WO2008040357A1

WO2008040357A1 - Cross-linking of foams of s-sulfonated keratin

Info

Publication number: WO2008040357A1
Application number: PCT/DK2007/050134
Authority: WO
Inventors: Jens Hoeg Truelsen; Pia Nielsen
Original assignee: Coloplast A/S
Priority date: 2006-10-02
Filing date: 2007-10-02
Publication date: 2008-04-10
Also published as: EP2079787A1; AU2007304585A1; CA2665253A1

Abstract

The present invention relates to a process for cross-linking of highly porous foams of S- sulfonated keratin, and to highly porous, cross-linked keratin foams (e.g. scaffolds) as such. Such scaffolds (foams) are particularly useful for wound care applications. Such porous keratin foams can be obtained through exchanging the aqueous solution of the reductant as the cross-linking media with an alcohol solution of the same reductant, and by exchanging the aqueous washing procedures with a similar alcohol washing procedure.

Description

CROSS-LINKING OF FOAMS OF S-SULFONATED KERATIN

FIELD OF THE INVENTION

The present invention relates to a process for cross-linking of highly porous foams of S- sulfonated keratin (in particular scaffolds), and to highly porous, cross-linked keratin foams (e.g. scaffolds) as such. Such scaffolds (foams) are particularly useful for wound care applications.

BACKGROUND OF THE INVENTION

Foams in the form of scaffolds are guiding structures used for wound care applications resulting in development of new tissue. They are usually made up of biomaterials and are added to tissue to guide the organization, growth and differentiation of cells in the process of forming functional tissue.

To achieve the goal of tissue reconstruction, scaffolds must meet some specific requirements. A high porosity and an adequate pore size are necessary to facilitate cell seeding and diffusion throughout the whole structure of both cells and nutrients. Biodegradability is essential since scaffolds need to be absorbed by the surrounding tissues without the necessity of a surgical removal. The rate at which degradation occurs has to coincide as much as possible with the rate of tissue formation : this means that while cells are fabricating their own natural matrix structure around themselves, the scaffold is able to provide structural integrity within the body and eventually it will break down leaving the neotissue, newly formed tissue which will take over the mechanical load. Injectability is also important for clinical uses.

Many different materials (natural and synthetic, biodegradable and permanent) have been investigated. Most of these materials have been known in the medical field before the advent of tissue engineering as a research topic, being already employed as bioresorbable sutures.

Examples of scaffolds are those manufactured from natural or synthetic polymers with good biocompatibility and biodegradability. Such materials are, e.g., gelatin, fibrin, hyalouronic acid, collagen, chitin, chitosan, keratin, alginate, poly(L-lactic acid) (PLLA), poly(D/L-lactic acid) (PDLLA), and poly(lactic-co-glycolic acid) (PLGA). By using such polymers it is possible to vary the physical characteristics (strengths, softness, flexibility) of the scaffold through combinations and modifications. One of such modification can be cross-linking, which will increase the strength and durability of the scaffold.

Preparation of cross-linked keratin foams has been described by Keratec Ltd. in WO 03/018673.

However, these foams were prepared by lyophilising fairly concentrated S-sulfonated keratin solutions that resulted in relative stiff materials. The keratin foams were afterwards cross- linked by different techniques, such as protonation of the S-sulfonated cystine or reduction of the S-sulfonated cystine resulting in the formation of disulfide linkages between two cystine groups. WO 03/018673 discloses the use of reductants such as ammonium thioglycolate or tributhylphosphine.

The application of these dry stiff keratin foams as wound dressings (e.g. scaffolds) may result in discomfort or even pain for the patient and it may be difficult/impossible for the wound care personnel to apply these dressings on difficult-to-access wounds, e.g. foot ulcers.

Thus, there is a need for soft, flexible, absorbent and coherent cross-linked keratin foam. Soft, flexible, absorbent and coherent keratin foams are in general needed for ensuring a comfortable and proper treatment of difficult-to-heal wounds and as releasing media for active ingredients such as growth factors.

SUMMARY OF THE INVENTION

The cross-linking procedure described in WO 03/018673 using an aqueous solution of a reductant, such as ammonium thioglycolate, has been used for keratin foams prepared from a 5% S-sulfonated keratin solution. However, when this cross-linking procedure is used for softer and more flexible keratin foams prepared from, e.g., a 2% or lower S-sulfonated keratin solution, the resultant cross-linked product collapses and results in a material with reduced/eliminated porous structure (see Example 2 (comparative example)).

