Layered collagen and ha scaffold suitable for osteochondral repair

Abstract

The invention relates to a method for producing a multi-layer collagen scaffold. The method generally comprises the steps of: preparing a first suspension of collagen and freezing or lyophilizing the suspension to provide a first layer; optionally preparing a further suspension of collagen and adding the further suspension onto the layer formed in the previous step to form a further layer, and freezing or lyophilizing the layers, wherein when the layer formed in the previous step is formed by lyophilization the lyophilized layer is re-hydrated prior to addition of the next layer; optionally, repeating the aforementioned step to form one or more further layers; and preparing a final suspension of collagen and pouring the final suspension onto the uppermost layer to form a final layer, and freeze-drying the layers to form the multi-layer collagen composite scaffold.

Claims

1. A cell seeded tissue engineering construct comprising cells from a host seeded onto a multi-layer collagen scaffold, wherein the cells are disposed within pores of the multi-layer scaffold and the multi-layer scaffold comprises a three freeze-dried layers in which an optional first layer comprises Type I collagen and HA; a second layer comprises Type I collagen, a Type II collagen, a polymer and/or biologic, and HA; and a third layer comprises Type I collagen, Type II collagen, and a polymer and/or biologic.

2. The cell seeded tissue engineering construct of claim 1, wherein the biologic is selected from the group consisting of a nucleic acid, a protein, a peptide, a cytokine, a hormone, a cell, and a growth factor.

3. The cell seeded tissue engineering construct of claim 1, wherein the polymer comprises a glycosaminoglycan (GAG).

4. A cell seeded tissue engineering construct comprising cells from a host seeded onto a multi-layer collagen scaffold, wherein the cells are disposed within pores of the multi-layer scaffold and the multi-layer scaffold comprises a plurality of freeze-dried layers in which an optional first layer consists of collagen and HA; a second layer consists essentially of a collagen, a polymer and/or biologic and HA; and a third layer consisting essentially of collagen, wherein a ratio of HA in the first layer to HA in the second layer is at least 1:1 (w/w), wherein the collagen component in the first layer comprises Type I collagen, and the collagen component of the second and third layers comprises Type I and II collagen.

5. The cell seeded tissue engineering construct of claim 4, wherein the ratio of HA in the first layer to HA in the second layer is at least 3:1 (w/w).

6. The cell seeded tissue engineering construct of claim 4, wherein the scaffold has a continuous pore architecture extending across the layers.

7. The cell seeded tissue engineering construct of claim 6, wherein the porous scaffold layers are seamlessly integrated.

8. The cell seeded tissue engineering construct of claim 6, wherein scaffold comprises a pore architecture gradient extending across the layers.

9. A cell seeded tissue engineering construct of claim 1, wherein the multi-layer scaffold comprises: (i) a plurality of porous freeze-dried layers in which the scaffold has a continuous pore architecture extending across the layers, and in which the pore architecture of at least two of the layers is typically different; (ii) a plurality of porous freeze-dried layers in which the interface between each layer is seamless, and in which the pore architecture of at least two of the layers is typically different; or (iii) continuous physical integration at the interface of each adjacent layer, wherein each layer comprises a pore architecture characteristic that is different to the other layers.

10. A method for producing a cell seeded tissue engineering construct, the method comprising seeding cells from a host onto a multi-layer collagen scaffold and culturing the cells on the multi-layer scaffold prior to implantation into a defect, wherein the cells are disposed within pores of the multi-layer scaffold and the multi-layer scaffold comprises: (i) a plurality of freeze-dried layers in which an optional first layer comprises Type I collagen and HA; a second layer comprises Type I collagen, a Type II collagen, a polymer and/or biologic, and HA; and a third layer comprises Type I collagen, Type II collagen, and a polymer and/or biologic; (ii) a plurality of freeze-dried layers in an optional first layer consists of collagen and HA; a second layer a collagen, a polymer and/or biologic and HA; and a third layer consisting essentially of collagen, wherein a ratio of HA in the first layer to HA in the second layer is at least 1:1 (w/w), wherein the collagen component in the first layer comprises Type I collagen, and the collagen component of the second and third layers comprises Type I and II collagen; (iii) a plurality of porous freeze-dried layers in which the scaffold has a continuous pore architecture extending across the layers, and in which the pore architecture of at least two of the layers is typically different; (iv) a plurality of porous freeze-dried layers in which the interface between each layer is seamless, and in which the pore architecture of at least two of the layers is typically different; or (v) continuous physical integration at the interface of each adjacent layer, wherein each layer comprises a pore architecture characteristic that is different to the other layers.

