for 12 weeks at 37 °C. Top row: SEMtopography of Matrix1 at 0, 4, 7, and 12 weeks of time intervals ofin vitro degradation. Spots (e) and (g) show the detailed 3D spherical

structures. Bottom row: SEM topography of Matrix2 under vitrodegradation at 0, 4, 7, and 12 weeks. An assemblage of microsphereswith interconnected porous structures was evident due to the uniquepolymer erosion of the blend system [50]


cartilage. For these tissues, the implanted scaffold musthave sufficient mechanical integrity to function from thetime of implantation to the completion of the remodelingprocess. If the mechanical properties, such as compressivestrength and tensile strength, are not comparable to thoseof natural tissues, problems with mismatch arise whichoften lead to failure of the tissue-engineered construct [5,7, 9, 11, 12, 60, 68, 72, 74, 75, 87, 92]. Studies on me-chanical properties were carried out by Laurencin and co-workers. The mechanical properties of alanine-basedpolyphosphazenes were investigated for their applicationas bone tissue engineering biomaterials. For these studies,polyphosphazenes were compared with the current stan-dard for bone tissue engineering applications, PLAGA(85% lactic acid/15% glycolic acid). The compressivestrengths of PNEA and PNEAmPh were comparable to thatof PLAGA (34.9 ± 5.7 MPa). PNEAPhPh, on the otherhand, had a compressive strength that was significantlyhigher than that of PLAGA due to the presence of thebulky aromatic groups which increases the steric hindranceand decreases the torsion of the polymer backbone.Consequently, these increase the rigidity of the material

and modulate its compressive properties [40]. Similar todegradation properties, it can be noted that the mechanicalproperties of polyphosphazene materials can be tailoredbased on their proposed applications by changing the sideg r o u p s u b s t i t u e n t s . F o r t h e L - a l a n i n e - b a s e dpolyphosphazene materials, it was shown that increasingthe steric bulk of the co-substituent increases both the ten-sile strength and elasticity of the material with more of animpact being observed as the side chain is changed from asmall amino acid such as glycine or alanine ethyl ester inPNEAEG and PNEA, respectively, to large aromatic sub-stituents, such as in PNEAmPh and PNEAPhPh. This isbecause the glass transition temperature and molecularweight of the polymer are affected by the presence of alarge aromatic ring, which in turn affects the mechanicalproperties of the material.

Deng et al. [14, 18, 42, 55] characterized the mechanicalp rope r t i e s o f mixed - subs t i t uen t b iodeg radab lepolyphosphazene poly[(glycine ethyl glycinato)1(phenylphenoxy)1phosphazene] (PNGEGPhPh) and its blends witha polyester (PLAGA–50% lactic acid/50% glycolic acid) andcompared it to the pristine PLAGA. Two dipeptide-based

Fig. 10 Graphs showing a 12-week study of in vitro degradation of theblend systems in pH 7.4 aqueous media at 37 °C. a Comparison of themolecular weight percentages of PLAGA in pristine PLAGA and the twoblend systems after degradation. bMass of the residual systems expressedin percentage. c Comparison of the molecular weight percentage ofpolyphosphazene in the residual blends matrices d pH profiles of the

media during degradation of pristine PLAGA and blend matrices.Slower degradation rate was evident in the blend systems than thepristine PLAGA. The trends for molecular weight changes of thePLAGA and phosphazene reasserted the similarities in the degradationsof the two components [50]


blends, namely, 25:75 (Matrix1) and 50:50 (Matrix2), wereproduced at two different weight ratios of PNGEGPhPh topoly(lactic acid-glycolic acid) (PLAGA). Both blends resultedin higher tensile modulus and strength than the polyester. Itwas revealed that the tensile modulus and ultimate tensilestrength increased significantly with the addition ofpolyphosphazene. The above studies showed that a changein the types and ratios of side group chemistries ofpolyphosphazene materials and blend compositions can havean influence on the mechanical properties [9, 18, 40, 42, 55,60]. Shan et al. [23] have synthesized a cross-linkablepoly(glycine ethyl ester-co-hydroxyethyl methacrylate)phosphazene (PGHP or PNEGHMP) and blended it withPLGA7525 or poly(L-lactide) (PLLA) using chloroform as amutual solvent. The photo-crosslinking was then done beforesolvent removal. The resulting PLGA (or PLLA)/PGHP com-posite films demonstrated no significant phase separation dueto the binding function of the crosslinked PGHP polymericnetwork. To obtain PLGA7525/PGHP porous composite scaf-folds, they used particle leaching/solvent casting with photo-crosslinking treatment performed before solvent evaporation.Both crosslinked PLGA (or PLLA)/PGHP films and scaffoldsdemonstrated a significant improvement in tensile moduli ascompared to the corresponding un-crosslinked blends.

Unique Tunability of Degradation and MechanicalProperties

One concern here is that changing the side groups affects notonly the mechanical properties but also the degradation rates,and therefore modulating the side chains to obtain suitablemechanical properties may alter the degradation rate of thematerial. This is undesirable. Thus, further research is neededinto other methods to control mechanical properties withouthaving an influence on erosion properties. For example, in-vestigation on the effects of different processing methods orscaffold preparation techniques (e.g., electrospinning versussolvent casting and particulate leaching) on mechanical prop-erties might be highly useful [9, 60]. In a sequel to this, Denget al. recently developed a mechanically competent 3D scaf-fold mimicking the bonemarrow cavity and the lamellar struc-ture of bone by orienting electrospun polyphosphazene-polyester blend nanofibers in a concentric manner with anopen central cavity (Fig. 12) [42]. The 3D biomimetic scaffoldexhibited mechanical attributes similar to native bone.Compressive modulus of the scaffold was found to be withinthe range of human trabecular bone. When tuned to havedesired properties, the concentric open macrostructures ofnanofibers that structurally and mechanically mimic the native

Fig. 11 Histological examination reveals the formation of pore systemthat stems from the arrangement of newly formed polymer spheres. Thisis capable of accommodating cell infiltration and tissue in-growth withinthe blend systems. a, b (H&E) The arrows show the formation ofpolymer sphere within Matrix1 and Matrix2 after 7 weeks ofimplantation, respectively; c, d (H&E) The arrows show the formation

of polymer sphere within Matrix1 and Matrix2 after 12 weeks ofimplantation, respectively. After 12 weeks of implantation, strongcollagen tissue infiltration via in situ formed pores within the matrixwas apparent in (c) and (d) with TRI. This reaffirms the availability andenhancement of cell infiltration and collagen tissue in-growth through theformation of in situ 3D interconnected porous structure [50]


bone can be a potential scaffold design for accelerated bonehealing.