It has surprisingly been shown (see Example 3) that by exchanging the aqueous solution of the reductant as the cross-linking media with an alcohol (e.g. ethanol) solution of the same reductant, and by exchanging the aqueous washing procedures with a similar ethanol based washing procedure, porous keratin foams are obtained. Hence, the present invention relates to a process for cross-linking of a highly porous foam of S-sulfonated keratin, cf. claim 1.

The invention further relates to a cross-linked keratin foam, cf. claim 7, various uses of such a cross-linked keratin foam, cf. claims 11 and 12, and to a method of supporting cell adhesion and/or the in-growth for regeneration of tissue utilizing said keratin foam, cf. claim 13.

In the present context, the term "foam" is intended to encompass light-weight structures conventionally known as foams, sponges, scaffolds, or "porous structures".

In the present context, the term "scaffold" is intended to mean porous structures into which cells may be incorporated (in-growth).

Scaffolds serve at least one of the following purposes:

• Allow cell attachment and migration

• Deliver and/or retain cells and biochemical factors

• Enable diffusion of vital cell nutrients and expressed products

• Exert certain mechanical and biological influences to modify the behaviour of the cell phase

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the present invention relates to a process for cross-linking of a highly porous foam of S-sulfonated keratin, said process comprising the steps of

(a) providing a highly porous foam of S-sulfonated keratin having a density of at the most 30

(b) contacting the keratin foam with a reductant dissolved in a solvent comprising at least 85% of a Ci-₆-alcohol for a period sufficient to obtain the desired degree of cross-linking;

(c) separating the cross-linked keratin foam from the reductant solution. Step (a)

In a first step, a highly porous foam of S-sulfonated keratin is provided. Typically, the highly porous foam is prepared by formation of a solution having a content of S-sulfonated keratin of at the most 30 img/mL, i.e. corresponding to a final foam having a density of at the most

The S-sulfonated keratin may be obtained via well-known procedures, e.g. as described in WO 03/011894 Al.

The solvent in which the S-sulfonated keratin is dissolved is typically an aqueous solvent, i.e. a solvent comprising at least 85% (w/w) of water. The S-sulfonated keratin is soluble only as the salt, which can be prepared by the addition of a base to the S-sulfonated keratin.

In one embodiment, the solvent is an aqueous solvent comprising at least 96% (w/w) of water and up to 4% (w/w) of organic constituents or salts. In one variant, the up to 4% (w/w) of non-aqueous constituent comprises at least one organic constituent selected from porosity improvers such as tert-butanol and softeners such as glycerol, low molecular weight polyethyleneglycol (i.e. liquid polyethyleneglycol), liquid Pluronic®, Tween® or other chemicals that may have softening properties. In one particularly interesting variant, the solvent consists of at least 97% of water in admixture with tert-butanol and glycerol.

In another embodiment, the solvent is water comprising a strong base. The base is typically present in an amount providing a pH of about 9 to 10. This may be obtained by adding 1 imL of a 1 M NaOH (or similar, e.g. 1 M KOH or ammonia) per gram of the S-sulfonated keratin. In this instance, the foam may be prepared as described in WO 03/018673 Al.

The concentration of the S-sulfonated keratin is typically at the most 30 img/mL, e.g. in the range of 5-30 img/mL, such as in the range of 10-25 img/mL.

The solution is subsequently cast onto a flat surface or in a mould so as to obtain a foam of the intended shape or a foam which can be later cut in suitable pieces. The aqueous solution is subsequently frozen so as to obtain a frozen solution of S-sulfonated keratin; and the frozen solution is subsequently freeze-dried so as to obtain a highly porous foam S- sulfonated keratin.

In one interesting embodiment, the foam is a scaffold adapted for wound care applications. Step (b)

In a step subsequent to step (a), the highly porous keratin foam is cross-linked by means of a reductant. The treatment with a suitable reductant causes the sulfonate groups to be removed from the S-sulfonated cystine groups of the keratin whereby two cystine groups react and form a disulfide linkage.

Suitable reductants are those selected from ammonium thioglycolate, tributylphosphine, and triscarboxyethylphosphine hydrochloride. Two or more reductants may be used in combination, if desired.

An important feature of step (b) is that the reductant is dissolved in a solvent comprising at least 85% of a Ci-₆-alcohol. The present inventors have found that this selection of solvent ensures that the physical shape of the foam of the S-sulfonated keratin as well as the resulting cross-linked keratin foam is substantially preserved, i.e. the foam will not collapse and become unsuited for the intended medical applications.

Ci-₆-alcohols comprise methanol, ethanol, 1-propanol, 2-propanol (iso-propanol), 1-butanol, 2-butanol, 2-methyl-l-butanol (iso-butanol), 2-methyl-2-butanol (tert-butanol), 1-pentanol, 1-hexanol, etc. The currently most preferred examples of Ci-₆-alcohols are ethanol and 2- propanol, in particular ethanol.

