The Effect of Negative Poisson's Ratio Polyurethane Scaffolds for Articular Cartilage Tissue

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2013 Article ID 853289 5 pageshttpdxdoiorg1011552013853289

Research ArticleThe Effect of Negative Poissonrsquos Ratio Polyurethane Scaffoldsfor Articular Cartilage Tissue Engineering Applications

Yeong Jun Park and Jeong Koo Kim

Department of Biomedical Engineering Graduate School Inje University Gimhae 621-749 Republic of Korea

Correspondence should be addressed to Jeong Koo Kim jkkiminjeackr

Received 16 May 2013 Accepted 21 August 2013

Academic Editor Hamdy Doweidar

Copyright copy 2013 Y J Park and J K Kim This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

An auxetic polyurethane (PU) scaffold was prepared to investigate chondrocyte proliferation under compressive stimulation forcartilage regeneration To give a negative Poissonrsquos ratio to the PU scaffold volumetric compression with a 3 1 ratio was appliedduring heat treatment For the control PU scaffold the Poissonrsquos ratio was 09 plusmn 025 with elongation at 20 of the strain rangePoissonrsquos ratio for experimental specimens was approximately minus04 plusmn 012 under the same conditions In cell proliferation tests cellswere cultivated within the prepared scaffold under compression with a 20 strain range With a 20 strain range elongation thecompressive load was approximately 03N The experimental group showed a 13 times higher cellular proliferation rate than thatof the control group after 3 days in culture At day 5 of culture however the rate of proliferation of the control group increased sothat there was no significant difference between groups However collagen content (produced by the cells) in the cell-proliferatedmedium was 15 times higher in the experimental group after 5 days in culture This may have been due to the effectiveness of theauxetic structure of the scaffold An isotropic compressive load was transmitted to the cells due to the negative Poisson ratio of thescaffold

1 Introduction

Bone and cartilage generation by autogenous celltissuetransplantation is one of the most promising techniques inorthopedic surgery and biomedical engineering [1] Due tothe inability of avascular and aneural tissues to self-healeven minor cartilage defects can result in mechanical jointinstability and progressive damage and cartilage damageis notoriously difficult to treat and cure [2] Autologouscell-based tissue engineering using three-dimensional (3D)porous scaffolds has provided an option for the repair of fullthickness defects in adult cartilage tissue [3] The scaffoldor 3D construct provides the necessary support for cells toproliferate and maintain their differentiated function and itsarchitecture defines the ultimate shape of the new bone andcartilage [4]

All these natural and artificial materials for 3D scaffoldshave a convex cell shape and exhibit a positive Poissonrsquosratio which is defined as the negative of the lateral straindivided by the longitudinal strain when a load is applied in

the longitudinal direction [5] On the other hand materialsor structures that contract in the transverse direction underuniaxial compression or expand laterally when stretched aresaid to have negative Poissonrsquos ratios [6 7]This phenomenonis the opposite of the behavior of a positive Poissonrsquos ratiodue to the effect of the material structure For negativePoissonrsquos ratio material the structures are commonly knownas auxetic structures which are similar to a reentrant hon-eycomb structure [8] This reentrant shape is fabricatedby transforming a conventional honeycomb shape into theconcave structure that finally produces the auxetic structureAs shown in Figure 1 the reentrant structure unfolds undertension and gives rise to a negative Poissonrsquos ratio Usuallyan allowable range of Poissonrsquos ratio is minus10 to 05 based onthe thermodynamic consideration of strain energy in thetheory of elasticity [9] In the last two decades the behaviorof the negative Poissonrsquos ratio was predicted discovered ordeliberately introduced in various materials such as foamand microstructure polymers [10 11] This sort of materialis expected to have interesting mechanical properties such

2 Advances in Materials Science and Engineering

x

y

(a)

x

y

(b)

Figure 1 Behaviour of positive Poissonrsquos ratio and negative Poissonrsquosratio when subjected to tensile loading The pores have a roundershape (a) while in (b) the pore space is larger giving rise to anegative Poissonrsquos ratio [11]

as high energy absorption fracture toughness indentationresistance and enhanced shear moduli which may be usefulin some applications [6]