Applications of Biodegradable Polyphosphazeneand its Blends

Scaffold-based regenerative engineering represents a clinicaltranslational strategy for tissue regeneration. Biodegradablescaffolds play a crucial role in scaffold-based regenerativeapproach, and their successful application is dependent ontheir inherent properties and compositions [2, 7, 9, 16, 31,60, 72, 75]. So far, a large number of polyphosphazene-based polymer systems have been developed for use in boneand soft tissue regeneration as well as in drug delivery [20,30, 70, 86, 88, 93, 94] (Table 2).

Bone Tissue Regeneration

Bone and skeletal tissue engineering applications have had thelion’s share of the research on biodegradable polyphosphazene. Laurencin and co-workers have extensively studiedthese materials and their interactions with osteoblast type cellsto determine their suitability for bone grafts and implants. Inone of the studies [78], a comparison was made between the3D and 2Dmatrices of amino acid-based polyphosphazene onwhich osteoblast cells were seeded. It was observed that thepores of the 3D constructs resembled, in shape and size, thoseof natural bone tissue, specifically trabecular bone. They alsonoted that the 3D polyphos phazene scaffolds were able topromote osteoblast adhesion and proliferation throughout theentire 21-week period of their study, whereas adhesion to the2D scaffolds was not as effective. Overall, the study was apromising one and showed that polyphosphazene materialswere suitable for bone tissue regeneration.

PLGA is renowned for its suitable compressive strength andsatisfactory in vitro cell performance, while the material compo-sition suffers from lack of bioactivity, uncontrolled bulk degrada-tion, and acidic degradation products. PPHOS-PLAGA blendswill combine the advantages of each parent polymer. Amino acid

ester-substituted polyphosphazenes offer a variety of functionalgroups that allow for controlled degradation into phosphate andammonia, thereby constituting a natural buffer. A series of studieshave been carried out by Deng at el. to evaluate various PPHOS-PLGA blends in the form of thin disks for bone tissue regenera-tion. In one study, the etheric biodegradable polyphosphazenePNEGMEEP was synthesized and blended with PLAGA atweight ratios of 25:75 and 50:50 (PNEGMEEP to PLGA ratio)using a mutual solvent approach [19].

The blend films, although not completelymiscible, were ableto nucleate bone-like apatite, degrade into near-neutral pH prod-ucts, support PRO growth and proliferation, and enhance oste-oblast phenotype expression due to the presence ofpolyphosphazene. In order to improve miscibility, a mixed-substituent biodegradable polyphosphazene PNGEGPhPh wassynthesized and blended with PLGA (50:50 lactide to glycolideratio) at weight ratios of 25:75 and 50:50 [18]. Completelymiscible PNGEGPhPh-PLGA blends with enhanced mechani-cal properties were obtained as a result of strong hydrogenbonding between the glycylglycine dipeptide side groups andPLGA. In vitro studies show significantly higher osteoblastgrowth on the blend films than on the PLGA film.Intriguingly, the novel blend system was able to develop an insitu porous structure, which enabled cell infiltration and colla-gen tissue in-growth throughout the void space between spheresas a result of polymer degradation [50]. The versatility of thisnovel polymer platform is confirmed by the robust tissue growthin the dynamic pore-forming scaffold. This has led to a newstrategy in regenerativemedicine, developing solid matrices thatbalance degradation with tissue formation. Furthermore, intro-duction of extra hydrogen-bonding sites by the incorporation ofdipeptide ester side groups to the polyphosphazene backbonehas resulted in the formulation of a wide range of misciblePPHOS-PLGA blend materials [86]. These biocompatible andmechanically competent blend systems have shown a great po-tential for bone regeneration [22, 84, 88]. Polymeric nanofibrousmatrices due to their similarity to natural extracellular matrix(ECM) are attractive candidates as bone regenerative scaffolds.Several polyphosphazene-based nanofibrous scaffolds havebeen successfully fabricated via an electrospinning process.

Fig. 12 a Electrospun polyphosphazene nanofibrous scaffolds; b SEM-SE image showing the topographical fixtures of 3D biomimetic scaffoldsseeded with cells after 28 days of culture. c Immunohistochemicalstaining for osteopontin (OPN), an important component of the

mineralized ECM, showing a homogenous ECM distributionthroughout the scaffold architecture at day 28. Asterisk (*) indicatesinterlamellar space, whereas double asterisk (**) indicates centralcavity [60]


Deng at el. [42] have designed a mechanically competent 3Dscaffold system using electrospun dipeptide-based PPHOS/PLGA (50:50) nanofibers which were orientated in a concentricmanner with an open central cavity. The biomimetic scaffoldshowed a comparable characteristic mechanical behavior to thatof native bone. Compressive modulus of the scaffold was foundto be within the range of human trabecular bone.