In a preferred embodiment, the solvent comprises at least 90%, such as at least 95% of the Ci-₆-alcohol.

With reference to the mole-to-mass amount of the reductant, the reductant is typically used in an amount of 0.1-40 mmol per gram of the S-sulfonated keratin. More particular, an amount corresponding to 0.5-20 mmol per gram, such as 0.8-9 mmol per gram, is used.

The reductant is allowed to interact with the foam of the S-sulfonated keratin for a period sufficient to obtain the desired degree of cross-linking. Typically, the cross-linking is allowed to take place somewhere between 1 minute to 24 hours, e.g. from 5 minutes to 12 hours. It will be evident for the skilled person, that the period can be reduced if the cross-linking takes place at elevated temperature, e.g. 25-50⁰C, and that prolonged reaction is necessary if the reaction takes place at a low temperature, e.g. -10 to 10⁰C.

A specific, illustrative example of a reductant solution is 0.003-0.05 M ammonium thioglycolate in 92-99.8% ethanol. Step (C)

In a step subsequent to step (b), the cross-linked keratin foam is separated from the reductant solution.

The reductant solution is typically removed by passive dripping off, by suction (moderate vacuum), or by compression of the foam.

However, it is often desirable to further reduce the presence of residual reductant by washing out the reductant solution, e.g. as described in optional step (d).

Step (d) (optional)

In a preferred embodiment, the process comprises the further step (d) of washing the cross- linked keratin foam with a solution comprising at least 90% of a Ci-₆-alcohol. Preferably, the solution comprises at least 95% of the Ci-₆-alcohol.

The Ci-₆-alcohol is as described above under step (b), and also in step (d), the currently most preferred examples of Ci-₆-alcohols are ethanol and 2-propanol, in particular ethanol.

The washing may be conducted by flushing, or by dipping, typically in 1 to 3 steps.

Particles and components of the extracellular matrix

The void space of the foam may be unoccupied so as to allow cell adhesion and/or in-growth for regeneration of tissue. In one embodiment, however, the pores of the material are at least partly occupied by particles or a component from the extracellular matrix or the foam is coated with particles or a component from the extracellular matrix. Such particles and components may facilitate the cell adhesion and/or in-growth for regeneration of tissue.

Examples of components from the extracellular matrix are chondroitin sulfate, hyaluronan, hyaluronic acid, heparin sulfate, heparan sulfate, dermatan sulfate, growth factors, fibrin, fibronectin, elastin, collagen, gelatin, and aggrecan.

Hence, the process of the invention also encompasses a variant wherein particles of the extracellular matrix or components from the extracellular matrix are dispersed or dissolved in the solution of the S-sulfonated keratin used in step (a) before the aqueous solution (dispersion) is frozen and freeze-dried. The process invention also encompasses the more specific variant wherein the components from the extracellular matrix are dissolved in a suitable solvent and then added to the solution of the S-sulfonated keratin. By mixing with the solvent of the S-sulfonated keratin, the components from the extracellular matrix will most likely precipitate so as to form a dispersion.

Further, the process of the invention encompasses a variant wherein the cross-linked keratin foam obtained in step (c), in a subsequent step (e.g. as an alternative or supplement to step (d)), is immersed in a solution of glucosaminoglycan (e.g. hyaluronan) and subsequently freeze-dried again.

Various uses

The foams (in particular scaffolds) prepared according to the process of the present invention are particularly useful for tissue regeneration. Besides the use of the foams (scaffolds) for tissue regeneration within wound care applications it may also be used within continence care and ostomy care.

As it will be obvious from the above, the foams have a multitude of uses within the field of medicine, healthcare, surgery, dentistry, etc., in particular uses where a biodegradable polymer is required, e.g. for wound dressings, scaffolds for cell attachment and in-growth for regeneration of tissue, hernia-mesh, cartilage, ligaments, implants, etc.

Hence, the present invention also relates to a cross-linked keratin foam as defined herein for use in therapy, dentistry or surgery.

More particular, the invention also relates to the use of a cross-linked keratin foam as defined herein for the preparation of a scaffold for supporting cell adhesion or the in-growth for regeneration of tissue, and to the use of a cross-linked keratin foam as defined herein for the preparation of a scaffold adapted for wound care applications.

The invention further relates to a method of supporting cell adhesion and/or the in-growth for regeneration of tissue, the method comprising the step of contacting a cross-linked keratin foam as defined herein with said tissue, e.g. soft tissue such as skin, ligament, tendon, cartilage, and bone. Alternative embodiments

It is envisaged that other S-sulfonated proteins may be used in a similar manner, i.e. by utilizing approximately the same means, to produce alternative, highly porous foams.