11. The method of claim 10, further comprising preparing the multi-layer scaffold by an iterative layering technique in which each layer is independently formed by freezing, or lyophilisation followed by rehydration with an acidic solvent, prior to addition of a following layer, wherein the final layer is formed by lyophilisation.

12. The method of claim 11, wherein all layers in the scaffold are formed by lyophilisation.

13. The method of claim 10, further comprising cross-linking one or more of the layers of the scaffold.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1: Toluidine blue stained 10 m transverse section of the bottom CHA layer of a 2-layer scaffold, showing the presence of hydroxyapatite (HA) particles within the scaffold struts. This confirms that HA remains within the scaffold during the iterative layering process. (Bottom layer CHA scaffold=200 wt % HA)

(2) FIG. 2: Pore structure of the 2-layer 200 wt % CHA/Col1 Scaffold. The scaffold pore structure was investigated by embedding the scaffold and then sectioning it to a thickness of 10 m using a microtome. The slices were mounted on a glass slides and the scaffold struts were stained using Toluidine blue stain. FIG. 3 shows a micrograph of histological sections of the 2 layer 200 wt % CHA (base layer)/Col1 (top layer) scaffold imaged using light microscopy. The micrographs indicate that the base 200 wt % layers retains its pore structure during the iterative freeze-drying process. The top layer also displays equiaxed pore morphology.

(3) FIG. 3: XRD patterns for the pure HA powder in blue and the bottom layer CHA scaffold in black, with the characteristic peaks for HA in red. (Bottom layer CHA scaffold=200 wt % HA). This shows that HA is successfully incorporated into the scaffold and that its phase purity is unaffected by the process.

(4) FIG. 4: Comparison of the mechanical properties of a standard collagen scaffold, the bottom or bone layer (Layer 3containing type I collagen and 200 wt % HA), the intermediate layer (Layer 2containing equal amounts of type I collagen, type II collagen and HA) and the top or cartilage layer (Layer 1containing equal amounts of type I and type II collagen), and a 3-layer scaffold following DHT treatment at 105 C. for 24 hours.

(5) FIG. 5: Permeability of the individual scaffold layers compared to that of a pure collagen scaffold. (Bottom layer CHA=200 wt % HA, Intermediate layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid:200 wt % HA in 0.5M acetic acid (1:1:1), Top layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid (1:1).

(6) FIG. 6: Representative micrographs of the pore structure of each of the component layers of the 3-layer scaffold produced in isolation showing the homogeneous pore architecture. (Bottom layer CHA=200 wt % HA, Intermediate layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid:200 wt % HA in 0.5M acetic acid (1:1:1), Top layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid (1:1)).

(7) FIG. 7: Porosity of a standard collagen scaffold, each of the component layers of the 3-layer scaffold produced in isolation, and a 3-layer scaffold, showing the high porosity of all scaffold variants. (Bottom layer CHA=200 wt % HA, Intermediate layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid:200 wt % HA in 0.5M acetic acid (1:1:1), Top layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid (1:1)).

(8) FIG. 8: Comparison of the pore diameters of each of the component layers of the 3-layer scaffold produced in isolation. The average pore diameters were found to vary from 112 m (intermediate layer scaffold) 136 m (bottom layer scaffold). (Bottom layer CHA=200 wt % HA, Intermediate layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid:200 wt % HA in 0.5M acetic acid (1:1:1), Top layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid (1:1)).

(9) FIG. 9: SEM micrographs of the 3-layer scaffold showing the highly porous structure, high degree of pore interconnectivity and seamless integration of the component layers. (Bottom layer CHA=200 wt % HA, Intermediate layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid:200 wt % HA in 0.5M acetic acid (1:1:1), Top layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid (1:1). This example was produced using the iterative layering technique and freeze-drying using a freezing temperature of 40 C.).