Auxetic materials are known to exhibit various enhancedphysical characteristics over their conventional counter-parts [10] They show increased indentation resistance andimproved acoustic damping propertiesThese enhanced char-acteristics make negative Poissonrsquos ratio materials performbetter in many practical applications however althoughthese enhanced properties have been studied in industrialand academic fields there have been few studies in tissueengineering Fozdar et al reported that there are possibilitiesto fabricate tunable negative Poissonrsquos ratio scaffold by digitalmicromirror device projection printing for tissue engineeringapplications [12]

In cartilage tissue engineering a focus has been onthe relationship between chondrocyte response and physicalproperties of the scaffold (ie stiffness hydrophilicity) How-ever knowledge is lacking about chondrocyte behavior withmechanical stimulation in a negative Poissonrsquos ratio scaffoldMechanical stimulation associated with normal body func-tions is crucial in properly reforming articular cartilage withtissue engineering [2]

In this study we form an auxetic structural polyurethane(PU) scaffold for cartilage regeneration and investigate chon-drocyte proliferation effectiveness within the auxetic scaffoldunder mechanical (compression) stimulation

2 Materials and Methods

21 Specimen (Scaffold) Preparation PU foamwas purchasedfrom Kumsung Sponge Co (Seoul Republic of Korea)

Volume ratioV0 V1 = 25 sim 3 1

Scaffold (V0) Compressed scaffold (V1)z

y

x

Figure 2 Schematic diagram of the preparation of an auxeticscaffold (compressive volume ratio = 25sim 30)

PU foams were treated with 70 ethanol for 30min forsterilization

Control specimens were prepared with 60 ppi PU foamPore size and shape of experimental specimens were con-trolled by heating and compression A metal mold withdimensions of 5 cm times 5 cm times 75 cm was used to preparespecimens To materialize the auxetic structure the desiredvolumetric ratio was estimated as 30 based on a predeter-mined calculation A schematic diagram of the process ispresented in Figure 2The initial sample size of the PU samplewas cut to approximately 8 cm times 8 cm times 9 cm giving aninitial volume to final volume ratio of 1 3 after applyingcompressionThe PU foamwas then placed carefully into themold using a tongue depressor to prevent wrinklesThe foamwas pulled and adjusted at each end to ensure that it fit themold exactlyThemetallic mold was then placed in the centerof a furnace (preheated to 150∘C) for 30min at 200∘C undertriaxial compression in order to produce a reentrant structureof the PU foam Then the mold was cooled slowly to roomtemperatureThemold containing the PU foamwas removedfrom the furnace and the heat-treated PU foamwas removedfrom the mold and cut into small cylindrical shapes

22 Estimation of Poissonrsquos Ratio Poissonrsquos ratio was esti-mated by image processing [13 14] A digital microscope(BX51 Olympus Corporation Tokyo Japan) was used tomeasure Poissonrsquos ratio of the specimens Compressive load-ing was applied to the specimens with a material testingmachine (LRX-PLUS Lloyd Instruments West Sussex UK)The experimental setupwith themicroscope for capturing thespecimen while applying a load to measure the displacementof the specimen is shown in Figure 3The relative positions ofpointed marks were used to estimate the strain and Poissonrsquosratio Images were processed with Photoshop 70 For eachspecimen five measurements were taken from a 005 to025 strain range as shown in Figure 4 In-built tools of thedigital microscope were used to measure the 119909-axis and 119910-axis displacements which were then calculated using (1) todetermine Poissonrsquos ratio

Advances in Materials Science and Engineering 3

Figure 3 Experimental setup with microscope for capturing thespecimen while applying a load to measure the displacement of thespecimen with the points (A to D)

Compression to 0 5 10 15 20 25

Take a photo

Strain

Figure 4 Strain intervals while applying load for taking photos tomeasure displacement of the prepared specimens

In order to investigate pore shapes and distributiona cross section of the scaffold was observed by ScanningElectron Microscope (S-4300DSE HITACHI Tokyo Japan)