EXAMPLES

Example 1 - Preparation of Keratin Scaffolds

Keratin scaffolds were prepared form 2% (w/w), 1.5% (w/w), 1.4% (w/w) and 0.8% (w/w) S-sulfonated keratin solutions. Table 1 shows the chemical compositions and the concentration of each component in the stock solutions prior to the lyophilisation.

Table 1 : Chemical composition of solutions prior to lyophilisation

All stock solutions were cooled to 0⁰C in a water/ice batch. 3x15 imL of a stock solution was transferred to 3 prenucleated 7.3x7.3 cm aluminium casts. The aluminium casts were prenucleated as described in Example 1 of WO 95/05204. Depending on the sample, the solutions were either frozen at -50⁰C or -80⁰C as indicated in Table 1. The frozen samples were transferred to a Heto fridge lyophilisor and lyophilised over night.

Example 2 - Cross-linking of keratin scaffolds as described in WO 03/018673 (comparative example)

A solution comprising 0.25 M ammonium thioglycolate and 0.1 M potassium phosphate buffer was prepared in a 1 L measuring flask. Keratin scaffolds prepared as described in Example 1 (one keratin scaffold from sample 1 and one keratin scaffold from sample 6 (see Table I)) were cross-linked using following procedure.

In a polystyrene Petri-dish, the keratin scaffold was immersed in 50 imL of the 0.25 M ammonium thioglycolate solution for 30 minutes. The cross-linked scaffold was afterwards washed 3 times by immersing in 50 imL demineralised water for 10 min. The cross-linked and washed keratin scaffolds was finally lyophilised.

The cross-linked keratin scaffolds obtained via this route were collapsed, had a significant reduced porosity and a highly increased stiffness making them less usable for would care applications.

Example 3 - Cross-linking of keratin scaffolds using ethanol

Three solutions of ammonium thioglycolate in 96% ethanol were prepared: 0.02 M, 0.01 M and 0.005 M.

The scaffolds prepared as described in Example 1 (samples 1-6, see Table 1) were cross- linked by immersion in 50 imL of the ammonium thioglycolate solution for 30 minutes (see Table 2). The cross-linked scaffolds were afterwards washed 3 times with 96% ethanol by immersing in 96% ethanol for 10 minutes per wash. The ethanol is exchanged between each wash. The cross-linked and washed keratin scaffolds were afterwards dried in a vacuum oven over night at room temperature.

The obtained cross-linked scaffolds were highly porous, flexible and water absorbent making them attractive for wound care application.

Table 2: Concentration of ammonium thioglycolate in cross-linking media

Claims

1. A process for cross-linking of a highly porous foam of S-sulfonated keratin, said process comprising the steps of

(c) separating the cross-linked keratin foam from the reductant solution.

2. The process according to any one of the preceding claims, wherein the solvent comprises at least 90%, such as at least 95% of the Ci-₆-alcohol.

3. The process according to any one of the preceding claims, wherein the Ci-₆-alcohol is ethanol.

4. The process according to any one of the preceding claims, wherein the process comprises the further step (d) of washing the cross-linked keratin foam with a solution comprising at least 90% of a Ci_-6-alcohol.

5. The process according to any one of the preceding claims, wherein the reductant is selected from the groups consisting of ammonium thioglycolate, tribuhylphosphine, and triscarboxyethylphosphine hydrochloride.

6. The process according to any one of the preceding claims, wherein the foam is a scaffold adapted for wound care applications.

7. A cross-linked keratin foam, said foam having a density of at the most 30 mg/cim³.

8. The keratin foam according to claim 7, wherein the density is in the range of 5-30 mg/cim³, such as in the range of 10-25 mg/cim³.

9. The keratin foam according to any one of the claims 7-8, which is prepared according to the process defined in any one of the claims 1-6.

10. The keratin foam according to any one of the claims 7-9, wherein the pores of the foam are at least partly occupied by particles of the extracellular matrix or by a component from the extracellular matrix.

11. The use of a cross-linked keratin foam as defined in any one of claims 7-10 for the preparation of a scaffold for supporting cell adhesion or the in-growth for regeneration of tissue.

12. The use of a cross-linked keratin foam as defined in any one of claims 7-10 for the preparation of a scaffold adapted for wound care applications.

13. A method of supporting cell adhesion and/or the in-growth for regeneration of tissue, the method comprising the step of contacting a cross-linked keratin foam as defined in any one of claims 7-10 with said tissue.