Soft Tissue Regeneration

Besides bone repair, polyphosphazenes have shown greatprospects in the regeneration of other types of tissue such asnerve, vessel, ligament, and tendon. These tissues arecategorized as soft tissues and their injuries account for 50%of the total musculoskeletal injuries reported in the USA peryear. Polyphos phazene materials with improved elastomericproperties are often utilized in this case. Studies on the influ-ence of alkyl ester chain length on mechanical properties anddegradation rates have been reported [9, 23, 27, 34, 37, 38, 40,45, 77, 82, 88, 95, 96]. Nichol et al. [97] investigated theinfluence of changing alkyl ester chain lengths between fiveand eight carbons on mechanical properties and degradationrates of L-alanine and L-phenylalanine alkyl esterpolyphosphazene materials for their application in tendonand ligament tissue engineering. They determined that theglass transition temperatures (Tg) of the materials decreasedwith increasing alkyl ester chain length due to increased flex-ibility of the alkyl side chain and improved elastomeric prop-erties of the polymer. It was also observed that the Tg’s of thephenylalanine materials were higher than those of the alaninecounterparts which is likely due to the increased bulkiness andsteric hindrance of the aromatic side chain that in turn in-creases the rigidity of the overall polymer. In vivo nerve repairstudies were performed using tubes of poly[(ethylalanato)1.4(imidazolyl)0.6phosphazene] (PEIP) to establishcontinuity of a completely lacerated sciatic nerve [98]. Thesescaffolds were found to have degraded extensively at 45 dayspost-implantation, leaving a tissue cable that bridged the lac-erated nerve stumps. The PEIP nerve guides appeared to bemore biocompatible than silicone tubes that are currentlyused, as significantly less scar tissue was induced and a thin-ner fibrotic capsule formed around these guides. Zhang et al.[99] have synthesized a novel electrically conductive and bio-degradable polyphosphazene polymer containing parent ani-line pentamer (PAP) and glycine ethyl ester (GEE) as sidechains for use in nerve tissue engineering. Conductivity isneeded in nerve tissue engineering considering that neuralsignals are propagated along nerve cells via electrical chargesand therefore the polymers that are used to regenerate thesetissues must be capable of transmitting waves of electricity.The polymer was formed into thin films for degradation andbiocompatibility testing. The poly[(glycine ethyl ester) (ani-line pentamer) phosphazene] (PGAP) polymer was shown to

have good electroactivity using cyclic voltammetry measure-ments indicating suitability of the material for propagatingneural signals. Thin films of both PGAP and poly[bis(glycineethyl ester)phosphazene] (PGEE) materials subjected to deg-radation studies in PBS at 37 °C showed a mass loss of ∼50and 70%, respectively, after 70 days. The mass loss of thePGAP was less than that of the PGEE due to the increasedhydrophobicity of the aniline pentamer side chain and there-fore decreased rate of hydrolysis since the hydrophobic sidechains sterically hinder the approach of water towards thebackbone. In vitro cytotoxicity of PGAP to the RSC96Schwann cells was evaluated using the cell viability assayand was compared to a thin film of poly-DL-lactic acid(PDLLA) as a control since it has extensively been shown tobe biocompatible with numerous types of cells. Schwann cellsbeing integral to the peripheral nervous system as supportivecells and neutral signals propagator, RSC96 Schwann cellswere selected in this study. The PGAP exhibited no cytotox-icity and improved cell adhesion in comparison to PDLLA.This work shows the versatility of polyphosphazene-basedmaterials as conductive and biodegradable polymers for nervetissue engineering applications.

Peach et al. [96] investigated the feasibility of usingpoly[(ethyl alanato)1(p-methyl phenoxy)1]phosphazene(PNEA-mPh) to modify the surface of electrospun PCL nano-fiber matrix for their application in tendon tissue engineering.Surface functionalization with PNEA-mPh significantly in-creased the PCL matrix hydrophilicity. An in vitro study wasconducted to examine the effect of surface functionalizationwith PNEA-mPh on human mesenchymal stem cells (hMSC)adhesion, cell construct infiltration, proliferation and tendondifferentiation, as well as long-term cellular construct mechan-ical properties [37]. Functionalized matrices showed a roughsurface morphology and which led to enhanced cell adhesionand infiltration as compared to the uncoated PCL fibers.Furthermore, functionalized matrices supported an enhancedtenogenic differentiation, possessing greater tenomodulin ex-pression and superior phenotypic maturity. Overall, this studywas able to show the in vitro biocompatibility of polyphosphazene-coated materials towards human mesenchymal stemcells as well as their ability to modulate appropriately thecells’ differentiation towards mature tendon cells. To the bestof our knowledge, polyphosphazene blends have not beenutilized much in soft tissue engineering.

Biodegradable Polyphosphazenes in Drug Delivery

Controlled release and local delivery of therapeutic drugs orgrowth factors play a key role in tissue engineering approach.Unlike the polyesters that exhibit bulk erosion and thepolyanhydrides that undergo surface erosion, polyphosphazenes have the unique capability to exhibit bulk, surface,or the combination of both. The side groups attached to the


backbone have a big influence on the type of erosion the poly-mer will undergo [84]. Furthermore, polyphosphazenes aresoluble in a wide range of solvents ranging from aqueous toorganics. These exceptional features, coupled with flexibledegradation kinetics that produces non-toxic degradation prod-ucts, make biodegradable polyphosphazenes attractive candi-dates for delivering a wide variety of drugs and growth factors[35]. Depending on the polyphosphazene used, drug releasefrom these formulations can be achieved either by diffusion(hydrogels) or erosion (hydrophobic) or a combination of both(hydrophilic-hydrophobic) [49, 74, 79, 81, 96, 100, 101].