We have

120576119909=

|119860 minus 119861| minus10038161003816100381610038161198600minus 1198610

100381610038161003816100381610038161003816100381610038161198600minus 1198610

1003816100381610038161003816

120576119910=


100381610038161003816100381610038161003816100381610038161198620minus 1198630

1003816100381610038161003816

] = minus120576119909

120576119910

(1)

where 120576119909= strain of 119909-axis 120576

119910= strain of 119910-axis ] = material

Poissonrsquos ratio1198600to1198630= initial point (load = 0) and119860 to119863

= moved point under load

23 Chondrocyte Isolation and Culture Porcine cartilage washarvested from the lateral and medial condyle of 6-month-old pig hinge legs The obtained cartilage was choppedinto small pieces with sterilized scissors The cartilage wasdigested for 4 h in Dulbeccorsquos Modified Eagle Medium(DMEMGibco) containing 2mgmL collagenase and filteredthrough a nylon sieve cell strainer (Falcon New JerseyUSA) having an 80120583m pore size The cells were rinsedwith phosphate buffered saline (PBS) solution (Lonza Walk-ersville USA) and then centrifuged at 1500 rpm for 5minThe centrifuged cell pellet was washed with PBS and thenumber of cells was determined by using a hemocytometerChondrocytes were then cultured in cell culture flasks withDMEM supplemented with 10 fetal bovine serum (Gibco)

and 1 antibiotics (Gibco) in a humidified incubator with 5CO2at 37∘CThe media were exchanged every 3 days

24 Cell Proliferation Estimation For sterilization of thespecimens in order to culture the prepared cells all specimenswere incubated with 70 ethanol for 1 h followed by washingthree times with PBS solution For cell seeding second tothird passage chondrocytes were used An initial cell densityof 20 times 105 (cellsspecimen) was seeded in a humidifiedincubator containing 5 CO

2at 37∘C Cells were cultured in

the prepared scaffold with static compression (compressionwith 20 strain) The exerted static load from 20 strainelongation of the prepared scaffold was about 03N Thecellular proliferation rate was measured by using the CellCountingKit (CCK-8 DojindoMolecular Technologies IncMaryland USA) and the Sircol collagen assay kit (BiocolorAntrim UK) Experiments were observed on days 1 3 and5 The absorbances of the CCK-8 solution (DMEM CCKsolution = 10 1) and Sircol collagen assay solution weremeasured by using a Fluorescence Multi-Detection Reader(Synergy HT BIO-TEK Instruments Inc Vermont USA) at450 nm and 550 nm respectively

25 Statistical Analysis Data were expressed as mean plusmnstandard error for all comparisons A 119905-test was used toevaluate differences between groups (119875 lt 005)

3 Results and Discussion

A PU scaffold with a negative Poissonrsquos ratio was formed bycreating a reentrant cell structure inside the foam The origi-nal microstructure of the PU foam is shown in Figure 5(a)the pores are well distributed and uniform in shape andsize For formation of negative Poissonrsquos ratio scaffolds acompressive load was applied with thermal treatment thisresulted in the transformation of the microstructure froma convex cell shape to a deformed cell shape as shown inFigure 5(b) The cell shapes were deformed and generatednew bonding and debonding between the cells [15] As aresult Poissonrsquos ratio of the control group was estimatedas 09 plusmn 025 on average with 20 strain elongation viacompressive loading In contrast the experimental group hada Poissonrsquos ratio of approximately minus04 plusmn 012 with 20 strainelongation via compressive loading (Figure 6)

In the cellular proliferation test the experimental grouphad a 13 times higher cellular proliferation rate comparedto the control group 3 days after cell seeding At day 5 inculture the experimental group had a 12 times higher cellularproliferation rate but there was no significant differencebetween groups as shown in Figure 7 However results ofthe Sircol collagen assay showed that the amount of collagenproduced by the cells after 3 and 5 days in culture was about15 times higher in the experimental group (119875 lt 005) asshown in Figure 8This is an important finding because colla-gen production is essential for proliferated cells to constructextracellular matrix in tissue engineeringThe results suggestthat isotropic compressive stimulation occurred due to theauxetic structure of the scaffold and isotropic mechanicalstimulation was effectively transmitted to the cells through


(a)

(a)

(b)

(b)