(10) FIG. 10: Cell numbers for 3-layer scaffolds compared to collagen scaffolds at 7 and 14 days showing an increase in cell number of approximately 50% from day 7 to day 14 for both the collagen and the 3-layer scaffolds. (Bottom layer CHA=200 wt % HA, Intermediate layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid:200 wt % HA in 0.5M acetic acid (1:1:1), Top layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid (1:1). This example was produced using the iterative layering technique and freeze-drying using a freezing temperature of 40 C.).

(11) FIG. 11: Scaffold contraction of the 3-layer scaffold compared to a standard collagen scaffold at 7 and 14 days post seeding with MC3T3-E1 mouse pre-osteoblast cells. The 3-layer scaffold was found to contract to a lesser extent than the standard collagen scaffold. (Bottom layer CHA=200 wt % HA, Intermediate layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid:200 wt % HA in 0.5M acetic acid (1:1:1), Top layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid (1:1). This example was produced using the iterative layering technique and freeze-drying using a freezing temperature of 40 C.).

(12) FIG. 12: Histologically prepared, haematoxylin and eosin (H&E) stained transverse sections of the 3-layered scaffold following 14 days in vitro culture with MC3T3-E1 mouse pre-osteoblast cells. (Bottom layer CHA=200 wt % HA, Intermediate layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid:200 wt % HA in 0.5M acetic acid (1:1:1), Top layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid (1:1). This example was produced using the iterative layering technique and freeze-drying using a freezing temperature of 40 C.).

(13) FIG. 13: Histological image of a type I collagen/hyaluronic (HyA) acid scaffold containing 10 mg/ml of HyA, following 21 days culture with rat MSCs in chondrogenic medium, stained with fast green, safranin-O and Haemotoxylin.

(14) FIG. 14: Layer adhesion strength test results for the 2-layer and 3-layer scaffolds. (2-layer scaffolds=Bottom layer CHA=200 wt % HA, Top layer=type I collagen in 0.05M acetic acid; 3-layer scaffolds=Bottom layer CHA=200 wt % HA, Intermediate layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid:200 wt % HA in 0.5M acetic acid (1:1:1), Top layer=type I collagen in 0.05M acetic acid:type II collagen in 0.05M acetic acid (1:1). Both produced using the iterative layering technique and freeze-drying using a freezing temperature of 40 C.).

(15) FIG. 15: 3-layered scaffold a) SEM image showing scaffold microstructure b) H&E stained sections at 14 days post seeding.

(16) FIG. 16: AChondroColl3 layered scaffold. BHistological images of normal cartilage superficial, intermediate and deep zones showing orientation of chondrocytes.

(17) FIG. 17: Histological images of normal cartilage superficial, intermediate and deep zones showing orientation of chondrocytes.

DETAILED DESCRIPTION OF THE INVENTION

Materials and Methods

Example 1

(18) Embodiment 1 relates to the production of a two layer scaffold using the iterative freeze-drying technique. The invention consists of a base bone layer composed of type I collagen and preferably 200 wt % hydroxyapatite, but this can range from 0 wt % HA up to 500 wt % HA. The top cartilage layer is composed of type I and type II collagen. The ratio of type I collagen to type II collagen can be from 0:1 to 1:0.

(19) The preferred 200 wt % collagen/HA (CHA) slurry for the bone layer of the scaffold is prepared as follows: 240 ml of preferably 0.5 M acetic acid (this can range from 0.05M to 10M) was prepared by adding 6.9 ml glacial acetic acid to 233.1 ml of distilled deionised water. 1.2 g of microfibrillar bovine tendon collagen (Collagen Matrix Inc., NJ, USA) was placed into a beaker and 100 ml of 0.5 M acetic acid solution was added. The beaker was refrigerated at 4 C. overnight to allow the collagen to swell and thus blend more easily. A WK1250 water cooling system (Lauda, Westbury, N.Y., USA) was used to cool a glass reaction vessel to 4 C. The collagen and acetic acid solution were added to the reaction vessel. 100 ml of the 0.5 M acetic acid solution was added to the beaker to remove any remaining collagen and then poured into the reaction vessel. The components were blended using an IKA Ultra Turrax T18 overhead blender (IKA Works Inc., Wilington, N.C.) at 15,000 rpm for 90 minutes. The slurry components were maintained at 4 C. during blending to prevent denaturation of the collagen as a result of the heat generated during the process.