In a study by Orsolini and co-workers, polyphosphazenespoly[(alanine ethyl ester)]phosphazene and poly[80% (phe-nylalanine ethylester)/20% (imidazolyl)]phos phazene wereused to design membranes and microcapsules for the treat-ment of periodontal diseases. These membranes and micro-capsules were loaded with anti-bacterial and anti-inflammatory drugs used for the treatment of periodontal dis-eases. Degradation rate that matched the healing of bone de-fects was achieved with the polyphosphazene membranes pre-pared with alanine ethyl ester and imidazole in a molar ratio of80:20. Effective healing of the rabbit tibia defects was moreevident in these membranes than the polytetrafluoroethylenemembranes. The release profiles of the two phosphazene poly-mers were suitable for treatment of periodontal diseases [102].

Laurencin et al. have carried out a study on the developmentof a musculoskeletal delivery system for the controlled releaseof anti-inflammatory agent colchicine to joints usingpolyphosphazenes poly[20% (imidazolyl)/80% (p-methylphenoxy)]phosphazene and poly[50% (ethyl glycinato)/50%(p-methylphenoxy)]phosphazene. Colchicine loaded with animidazolyl content of 20% and poly[50% (ethyl glycinato)/50% (p-methoxyphenol)]phosphazene were used to carry outpolymer degradation and drug release studies. Higher rate of

degradation at physiological pH was observed for the 50%ethyl glycinato-substituted polyphos phazene than the onefor 20% for imidazolyl-substituted polymer at physiologicalpH. Over a 21-day period of the study, the release of colchicinewas found to be 20% from the imidazole-substituted polymerand 60% from the ethyl glycinato-substituted polymer. Theresult confirmed the strong agreement between the colchicinerelease profiles from the polymers and the degradation of therespective polymers [103].

As aforementioned, blending can be used to manipulate thedegradation rate and hence impact a drug’s release profile.Schacht et al. performed a study that investigated the degrada-tion of blends that contain two polyphosphazenes with differentrates of degradation as a matrix for controlled drug release. Thesingle-substituent polyphosphazenes that have ethyl glycinateand phenylalanate as side groups as well as their blends wereused to study the in vitro release of the antitumor agent mito-mycin C (MMC) [104]. Results showed a faster release ofMMC from poly[bis(ethylglycinato)]phosphazene than frompoly[bis(phenylalanato)]phosphazene. For their blend systems,a direct correlation exists between the release rate and the com-position of poly[bis(ethylglycinato)]phosphazene. The releaserate increased with increasing ethylglycinato content and de-creased with increasing phenylalanato content. The release pro-file exhibited a diffusion mechanism. The drop in release that isassociated with increase in phenylalanato content may be at-tributed to the presence of a hydrophobic group in the matrix,which would hinder the permeation of water into the matrix.

Conclusion and Future Outlook

Regenerative engineering is a growing and nascent interdisci-plinary field, which puts forward a convergence approach to

Table 2 Summary ofrepresentative polyphosphazene-based blends used in tissueengineering

Polyphosphazene component Blend system

Poly[(ethyl glycinato) phosphazene] (PNEG) PNEG/PLAGA

Poly[(ethyl alanato)polyphosphazene] (PNEA) PNEA/PLAGA

Poly[(glycine ethyl glycinato)(phenylphenoxy)phosphazene] (PNGEG-PhPh) PNGEGPhPh/PLAGA

Poly[(ethyl alanato)(p-phenyl phenoxy)phosphazene] (PNEA-PhPh) PNEAPhPh/PLAGA

Poly[(ethyl glycinato)(p-methylphenoxy)phosphazene] (PNEG-mPh) PNEGmPh/PLAGA

Poly[(ethyl alanato)(p-methylphenoxy)phosphazene] (PNEA-mPh) PNEAmPh/PLAGA

Poly[(ethyl alanato)(ethyl glycinato)phosphazene] (PNEAEG) PNEAEG/PLAGA

Poly[(ethyl glycinato)(hydroxyethylmethacrylate)phosphazene] (PNEGH-mP) PNEGHMP/PLAGA

Poly[(ethyl glycinato)(methoxyethoxyethoxy)phosphazene] (PNEGMEEP) PNEGMEEP/PLAGA

Poly[(ethyl alanato)]phosphazene (PNEA)

Poly[(ethyl phenylalanato)(imidazolyl)]phosphazene (PNEPhAIL)

PNEA/PNEPhAIL

Poly[(ethyl glycinato)(p-methylphenoxy)]phosphazene (PNEG-mPh)

Poly[(imidazolyl)(p-methylphenoxy)]phosphazene (PNIL-mPh)

PNEG-mPh/PNIL-mPh

Poly[bis(ethylglycinato)]phosphazene (PNEG)

Poly[bis(phenylalanato)]phosphazene (PNPhA)

PNEG/PNPhA


create a regenerative toolbox to move beyond individual tis-sue repair to the regeneration of complex tissues and organsystems. It aims to completely regenerate complex tissues andorgan systems such as a knee or a whole limb. Biodegradablepolyphosphazenes represent polymers with high potentials forregenerative engineering. The synthetic flexibility presents awide door of opportunities to producing a library of degrad-able biomaterials with significant prospects for musculoskel-etal tissue regeneration. The substitution of certain groups caninfluence degradation rate and mechanical properties.Polyphosphazene polymers provide a promising matrix plat-form to create miscible blend systems that mimic the nativeextracellular macromolecules. Much progress has been madeon the development of polyphosphazene blends with polyes-ters. The tunability and the pH-buffering effect of thepolyphosphazene hydrolysis products combined with the highstrength of the polyester allows the development of biomate-rials with more useful properties than either of the parentpolymers. Nevertheless, several biological and engineeringchallenges remain for polymeric scaffolds to make a signifi-cant clinical impact for musculoskeletal tissue regeneration.The tissues in the human body are incredibly complex with3D hierarchical structures, anisotropic properties, and cell het-erogeneity. The successful translation of this promising tech-nology requires a further understanding of the scaffold designand close collaboration among chemists, biologists, chemicalengineers, material scientists, and clinicians.