Figure 5 Microstructures of polyurethane scaffolds (a) a control specimen and (b) an experimental specimen

minus15

minus1

minus05

0

05

1

15

0 005 01 015 02 025

Poiss

onrsquos

ratio

Compressive strain

ControlExperimental

lowast

lowast

lowast

lowast

Figure 6 Poissonrsquos ratio variation of prepared polyurethane scaf-folds with strain ranges with compressive loading (lowast119875 lt 005 119899 = 5)

the scaffold Isotropic compressive stimulation was favorablefor chondrocyte proliferation and collagen production Theresults indicate that isotropic compressive stimulation couldbemore effective on collagen production during chondrocyteproliferation which is a good indicator for the constructionof extracellular matrix as well as cartilage tissue and thatchondrocytes preferentially proliferate on a negative Poissonrsquosratio PU scaffold rather than a conventional scaffold becauseof isotropic mechanical stimulation

The cellular proliferation decrease after 5 days in culturecould be affected by viscoelastic properties of the scaffoldTheapplied load to the scaffold would be transmitted sufficientlyto the cells during the initial 3 days however on day 5 thecompressive load should be decreased due to stress relaxationof the PU scaffold Hence it is inferred that the increase incellular proliferation decreased at day 5 compared with theinitial 3-day cultivation period

1 3 50

20

40

60

80

100

120

140

160

Cel

lula

r pro

lifer

atio

n ra

te (

)

ControlExperimental

Time (day)

lowast

Figure 7 Chondrocyte proliferation rate on polyurethane scaffoldsValues were normalized (119899 = 5 lowast119875 lt 005)

4 Conclusions

It is well known that physical andor chemical stimulationis effective for cellular proliferation during cell cultivationWe used PU foam to investigate the formation of an auxeticstructural scaffold and the effectiveness of cell proliferationin the scaffold with mechanical stimulation With compres-sive load stimulation the auxetic PU scaffold successfullytransferred the compressive load isotropically to the cellsHence the auxetic scaffold could be effective for chondrocyteproliferation with compressive load stimulation From thisstudy we confirmed that mechanical stimulation is effectivefor chondrocytes proliferation and that isotropic stimulationcould bemore effective for collagen production from the cells

Biodegradable polymeric scaffolds are widely used intissue engineering but there have been few studies relatedto auxetic biodegradable scaffolds Hence we suggest that


020406080

100120140160180200220240260280300

1 3 5

Relat

ive c

olla

gen

amou

nt (

)

Time (day)

ControlExperimental

lowast

lowast

Figure 8 Relative amount of collagen within the PU scaffold duringcell cultivation Values were normalized (119899 = 5 lowast119875 lt 005)

biodegradable polymeric scaffolds should be considered forforming an auxetic structure as well as for isotropic mechan-ical stimulation in cartilage tissue engineering

Acknowledgment

This work was supported by a 2013 Inje University researchgrant

References

[1] CW Patrick Jr A GMikos and L VMcIntire ldquoProspectus oftissue engineeringrdquo in Frontiers in Tissue Engineering pp 3ndash14Elsevier Science New York NY USA 1998

[2] Y M Jung M S Park J W Lee et al ldquoCartilage regenerationwith highly-elastic three-dimensional scaffolds prepared frombiodegradable poly(l-lactide-co-120576-caprolactone)rdquo Biomaterialsvol 29 no 35 pp 4630ndash4636 2008

[3] Y Wang D J Blasioli H J Kim H S Kim and D L KaplanldquoCartilage tissue engineering with silk scaffolds and humanarticular chondrocytesrdquo Biomaterials vol 27 no 25 pp 4434ndash4442 2006

[4] D W Hutmacher ldquoScaffolds in tissue engineering bone andcartilagerdquo Biomaterials vol 21 no 24 pp 2529ndash2543 2000

[5] J B Choi and R S Lakes ldquoAnalysis of elastic modulus ofconventional foams and of re-entrant foam materials with anegative Poissonrsquos ratiordquo International Journal of MechanicalSciences vol 37 no 1 pp 51ndash59 1995

[6] H Wan H Ohtaki S Kotosaka and G Hu ldquoA study ofnegative Poissonrsquos ratios in auxetic honeycombs based on a largedeflection modelrdquo European Journal of Mechanics ASolids vol23 no 1 pp 95ndash106 2004

[7] B Brandel and R S Lakes ldquoNegative Poissonrsquos ratio polyethy-lene foamsrdquo Journal of Materials Science vol 36 no 24 pp5885ndash5893 2001