(20) 2.4 g hydroxyapatite (HA) powder (Captal R Reactor Powder, Plasma Biotal, UK) was added to 40 ml of the 0.5 M acetic acetic solution. The HA acetic acid suspension was mixed in a syringe-type delivery device creating a homogenous suspension of HA particles within the acetic acid solution. 10 ml of the HA suspension was added to the collagen slurry during blending by placing the tip of the HA delivery device tube into the vortex centre created by the blender. This ensured rapid dispersion of the HA suspension through the collagen slurry. 10 ml of the HA suspension were added to the slurry every hour (4 additions of 10 ml of HA suspension in total). The slurry was blended for a further 60 minutes following addition of the final 10 ml of HA giving a total blend time of 330 minutes (5 hours). The interval between the addition of the aliquots of the HA suspension can be varied from 30 to 240 minutes. The number of additions can also be varied, preferably HA is added in at least 2 aliquots.

(21) CHA slurries containing other quantities HA can be produced by varying the quantity of HA added, for example, for a 100 wt % CHA slurry, 1.2 g of HA powder would be added to 40 ml of 0.5M acetic acid. Examples of such slurries are described in International Patent Application (Publication) No. WO2008/096334 (Royal College of Surgeons in Ireland).

(22) Following blending, the slurry was degassed in a conical flask connected to a vacuum pump for 30 minutes to remove unwanted air bubbles within the slurry. 15.6 ml of the slurry was then placed in a 60 mm60 mm square 304 grade stainless steel tray. The slurry was then freeze-dried in a Virtis (VirTis Co., Gardiner, N.Y., USA) freeze-drier. The freeze-drying cycle used can be varied in order to produce scaffolds with different pore structures. This is achieved by varying the freezing temperature used from 10 C. to 70 C. The 40 C. freeze-drying cycle consisted of the following steps: The tray was placed on a shelf in the freeze drier at 20 C. The cycle involved cooling the shelf to 40 C. at a preferred constant rate of 0.9 C./min, based on the findings of a previous study (O'Brien F J; 2004). The cooling rate selected can range from 0.1 C./min to 10 C./min. The shelf temperature was then held constant for 60 minutes to complete the freezing process. The shelf temperature was then ramped up to 0 C. over 160 minutes. The ice phase was then sublimated under a vacuum of 200 mTorr at 0 C. for at least 17 hours to produce the porous CHA scaffold.

(23) A type I collagen (Col1) slurry was produced as follows:

(24) The preferred Col1 slurry contains 5 g/l of type I collagen suspended in 0.05 M acetic acid. The quantity of Col1 can be varied between 5 g/l and 50 g/l. The acetic acid concentration used can range between 0.01 M and 10 M. 240 ml of 0.05 M acetic acid was prepared by adding 0.69 ml glacial acetic acid to 239.31 ml of distilled deionised water. 160 ml of 0.05 M acetic acid was added to 0.8 g of Col1 and left to swell overnight in the refrigerator at 4 C. A WK1250 water cooling system (Lauda, Westbury, N.Y., USA) was used to cool a glass reaction vessel to 4 C. The collagen and acetic acid solution was added to the reaction vessel and the components were blended for 90 minutes. Following blending, the slurry was degassed in a conical flask connected to a vacuum pump for 30 minutes to remove unwanted air bubbles within the slurry. The slurry was placed in a bottle and stored in a refrigerator at 4 C. The acetic acid concentration used to produce the Col1 slurry can be varied from 0.01M and 10M.

(25) Iterative Layering Process

(26) The bone layer CHA scaffold was rehydrated in an acetic acid solution in order to prevent collapse of the scaffold following addition of the second layer slurry and also to prevent excessive infiltration of the second layer slurry into the base scaffold. The concentration of the acetic acid solution can be varied from 0.001M acetic acid to 5M, with 0.025 M acetic acid solution being the preferred concentration. 800 ml of 0.025 M acetic acid was prepared by adding 1.1 ml glacial acetic acid to 798.9 ml of distilled deionised water. Rehydration involved filling the 60 mm60 mm freeze-dying tray with acetic acid and placing the scaffold into it. This was then placed under vacuum until the scaffold was fully rehydrated and air bubbles had been removed from the scaffold. Excess acetic acid was removed using a pipette. 15.6 ml of the top layer collagen slurry was carefully pipetted on top. The two layer construct was then returned to the freeze-dryer and freeze-dried using the freeze-drying process described above.