References

1. Laurencin CT, KhanY. Regenerative engineering. Sci Transl Med.2012;4(160):160ed169.

2. Laurencin CT, Nair LS. Regenerative engineering: approaches tolimb regeneration and other grand challenges. Regen Eng TranslMed. 2015;1(1):1–3.

3. Reichert WM, Ratner BD, Anderson J, Coury A, Hoffman AS,Laurencin CT, et al. 2010 Panel on the biomaterials grand chal-lenges. J Biomed Mater Res A. 2011;96(2):275–87.

4. Polyphosphazenes for biomedical applications (1). Hoboken, US:Wiley; 2009.

5. Allcock HR, Morozowich NL. Bioerodible polyphosphazenesand their medical potential. Polym Chem. 2012;3(3):578–90.

6. Laurencin CT, Norman ME, Elgendy HM, El‐Amin SF, AllcockHR, Pucher SR, et al. Use of polyphosphazenes for skeletal tissueregeneration. J Biomed Mater Res. 1993;27(7):963–73.

7. Nair LS, Laurencin CT. Biodegradable polymers as biomaterials.Prog Polym Sci. 2007;32(8–9):762–98.

8. Allcock HR. The synthesis of functional polyphosphazenes andtheir surfaces. Appl Organomet Chem. 1998;12(10–11):659–66.

9. Baillargeon AL, Mequanint K. Biodegradable polyphosphazenebiomaterials for tissue engineering and delivery of therapeutics.Biomed Res Int. 2014;2014:761373.

10. Rothemund S, Teasdale I. Preparation of polyphosphazenes: atutorial review. Chem Soc Rev. 2016.

11. Borden M, Attawia M, Khan Y, El-Amin S, Laurencin C. Tissue-engineered bone formation in vivo using a novel sintered polymer-ic microsphere matrix. Bone Joint J. 2004;86(8):1200–8.

12. Cao H, Kuboyama N. A biodegradable porous composite scaffoldof PGA/β-TCP for bone tissue engineering. Bone. 2010;46(2):386–95.

13. Carampin P, Conconi MT, Lora S, Menti AM, Baiguera S, BelliniS, et al. Electrospun polyphosphazene nanofibers for in vitro ratendothelial cells proliferation. J BiomedMater Res A. 2007;80(3):661–8.

14. Deng M, James R, Laurencin CT, Kumbar SG. Nanostructuredpolymeric scaffolds for orthopaedic regenerative engineering.IEEE Trans NanoBiosci. 2012;11(1):3–14.

15. Deng M, Kumbar SG, Wan Y, Toti US, Allcock HR, LaurencinCT. Polyphosphazene polymers for tissue engineering: an analysisof material synthesis, characterization and applications. SoftMatter. 2010;6(14):3119–32.

16. Jabbarzadeh E, Deng M, Lv Q, Jiang T, Khan YM, Nair LS, et al.VEGF-incorporated biomimetic poly(lactide-co-glycolide)sintered microsphere scaffolds for bone tissue engineering. JBiomed Mater Res B Appl Biomater. 2012;100B(8):2187–96.

17. Li W-J, Laurencin CT, Caterson EJ, Tuan RS, Ko FK. Electrospunnanofibrous structure: a novel scaffold for tissue engineering. JBiomed Mater Res. 2002;60(4):613–21.

18. Deng M, Nair LS, Nukavarapu SP, Jiang T, Kanner WA, Li X,et al. Dipeptide-based polyphosphazene and polyester blends forbone tissue engineering. Biomaterials. 2010;31(18):4898–908.

19. Deng M, Nair LS, Nukavarapu SP, Kumbar SG, Brown JL,Krogman NR, et al . Biomimet ic , b ioact ive ether icpolyphosphazene-poly(lactide-co-glycolide) blends for bone tis-sue engineering. J Biomed Mater Res A. 2010;92(1):114–25.

20. Ibim SE, Ambrosio AM, Kwon MS, El-Amin SF, Allcock HR,Laurencin CT. Novel polyphosphazene/poly(lactide-co-glycolide)blends: miscibility and degradation studies. Biomaterials.1997;18(23):1565–9.

21. Krogman NR, Steely L, Hindenlang MD, Nair LS, Laurencin CT,Allcock HR. Synthesis and characterization of polyphosphazene-block-polyester and polyphosphazene-block-polycarbonate mac-romolecules. Macromolecules. 2008;41(4):1126–30.

22. Krogman NR, Weikel AL, Nguyen NQ, Kristhart KA,Nukavarapu SP, Nair LS, et al. Hydrogen bonding in blends ofpolyesters with dipeptide‐containing polyphosphazenes. J ApplPolym Sci. 2010;115(1):431–7.

23. Shan D, Huang Z, Zhao Y, Cai Q, Yang X. Improving the misci-bility of biodegradable polyester/polyphosphazene blends usingcross-linkable polyphosphazene. Biomed Mater. 2014;9(6):061001.

24. Bostman O, Pihlajamaki H. Clinical biocompatibility of biode-gradable orthopaedic implants for internal fixation: a review.Biomaterials. 2000;21(24):2615–21.

25. Landes CA, Ballon A, Roth C. Maxillary and mandibularosteosyntheses with PLGA and P (L/DL) LA implants: a 5-yearinpatient biocompatibility and degradation experience. PlastReconstr Surg. 2006;117(7):2347–60.

26. Conconi MT, Lora S, Menti AM, Carampin P, Parnigotto PP.In vitro evaluation of poly[bis(ethyl alanato)phosphazene] as ascaffold for bone tissue engineering. Tissue Eng. 2006;12(4):811–9.

27. Nair LS, Bhattacharyya S, Bender JD, Greish YE, Brown PW,Allcock HR, et al. Fabrication and optimization of methylphenoxysubstituted polyphosphazene nanofibers for biomedical applica-tions. Biomacromolecules. 2004;5(6):2212–20.