[8] D Prall and R S Lakes ldquoProperties of a chiral honeycombwith a Poissonrsquos ratio of -1rdquo International Journal of MechanicalSciences vol 39 no 3 pp 305ndash314 1997

[9] H Ohtaki G Hu Y Nagasaka and S Kotosaka ldquoAnalysisof negative Poissonrsquos ratios of re-entrant honeycombsrdquo JSMEInternational Journal A vol 47 no 2 pp 113ndash121 2004

[10] J N Grima R Gatt N Ravirala A Alderson and K E EvansldquoNegative Poissonrsquos ratios in cellular foam materialsrdquoMaterialsScience and Engineering A vol 423 no 1-2 pp 214ndash218 2006

[11] W Yang Z M LiW Shi B H Xie andM B Yang ldquoReview onauxetic materialsrdquo Journal of Materials Science vol 39 no 10pp 3269ndash3279 2004

[12] D Y Fozdar P Soman J W Lee L-H Han and S ChenldquoThree-dimensional polymer constructs exhibiting a tunablenegative Poissonrsquos ratiordquoAdvanced Functional Materials vol 21no 14 pp 2712ndash2720 2011

[13] J B Choi and R S Lakes ldquoNonlinear properties of polymercellular materials with a negative Poissonrsquos ratiordquo Journal ofMaterials Science vol 27 no 17 pp 4678ndash4684 1992

[14] R DWiddle Jr A K Bajaj and P Davies ldquoMeasurement of thePoissonrsquos ratio of flexible polyurethane foam and its influenceon a uniaxial compression modelrdquo International Journal ofEngineering Science vol 46 no 1 pp 31ndash49 2008

[15] A Butenhoff R S Lakes and Y C Wang ldquoInfluence of cellsize on re-entrant transformation of negative Poissonrsquos ratioreticulated polyurethane foamsrdquo Cellular Polymers vol 20 no4ndash6 pp 373ndash385 2001

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Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


x

y

(a)

x

y

(b)

Figure 1 Behaviour of positive Poissonrsquos ratio and negative Poissonrsquosratio when subjected to tensile loading The pores have a roundershape (a) while in (b) the pore space is larger giving rise to anegative Poissonrsquos ratio [11]

as high energy absorption fracture toughness indentationresistance and enhanced shear moduli which may be usefulin some applications [6]

Auxetic materials are known to exhibit various enhancedphysical characteristics over their conventional counter-parts [10] They show increased indentation resistance andimproved acoustic damping propertiesThese enhanced char-acteristics make negative Poissonrsquos ratio materials performbetter in many practical applications however althoughthese enhanced properties have been studied in industrialand academic fields there have been few studies in tissueengineering Fozdar et al reported that there are possibilitiesto fabricate tunable negative Poissonrsquos ratio scaffold by digitalmicromirror device projection printing for tissue engineeringapplications [12]

In cartilage tissue engineering a focus has been onthe relationship between chondrocyte response and physicalproperties of the scaffold (ie stiffness hydrophilicity) How-ever knowledge is lacking about chondrocyte behavior withmechanical stimulation in a negative Poissonrsquos ratio scaffoldMechanical stimulation associated with normal body func-tions is crucial in properly reforming articular cartilage withtissue engineering [2]

In this study we form an auxetic structural polyurethane(PU) scaffold for cartilage regeneration and investigate chon-drocyte proliferation effectiveness within the auxetic scaffoldunder mechanical (compression) stimulation

2 Materials and Methods

21 Specimen (Scaffold) Preparation PU foamwas purchasedfrom Kumsung Sponge Co (Seoul Republic of Korea)

Volume ratioV0 V1 = 25 sim 3 1

Scaffold (V0) Compressed scaffold (V1)z

y

x

Figure 2 Schematic diagram of the preparation of an auxeticscaffold (compressive volume ratio = 25sim 30)