Example 2

(27) Embodiment 2 relates to a three layer scaffold, the base layer of the scaffold has similar structural and compositional properties to the subchondral bone layer and consists of the primary constituents of bone; type I collagen (the organic phase) and hydroxyapatite (the mineral phase). The intermediate layer has a similar composition to calcified cartilage and consists of type II collagen which is present in cartilage and also type I collagen and hydroxyapatite (present in bone). The top layer, modelled on the superficial to the deep zones of articular cartilage, comprises type I and type II collagen.

(28) Bone Layer:

(29) The bone layer consisted of a CHA scaffold, with the amount of HA present varying between 0 wt % and 500 wt %. The CHA slurry was fabricated and freeze-dried as described in embodiment 1 above.

(30) Intermediate Layer:

(31) The intermediate layer consisted of type I collagen (Col1) (Collagen Matrix Inc., NJ, USA), type II collagen (Col2) (Porcine type II collagen, Biom'up, Lyon, France) and hydroxyapatite (HA) (Captal R Reactor Powder, Plasma Biotal, UK).

(32) A type I (Col1) slurry was produced as described in embodiment 1. The type II collagen (Col2) slurry can contain from 5 g/l to 50 g/l type II collagen. The 5 g/l Col2 slurry is produced by placing 0.2 g of Col2 into a glass beaker and then adding 40 ml of acetic acid solution. The acetic acid concentration used can be varied from 0.01 M to 0.5 M. The solution was refrigerated at 4 C. overnight to allow the collagen to swell. The solution was placed on ice and blended using a homogeniser for 30 minutes to produce a homogenous slurry. The slurries containing the greater quantities of Col2 are produced by increasing the amount of Col2 added, for example a 1% Col2 slurry contains 0.4 g of Col2 in 40 ml of 0.05 M acetic acid.

(33) The intermediate layer slurry was produced by combining the CHA slurry, Col1 slurry and Col2, produced as described in embodiment 1 and 2, slurry into a glass beaker. The 3 slurries were mixed by blending using a homogeniser for 30 minutes until a homogenous solution was produced. The homogenous slurry was then degassed to remove air bubbles by placing the beaker in a vacuum chamber connected to a vacuum pump. The amount of each component slurry in the intermediate layer can be varied between 0% and 100%.

(34) Prior to addition of the intermediate layer slurry to the bone layer scaffold, the bone layer scaffold was rehydrated. This is necessary in order to prevent scaffold collapse. The preferred rehydration medium was a 0.025 M acetic acid solution. 800 ml of 0.025 M acetic acid was prepared by adding 1.1 ml glacial acetic acid to 798.9 ml of distilled deionised water.

(35) A 60 mm60 mm square 304 grade stainless steel tray was used for producing the layered scaffolds. A CHA bone layer scaffold was produced and rehydrated as described in embodiment 1. 15.6 ml of the intermediate layer slurry was pipetted on top of the rehydrated CHA bone layer. The quantity added to each one can be varied to give an intermediated layer thickness of between 1 mm and 15 mm. The 2-layer construct was then freeze-dried as described in embodiment 1.

(36) Cartilage Layer:

(37) The cartilage layer slurry was produced by placing the Col1 slurry and Col2 slurry, produced as described above, into a beaker, placing the beaker on ice and then blending until a homogenous solution was produced. The homogenous slurry was then degassed to remove air bubbles by placing the beaker in a vacuum chamber connected to a vacuum pump. The ratio of the Col1 slurry to the Col2 slurry (Col1:Col2) can vary from 0:1 to 1:0.

(38) Prior to addition of the cartilage layer, the 2-layer scaffold was rehydrated in acetic acid as previously described. The cartilage layer slurry was pipetted on top of the rehydrated 2-layer scaffold, the quantity used ranging from 3 ml to 60 ml, to give a scaffold ranging from 1 mm to 15 mm, depending on the thickness required. The entire structure was freeze-dried again to produce a 3-layer scaffold.