28. Sethuraman S, Nair LS, El-Amin S, Farrar R, Nguyen MT, SinghA, et al. In vivo biodegradability and biocompatibility evaluationof novel alanine ester based polyphosphazenes in a rat model. JBiomed Mater Res A. 2006;77(4):679–87.


29. Duan S, Yang X, Mao J, Qi B, Cai Q, Shen H, et al.Osteocompatibility evaluation of poly(glycine ethyl ester-co-alanine ethyl ester)phosphazene with honeycomb-patterned sur-face topography. J Biomed Mater Res A. 2013;101(2):307–17.

30. Rothemund S, Aigner TB, Iturmendi A, Rigau M, Husar B,Hildner F, et al. Degradable glycine-based photo-polymerizablepolyphosphazenes for use as scaffolds for tissue regeneration.Macromol Biosci. 2015;15(3):351–63.

31. Allcock HR. The expanding field of polyphosphazene high poly-mers. Dalton Trans. 2016;45(5):1856–62.

32. Allcock H. Phosphorus-nitrogen compounds: cyclic, linear, andhigh polymeric system. Elsevier; 2012

33. Allcock HR. Heteroatom ring systems and polymers. 1967.34. Allcock HR. Chemistry and applications of polyphosphazenes.

Wiley-Interscience; 2003.35. Oredein-McCoy O, Krogman NR, Weikel AL, Hindenlang MD,

Allcock HR, Laurencin CT. Novel factor-loaded polyphosphazenematrices: potential for driving angiogenesis. J Microencapsul.2009;26(6):544–55.

36. Nair LS, Lee DA, Bender JD, Barrett EW, Greish YE, Brown PW,et al. Synthesis, characterization, and osteocompatibility evalua-tion of novel alanine-based polyphosphazenes. J Biomed MaterRes A. 2006;76(1):206–13.

37. Peach MS, James R, Toti US, Deng M, Morozowich NL, AllcockHR, et al. Polyphosphazene functionalized polyester fiber matri-ces for tendon tissue engineering: in vitro evaluation with humanmesenchymal stem cells. Biomed Mater. 2012;7(4):045016.

38. Singh A, Krogman NR, Sethuraman S, Nair LS, Sturgeon JL,Brown PW, et al. Effect of side group chemistry on the propertiesof biodegradable L-alanine cosubstituted polyphosphazenes.Biomacromolecules. 2006;7(3):914–8.

39. Allcock HR. Inorganic–organic polymers. Adv Mater. 1994;6(2):106–15.

40. Sethuraman S, Nair LS, El-Amin S, Nguyen M-T, Singh A,Krogman N, et al. Mechanical properties and osteocompatibilityof novel biodegradable alanine based polyphosphazenes: sidegroup effects. Acta Biomater. 2010;6(6):1931–7.

41. Allcock HR, Crane CA, Morrissey CT, Nelson JM, Reeves SD,Honeyman CH. Manners I: BLiving^ cationic polymerization ofphosphoranimines as an ambient temperature route topolyphosphazenes with controlled molecular weights.Macromolecules. 1996;29(24):7740–7.

42. Deng M, Kumbar SG, Nair LS, Weikel AL, Allcock HR,Laurencin CT. Biomimetic structures: biological implications ofdipeptide-substituted polyphosphazene–polyester blend nanofibermatrices for load-bearing bone regeneration. Adv Funct Mater.2011;21(14):2641–51.

43. Allcock H. Recent advances in phosphazene (phosphonitrilic)chemistry. Chem Rev. 1972;72(4):315–56.

44. Allcock H, Fuller T, Matsumura K. Hydrolysis pathways foraminophosphazenes. Inorg Chem. 1982;21(2):515–21.

45. Allcock HR, Pucher SR, Scopelianos AG. Poly [(amino acid ester)phosphazenes] as substrates for the controlled release of smallmolecules. Biomaterials. 1994;15(8):563–9.

46. Ulery BD, Nair LS, Laurencin CT. Biomedical applications ofbiodegradable polymers. J Polym Sci B Polym Phys.2011;49(12):832–64.

47. DengM,Nair LS, Nukavarapu SP, Kumbar SG, Jiang T, KrogmanNR, et al. Miscibility and in vitro osteocompatibility of biodegrad-able blends of poly [(ethyl alanato)(p-phenyl phenoxy)phosphazene] and poly (lactic acid-glycolic acid). Biomaterials.2008;29(3):337–49.

48. Heyde M, Moens M, Van Vaeck L, Shakesheff KM, Davies MC,Schacht EH. Synthesis and characterization of novelpoly[(organo)phosphazenes] with cell-adhesive side groups.Biomacromolecules. 2007;8(5):1436–45.

49. Veronese F, Marsilio F, Caliceti P, De Filippis P, Giunchedi P, LoraS. Polyorganophosphazene microspheres for drug release: poly-mer synthesis, microsphere preparation, in vitro and in vivonaproxen release. J Control Release. 1998;52(3):227–37.

50. Deng M, Nair LS, Nukavarapu SP, Kumbar SG, Jiang T, WeikelAL, et al. In situ porous structures: a unique polymer erosionmechanism in biodegradable dipeptide-based polyphosphazeneand polyester blends producing matrices for regenerative engi-neering. Adv Funct Mater. 2010;20(17):2743–957.

51. Ramani R, Alam S. Composition optimization of PEEK/PEI blendusing model-free kinetics analysis. Thermochim Acta.2010;511(1–2):179–88.

52. Utracki L, Favis B. Polymer alloys and blends, vol. 4. New York:Marcel Dekker; 1989.

53. Parameswaranpillai J, Thomas S, Grohens Y. Polymer blends:state of the art , new challenges, and opportunit ies.Characterization of polymer blends: miscibility, morphology andinterfaces. 2014. p. 1–6.