PU foams were treated with 70 ethanol for 30min forsterilization

Control specimens were prepared with 60 ppi PU foamPore size and shape of experimental specimens were con-trolled by heating and compression A metal mold withdimensions of 5 cm times 5 cm times 75 cm was used to preparespecimens To materialize the auxetic structure the desiredvolumetric ratio was estimated as 30 based on a predeter-mined calculation A schematic diagram of the process ispresented in Figure 2The initial sample size of the PU samplewas cut to approximately 8 cm times 8 cm times 9 cm giving aninitial volume to final volume ratio of 1 3 after applyingcompressionThe PU foamwas then placed carefully into themold using a tongue depressor to prevent wrinklesThe foamwas pulled and adjusted at each end to ensure that it fit themold exactlyThemetallic mold was then placed in the centerof a furnace (preheated to 150∘C) for 30min at 200∘C undertriaxial compression in order to produce a reentrant structureof the PU foam Then the mold was cooled slowly to roomtemperatureThemold containing the PU foamwas removedfrom the furnace and the heat-treated PU foamwas removedfrom the mold and cut into small cylindrical shapes

22 Estimation of Poissonrsquos Ratio Poissonrsquos ratio was esti-mated by image processing [13 14] A digital microscope(BX51 Olympus Corporation Tokyo Japan) was used tomeasure Poissonrsquos ratio of the specimens Compressive load-ing was applied to the specimens with a material testingmachine (LRX-PLUS Lloyd Instruments West Sussex UK)The experimental setupwith themicroscope for capturing thespecimen while applying a load to measure the displacementof the specimen is shown in Figure 3The relative positions ofpointed marks were used to estimate the strain and Poissonrsquosratio Images were processed with Photoshop 70 For eachspecimen five measurements were taken from a 005 to025 strain range as shown in Figure 4 In-built tools of thedigital microscope were used to measure the 119909-axis and 119910-axis displacements which were then calculated using (1) todetermine Poissonrsquos ratio




Take a photo

Strain



We have

120576119909=


100381610038161003816100381610038161003816100381610038161198600minus 1198610

1003816100381610038161003816

120576119910=


100381610038161003816100381610038161003816100381610038161198620minus 1198630

1003816100381610038161003816

] = minus120576119909

120576119910

(1)















(a)

(a)

(b)

(b)


minus15

minus1

minus05

0

05

1

15

0 005 01 015 02 025

Poiss

onrsquos

ratio

Compressive strain

ControlExperimental

lowast

lowast

lowast

lowast




1 3 50

20

40

60

80

100

120

140

160

Cel

lula

r pro

lifer

atio

n ra

te (

)

ControlExperimental

Time (day)

lowast


4 Conclusions




020406080

100120140160180200220240260280300

1 3 5

Relat

ive c

olla

gen

amou

nt (

)

Time (day)

ControlExperimental

lowast

lowast



Acknowledgment


References























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






Take a photo

Strain



We have

120576119909=


100381610038161003816100381610038161003816100381610038161198600minus 1198610

1003816100381610038161003816

120576119910=


100381610038161003816100381610038161003816100381610038161198620minus 1198630

1003816100381610038161003816

] = minus120576119909

120576119910

(1)















(a)

(a)

(b)

(b)


minus15

minus1

minus05

0

05

1

15

0 005 01 015 02 025

Poiss

onrsquos

ratio

Compressive strain

ControlExperimental

lowast

lowast

lowast

lowast




1 3 50

20

40

60

80

100

120

140

160

Cel

lula

r pro

lifer

atio

n ra

te (

)

ControlExperimental

Time (day)

lowast


4 Conclusions




020406080

100120140160180200220240260280300

1 3 5

Relat

ive c

olla

gen

amou

nt (

)

Time (day)

ControlExperimental

lowast

lowast



Acknowledgment


References























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a)

(a)

(b)

(b)


minus15

minus1

minus05

0

05

1

15

0 005 01 015 02 025

Poiss

onrsquos

ratio

Compressive strain

ControlExperimental

lowast

lowast

lowast

lowast




1 3 50

20

40

60

80

100

120

140

160

Cel

lula

r pro

lifer

atio

n ra

te (

)

ControlExperimental

Time (day)

lowast


4 Conclusions




020406080

100120140160180200220240260280300

1 3 5

Relat

ive c

olla

gen

amou

nt (

)

Time (day)

ControlExperimental

lowast

lowast



Acknowledgment


References























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




020406080

100120140160180200220240260280300

1 3 5

Relat

ive c

olla

gen

amou

nt (

)

Time (day)

ControlExperimental

lowast

lowast



Acknowledgment


References























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Documents

The Effect of Negative Poisson's Ratio Polyurethane Scaffolds for Articular Cartilage Tissue