(39) Following freeze-drying the 3-layer porous structure was crosslinked using a dehydrothermal cross-linking process (DHT). This involved placing the structure in a vacuum oven (Fisher IsoTemp 201, Fisher Scientific, Boston, Mass.) to crosslink the collagen and thus provide an increase in the mechanical strength of the structure. Cross-linking can carried out at a temperature of from 105 C. to 180 C. under a vacuum of 50 mTorr for 24 hours.

Example 3

(40) In another example, a 3-layered scaffold is produced where the base layer is crosslinked via a chemical cross-linking method described earlier (EDAC) prior to addition of the 2.sup.nd layer in order to improve mechanical stiffness of the scaffold and maintain a equiaxed pore structure when additional layers are added to the scaffold. The degree of cross-linking used can be controlled based on the structural requirements.

Example 4

(41) Embodiment 4 relates to an alternative method for the production of layered tissue engineering scaffolds. The process involves producing a collagen based slurry as above, and pipetting the 67.5 ml of the slurry into a 127 mm127 mm square 304 grade stainless steel tray. This tray is then placed in the freeze-dryer and the slurry is frozen to a temperature of between 10 C. and 70 C. at a preferred constant rate of 0.9 C./min. This freezing rate can be varied between 0.1 C./min and 10 C./min. The shelf temperature was then held constant for 60 minutes to complete the freezing process. The frozen scaffold was then removed from the freeze-dryer and 67.5 ml of a second slurry layer was applied on top. The 2-layer structure was then freeze-dried. The cycle involves cooling the shelf to a temperature of between 10 C. and 70 C. at a constant rate of 0.9 C./min. The shelf temperature was then held constant for 60 minutes to complete the freezing process. The shelf temperature was then ramped up to 0 C. over 160 minutes. The ice phase was then sublimated under a vacuum of 200 mTorr at 0 C. for at least 17 hours to produce the 2-layer porous scaffold.

Example 5

(42) Embodiment 5 relates to a 3-layered scaffold and the method of fabrication. The process involves the production of a collagen based slurry as above, and pipetting 67.5 ml of the slurry into stainless steel tray as described above. The tray is placed into the freeze-dryer and the slurry is frozen to a temperature of between 10 C. and 70 C. at a suitable constant freezing rate, preferable 0.9 C./min. The shelf temperature was held constant for at least 60 minutes to complete the freezing process. A second slurry layer was applied to this frozen slurry and the >60 minute freezing step was repeated. The frozen 2-layer material was then removed from the freeze-dryer and a 3.sup.rd collagen-based slurry was again pipetted on top. This was then returned to the freeze-drying and freeze-dried using a freeze-drying cycle where the shelf was cooled to a temperature of between 10 C. and 70 C. at a constant rate of 0.9 C./min. The shelf temperature was then held constant for 60 minutes to complete the freezing process. The shelf temperature was then ramped up to 0 C. over 160 minutes. The ice phase was then sublimated under a vacuum of 200 mTorr at 0 C. for at least 17 hours to produce a 3-layer scaffold.

Example 6

(43) A further embodiment relating to the scaffold disclosed here relates to the use of the scaffold as a growth factor delivery carrier system. Growth factors that could be incorporated into the scaffold include the TGF- superfamily, IFG, FGF, BMP, PDGF, EGF and cannabinoids. These growth factors could be included into the scaffold in a number of ways, including by soaking the prepared scaffold in a solution containing the growth factor of interest, through cross-linking, using transcription, or through other methods.

(44) Characterisation of Scaffolds

(45) The properties of the individual scaffold layers and layered scaffolds produced in this study were compared to a control scaffold made of type I collagen, fabricated using the standard protocol as detailed above. Briefly, a slurry was produced using 5 g/1 type I collagen in 0.05M acetic acid solution and lyophilised at a constant cooling rate to a final freezing temperature of 40 C.