54. Zhang QS, Yan YH, Li SP, Feng T. Synthesis of a novel biode-gradable and electroactive polyphosphazene for biomedical appli-cation. Biomed Mater. 2009;4(3):035008.

55. Deng M. Novel biocompatible polymeric blends for bone regen-eration: material and matrix design and development. 2010.

56. Allcock HR. Polyphosphazenes: new polymers with inorganicbackbone atoms. Science (New York, NY). 1976;193(4259):1214–9.

57. Allcock HR, Ambrosio AM. Synthesis and characterization ofpH-sensitive poly (organophosphazene) hydrogels. Biomaterials.1996;17(23):2295–302.

58. Chaubal MV, Gupta AS, Lopina ST, Bruley DF. Polyphosphatesand other phosphorus-containing polymers for drug delivery ap-plications. Crit Rev Ther Drug Carrier Syst. 2003;20(4):295–315.

59. Morozowich NL, Modzelewski T, Allcock HR. Synthesis ofphosphonated polyphosphazenes via two synthetic routes.Macromolecules. 2012;45(19):7684–91.

60. Kumbar S, Laurencin C, Deng M. Natural and synthetic biomed-ical polymers. Newnes; 2014.

61. Allcock HR. Recent developments in polyphosphazene materialsscience. Curr Opinion Solid State Mater Sci. 2006;10(5–6):231–40.

62. Allcock HR. Expanding options in polyphosphazene biomedicalresearch. In: Polyphosphazenes for biomedical applications.Wiley; 2008. p. 15–43.

63. Henke H, Wilfert S, Iturmendi A, Brüggemann O, Teasdale I.Branched polyphosphazenes with controlled dimensions. JPolym Sci A Polym Chem. 2013;51(20):4467–73.

64. Krogman NR, Weikel AL, Kristhart KA, Nukavarapu SP, DengM, Nair LS, et al. The influence of side group modification inpolyphosphazenes on hydrolysis and cell adhesion of blends withPLGA. Biomaterials. 2009;30(17):3035–41.

65. Tian Z, Zhang Y, Liu X, Chen C, Guiltinan MJ, Allcock HR.Biodegradable polyphosphazenes containing antibiotics: synthe-sis, characterization, and hydrolytic release behavior. PolymChem. 2013;4(6):1826–35.

66. Andrianov AK, Svirkin YY, LeGolvan MP. Synthesis and biolog-ically relevant properties of polyphosphazene polyacids.Biomacromolecules. 2004;5(5):1999–2006.

67. Cohen S, Bano MC, Cima LG, Allcock HR, Vacanti JP, VacantiCA, et al. Design of synthetic polymeric structures for cell trans-plantation and tissue engineering. ClinMater. 1993;13(1–4):3–10.

68. Deng M, Nair LS, Krogman NR, Allcock HR, Laurencin CT.Biodegradable polyphosphazene blends for biomedical applica-tions. In: Polyphosphazenes for biomedical applications. Wiley;2008. p. 139–154.


69. Stone DA, Allcock HR. A new polymeric intermediate for thesyn thes i s o f hyb r id ino rgan i c–o rgan i c po lymer s .Macromolecules. 2006;39(15):4935–7.

70. Ambrosio AMA, Allcock HR, Katti DS, Laurencin CT.Degradable polyphosphazene/poly(α-hydroxyester) blends: deg-radation studies. Biomaterials. 2002;23(7):1667–72.

71. Morozowich NL, Weikel AL, Nichol JL, Chen C, Nair LS,Laurencin CT, et al. Polyphosphazenes containing vitamin sub-stituents: synthesis, characterization, and hydrolytic sensitivity.Macromolecules. 2011;44(6):1355–64.

72. O’Brien FJ. Biomaterials & scaffolds for tissue engineering.MaterToday. 2011;14(3):88–95.

73. Andrianov AK. Water-soluble polyphosphazenes for biomedicalapplications. J Inorg Organomet Polym Mater. 2006;16(4):397–406.

74. Lakshmi S, Katti DS, Laurencin CT. Biodegradablepolyphosphazenes for drug delivery applications. Adv DrugDeliv Rev. 2003;55(4):467–82.

75. Nukavarapu SP, Kumbar SG, Allcock HR, Laurencin CT.Biodegradable polyphosphazene scaffolds for tissue engineering.In: Polyphosphazenes for biomedical applications. Wiley; 2008.p. 117–138.

76. Allcock H, Fuller T, Mack D, Matsumura K, Smeltz KM.Synthesis of poly[(amino acid alkyl ester) phosphazenes].Macromolecules. 1977;10(4):824–30.

77. Allcock HR, Pucher SR. Polyphosphazenes with glucosyl andmethylamino, trifluoroethoxy, phenoxy, or (methoxyethoxy) eth-oxy side groups. Macromolecules. 1991;24(1):23–34.

78. Laurencin CT, El‐Amin SF, Ibim SE,Willoughby DA, AttawiaM,Allcock HR, et al . A highly porous 3 ‐dimensionalpolyphosphazene polymer matrix for skeletal tissue regeneration.J Biomed Mater Res. 1996;30(2):133–8.

79. Modzelewski T, Wonderling NM, Allcock HR. Polyphosphazeneelastomers containing interdigitated oligo-p-phenyleneoxy sidegroups: synthesis, mechanical properties, and x-ray scatteringstudies. Macromolecules. 2015;48(14):4882–90.

80. Weikel AL, Krogman NR, Nguyen NQ, Nair LS, Laurencin CT,Allcock HR. Polyphosphazenes that contain dipeptide sidegroups: synthesis, characterization, and sensitivity to hydrolysis.Macromolecules. 2009;42(3):636–9.