(46) Mechanical Stiffness

(47) In order to ensure survival following implantation the mechanical properties of the implant must be sufficient to withstand the forces experienced during load bearing. The mechanical properties of the scaffold have also been shown to affect cellular response (Engler et al.; 2006). The differentiation lineage for MSCs was found by Engler et al. to vary depending on substrate elasticity, with a neuronal phenotype resulting on soft substrates and osteoblasts resulting on high stiffness substrates. The mechanical properties would thus have particular importance in applications where defect healing occurs due to the infiltration of MSCs, for example, if the scaffold was to be used in combination with the microfracture technique. Mechanical testing was carried out on 9.7 mm diameter samples using the Zwick Z050 Mechanical Testing Machine (Zwick/Roell, Germany). Prior to testing samples were pre-hydrated with phosphate buffered saline (PBS). The scaffolds were loaded at a strain rate of 10% per minute and the modulus was defined as the slope of a linear fit to the stress-strain curve over the 2-5% strain range.

(48) The mechanical properties of each individual layer of the 3-layer scaffold produced in isolation and of the 3-layer scaffold were determined and compared to a standard collagen scaffold. The results are shown in FIG. 4. The bottom layer was found to have the highest compressive modulus of approximately 0.95 kPa, significantly higher than the other two groups (p<0.05). This is due to the presence of the HA mineral phase. The compressive moduli of the intermediate layer and the top layer were found to be between approximately 0.3 kPa and 0.4 kPa, with no significant difference being found between the two groups. The compressive modulus of the 3-layer scaffold was found to be similar to that of the collagen scaffold, with no statistically significant difference (p>0.05) being found between the two groups.

(49) Distribution of Hydroxyapatite (HA)

(50) The distribution of hydroxyapatite (HA) within a 2-layer scaffold was investigated by embedding the scaffold in a polymer, carrying out histological preparation and then staining with Toluidine blue stain. Microscopic analysis enabled HA distribution to be examined. The presence of HA particles within the collagen struts is evident, as shown in FIG. 1. X-ray diffraction (XRD) was used to analyse the effect of the fabrication process on the chemical composition of the bottom layer CHA scaffold. The XRD pattern for the bottom layer scaffold was compared to that of the pure HA powder and to the standard XRD pattern for HA (JCPDS 72-1243). The results, shown in FIG. 3, confirm the presence of HA in the bottom layer CHA scaffold. No other calcium phosphate phases were identified, thus confirming that degradation of the HA component has not occurred.

(51) Scaffold Permeability

(52) The permeability of a porous material is the ease with which a fluid can flow through it. High permeability is essential for tissue engineered scaffold materials in order to allow cellular migration into their centre. The permeability of the individual layers of the 3-layer scaffold is compared in FIG. 5. Scaffold permeability can be seen to relate to substrate stiffness, with the scaffolds which were found to have a higher compressive modulus displaying the greatest permeability. The permeability of our 3-layer scaffold can be controlled by altering the mechanical properties of the individual scaffold layers, in order to produce an optimal scaffold.

(53) Porosity and Pore Structure

(54) A high level of porosity is a vital requirement for scaffolds used for tissue regeneration in order to allow the infiltration of cells, diffusion of nutrients and removal of waste. If the porosity is insufficient avascular necrosis will occur at the centre of the implanted material, leading to failure of the construct. One of the main advantages of the present invention is the high degree of porosity within all regions of the scaffold. The porosity of each of the component layers of the 3-layer scaffold was determined using the density method as per F2450-04: Standard Guide for Assessing Microstructure of Polymeric Scaffolds for Use in Tissue Engineered Medical Products, using the following formulae

(55) $V_p=V_T-\frac{m_s}{{}_s}$$\%\mathrm{Porosity}=V_p/V_{T}100$

(56) Where Vp is the volume of the pores in the scaffold, V.sub.T is the total volume of the scaffold, determined by measuring the sample dimensions, m.sub.s is the mass of the scaffold, and .sub.s is the density of the material.

(57) The average porosity of each layer is shown in FIG. 7. The top layer was found to have the highest porosity and the bottom layer the lowest. The difference between groups was found to be statistically significant (p<0.05). A reduction in porosity was seen due to the addition of HA particles but this is negligible in real terms (99.5-98.8%). A high level of porosity is necessary in order to ensure the infiltration of cells into the centre of the scaffold and also the supply of nutrients and removal of waste from these cells. If porosity is insufficient, areas of necrosis will result within the scaffold.

(58) The pore size and pore structure of scaffold materials is also important. A homogenous interconnecting pore structure with optimal pore size is necessary in order to successfully generate repair tissue. If pores are too small cell migration is limited, whereas if pores are too large there is a decrease in surface area, limiting cell adhesion (O'Brien F J, 2005; Murphy C M, 2009). One of the advantages of the freeze-drying process used to produce the scaffolds detailed in this invention is the ability to precisely control pore size and pore homogeneity. The pore structure of the individual scaffold layers and of 2-layer scaffolds was analysed by embedding the scaffold in JB4 glycomethacrylate (Polysciences, Germany), in both longitudinal and transverse plane, preparing the scaffolds histologically and staining them with toluidine blue prior to microscopic analysis. Representative micrographs demonstrating the pore homogeneity of the individual scaffold layers are shown in FIG. 6. These micrographs demonstrate that the addition of type II collagen and hydroxyapatite to the various layers does not effect pore homogeneity.

(59) Pore size was determined using a linear intercept method. The average pore sizes of the individual scaffold layers produced in isolation using a 40 C. freeze-drying cycle, are shown in FIG. 8. The average pore diameters were found to range from 112 m for the intermediate scaffold to 136 m for the bottom scaffold. The pore size of each individual layer can be controlled by altering the freezing temperature used during the freeze-drying process.

(60) A homogenous pore structure is also obtained when producing layered scaffolds. The microscope images of sections from both the top and bottom layers of a 2-layer scaffold shown in FIG. 2 demonstrate the capability of the iterative layering method to produce a layered construct with a highly porous structure and homogenous pore size distribution throughout.

(61) Scanning Electron Microscopy (SEM) analysis of the 3-layered scaffold (FIG. 9) demonstrates the high degree of pore interconnectivity throughout the construct. Structural continuity at the interfaces is evident, with the individual layers being seamlessly integrated. This seamless integration of the scaffold layers is vital in order to promote the regeneration of anatomical repair tissue. This type of continuous structure cannot be achieved using other layered scaffold production methods which involve for example the suturing or gluing together of the scaffold layers.

(62) In-Vitro Bioactivity

(63) The ability of cells to attach to, infiltrate through, and proliferate within the 3-layered scaffold described in this invention was investigated through in vitro culture. Scaffold discs, 12.7 mm () in diameter and 4 mm in height, were cut from pre-fabricated scaffold sheets of the 3-layered scaffold material. The scaffolds were seeded with MC3T3-E1 mouse pre-osteoblast cells at a density of 210.sup.6 cells per scaffold. Scaffolds were evaluated at 7 and 14 days post seeding. FIG. 12 shows transverse sections of a 3-layer scaffold, following 14 days in culture, prepared histiologically and stained using haematoxylin and eosin (H&E) staining. Cells were seen to infiltrate into scaffold and adhere to the collagen struts.

(64) Cell number was determined by DNA quantification using Hoechst DNA assay. Qiazol Lysis Reagent was used to allow dissociation of nucleoprotein complexes. Hoechst 33258 dye was then added to fluorescently label DNA and fluorescent emission was read using a fluorescence spectrophotometer. Readings were converted to cell number using a standard curve. Cells numbers for the collagen and 3-layer scaffolds at 7 and 14 days are shown in FIG. 10. There was an increase in cell number of approximately 50% from day 7 to day 14 for both the collagen and the 3-layer scaffolds, indicating that cells are readily proliferating within both scaffold types. This demonstrates that the scaffolds are highly biocompatible, providing an excellent environment for the growth and differentiation of the MC3T3 cells.

(65) Interfacial Adhesion Strength

(66) The interfacial adhesion strength between the layers of the multi-layer construct described here is an important property. If adhesion strength is insufficient delamination will occur at the layer interfaces. Interfacial adhesion strength of both 2-layer and 3-layer constructs was determined using a custom designed rig fitted to the Zwick Z050 Mechanical Testing Machine (Zwick/Roell, Germany). Testing involved adhering the scaffold to test stubs using a high viscosity adhesive. A tensile load was applied to samples at a strain rate of 10% per minute. The samples were tested to failure. Pre-hydration of samples in PBS was carried out prior to testing and testing was carried out in a bath of PBS to maintain hydration during the test period. Fibre pullout was observed on the fracture surface following testing indicating true integration of the scaffold layers.

(67) The invention is not limited to the embodiment hereinbefore described which may be varied in construction and detail without departing from the spirit of the invention.

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