81. Weikel AL, Owens SG, Morozowich NL, Deng M, Nair LS,Laurencin CT, et al. Miscibility of choline-substitutedpolyphosphazenes with PLGA and osteoblast activity on resultingblends. Biomaterials. 2010;31(33):8507–15.

82. Adibi S. Glycyl-dipeptides: new substrates for protein nutrition. JLab Clin Med. 1989;113(6):665–73.

83. El-Amin SF, Kwon MS, Starnes T, Allcock HR, Laurencin CT.The biocompatibility of biodegradable glycine containingpolyphosphazenes: a comparative study in bone. J InorgOrganomet Polym Mater. 2006;16(4):387–96.

84. Kumbar SG, Bhattacharyya S, Nukavarapu SP, Khan YM, NairLS, Laurencin CT. In vitro and in vivo characterization of biode-gradable poly(organophosphazenes) for biomedical applications. JInorg Organomet Polym Mater. 2006;16(4):365–85.

85. Allcock HR, Pucher SR, Scopelianos AG. Poly[(amino acid ester)phosphazenes]: synthesis, crystallinity, and hydrolytic sensitivityin solution and the solid state. Macromolecules. 1994;27(5):1071–5.

86. Krogman NR, Singh A, Nair LS, Laurencin CT, Allcock HR.Miscibility of bioerodible polyphosphazene/poly(lactide-co-glycolide) blends. Biomacromolecules. 2007;8(4):1306–12.

87. Laurencin C, Morris C, Pieere-Jaques H, Schwartz E, Zou L. Thedevelopment of bone bioerodible polymer composites for skeletal

tissue regeneration: studies of initial cell adhesion and spread.Tran Orthop Res Soc. 1990;36:383.

88. Nair LS, Bender JD, Singh A, Sethuraman S, Greish YE, BrownPW, et al. Biodegradable poly[bis (ethyl alanato) phosphazene]-poly ( lact ide-co-glycol ide) blends: miscibi l i ty andosteocompatibility evaluations. In: MRS Proceedings.Cambridge Univ Press; 2004. Y9. 7.

89. Deng M, Nair LS, Nukavarapu SP, Kumbar SG, Jiang T, WeikelAL, et al. In situ porous structures: a unique polymer erosionmechanism in biodegradable dipeptide‐based polyphosphazeneand polyester blends producing matrices for regenerative engi-neering. Adv Funct Mater. 2010;20(17):2794–806.

90. Borden M, Attawia M, Khan Y, Laurencin CT. Tissue engineeredmicrosphere-based matrices for bone repair: design and evalua-tion. Biomaterials. 2002;23(2):551–9.

91. Borden M, El-Amin SF, Attawia M, Laurencin CT. Structural andhuman cellular assessment of a novel microsphere-based tissueengineered scaffold for bone repair. Biomaterials. 2003;24(4):597–609.

92. Thavornyutikarn B, Chantarapanich N, Sitthiseripratip K, ThouasGA, Chen Q. Bone tissue engineering scaffolding: computer-aided scaffolding techniques. Prog Biomater. 2014;3(2):61–102.

93. Qiu LY. In vitro and in vivo degradation study on novel blendscomposed of polyphosphazene and polyester or polyanhydride.Polym Int. 2002;51(6):481–7.

94. Qiu LY, Zhu KJ. Novel blends of poly[bis (glycine ethyl ester)phosphazene] and polyesters or polyanhydrides: compatibility anddegradation characteristics in vitro. Polym Int. 2000;49(11):1283–8.

95. Ambrosio AM, Sahota JS, Runge C, Kurtz SM, Lakshmi S,Allcock HR, et al. Novel polyphosphazene-hydroxyapatite com-posites as biomaterials. IEEE EngMed BiolMag. 2003;22(5):18–26.

96. Peach MS, Kumbar SG, James R, Toti US, Balasubramaniam D,DengM, et al. Design and optimization of polyphosphazene func-tionalized fiber matrices for soft tissue regeneration. J BiomedNanotechnol. 2012;8(1):107–24.

97. Nichol JL, Morozowich NL, Allcock HR. Biodegradable alanineand phenylalanine alkyl ester polyphosphazenes as potential liga-ment and tendon tissue scaffolds. Polym Chem. 2013;4(3):600–6.

98. Langone F, Lora S, Veronese FM, Caliceti P, Parnigotto PP,Valenti F, et al. Peripheral nerve repair using a poly (organo)phosphazene tubular prosthesis. Biomaterials. 1995;16(5):347–53.

99. Zhang Q, Yan Y, Li S, Feng T. The synthesis and characterizationof a novel biodegradable and electroactive polyphosphazene fornerve regeneration. Mater Sci Eng C. 2010;30(1):160–6.

100. Modzelewski T, Wilts E, Allcock HR. Elastomericpolyphosphazenes with phenoxy-cyclotriphosphazene sidegroups. Macromolecules. 2015;48(20):7543–9.

101. Xu J, Zhu X, Qiu L. Polyphosphazene vesicles for co-delivery ofdoxorubicin and chloroquinewith enhanced anticancer efficacy bydrug resistance reversal. Int J Pharm. 2016;498(1–2):70–81.

102. Veronese FM, Marsilio F, Lora S, Caliceti P, Passi P, Orsolini P.Polyphosphazene membranes and microspheres in periodontaldiseases and implant surgery. Biomaterials. 1999;20(1):91–8.

103. Ibim SM, El-Amin SF, Goad ME, Ambrosio AM, Allcock HR,Laurencin CT. In vi t ro release of colchicine usingpoly(phosphazenes): the development of delivery systems formusculoskeletal use. Pharm Dev Technol. 1998;3(1):55–62.

104. Schacht E, Vandorpe J, Lemmouchi Y, Dejardin S, Seymour L.Degradable polyphosphazenes for biomedical applications.Frontiers in biomedical polymer applications of polymers.Lancaster PA: Technomic; 1998. p. 7–42.


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