WO2013095138A1

WO2013095138A1 - Activated or biologically functionalised polymer network

Info

Publication number: WO2013095138A1
Application number: PCT/NL2012/050911
Authority: WO
Inventors: Bhushan CHANDRASHEKHAR-BHAT; Pier Buma; Eric Leonardus Winfriedus DE MULDER; Robbert Arnold DE GRAAF
Original assignee: Polyganics B.V.
Priority date: 2011-12-23
Filing date: 2012-12-21
Publication date: 2013-06-27
Also published as: NL2008038C2

Abstract

The invention is directed to a biodegradable porous polymer network, to a process for preparing a biodegradable porous polymer network, and to the use of a biodegradable polymer network. More in particular, the biodegradable porous polymer network of the invention may be activated or functionalised with a biological compound. The biodegradable porous polymer network of the invention comprises a phase-separated biodegradable polymer, said polymer comprising an amorphous segment and a crystalline segment, at least said amorphous segment comprising a hydrophilic segment, wherein the polymer network comprises free groups of the following structure (I), wherein Y is selected from CH₂ or NH, R¹ is selected from the groups consisting of an optionally substituted linear or branched C₁-C₁₈ alkyl or alkenyl group, an optionally substituted C₃-C₈ cycloalkyl group, an optionally substituted aryl group, and an optionally substituted C₇-C₁₀ aralkyl group, and R² is hydrogen, or comprises a biological compound.

Description

Title: Activated or biologically functionalised polymer network

The invention is directed to a biodegradable porous polymer network, to a process for preparing a biodegradable porous polymer network, and to the use of a biodegradable polymer network. More in particular, the biodegradable porous polymer network of the invention may be activated, or functionalised with a biological compound.

Tissue engineering is a multidisciplinary field which involves the application of the principles and methods of engineering and life sciences towards the fundamental understanding of structure -function relationships in normal and pathological mammalian tissues and the development of biological substitutes that restore, maintain, or improve tissue function. The goal of tissue engineering is to surpass the limitations of conventional treatments based on organ transplantation and biomaterial implantation. It has the potential to produce a supply of immunologically tolerant 'artificial' organ and tissue substitutes that can grow with the patient. This should lead to a permanent solution to the damaged organ or tissue without the need for supplementary therapies, thus making it a cost-effective treatment in the long term. As an example, a scaffold for replacing damaged meniscus tissue can be mentioned. Such a scaffold could guide new tissue ingrowth into the lost meniscus and accordingly meniscus regeneration could be established.

One of the principle methods behind tissue engineering involves growing the relevant cell(s) in vitro into the required three-dimensional (3D) organ or tissue. But cells lack the ability to grow in favoured 3D orientations and thus define the anatomical shape of the tissue. Instead, they randomly migrate to form a two-dimensional (2D) layer of cells. However, 3D tissues are required and this can be achieved by seeding the cells onto porous matrices, known as scaffolds, to which the cells attach and colonise. The scaffold therefore is a very important component for tissue engineering. Several requirements have been identified as crucial for the production of tissue engineering scaffolds: (1) the scaffold should possess interconnecting pores of appropriate scale to favour tissue integration and vascularisation, (2) the scaffold should be made from material with controlled biodegradability or bioresorb ability so that tissue will eventually replace the scaffold, (3) the scaffold should have appropriate surface chemistry to favour cellular attachment, differentiation and proliferation, (4) the scaffold should possess adequate mechanical properties to match the intended site of implantation and handling, (5) the scaffold should not induce any adverse response and, (6) the scaffold should be easily fabricated into a variety of shapes and sizes. Bearing these requirements in mind, several materials have been adopted or

synthesised and fabricated into scaffolds.

Investigations into synthetic and natural inorganic ceramic materials (e.g. hydroxy apatite and tricalcium phosphate) as candidate scaffold material have been aimed mostly at bone tissue engineering. This is because these ceramics resemble the natural inorganic component of bone and have osteoconductive properties. However, these ceramics are inherently brittle and cannot match the mechanical properties of bone. It should be mentioned that bone is a composite comprising a polymer matrix reinforced with ceramic particles. The polymer is the protein collagen, 30 % dry weight, and

hydroxy apatite (HA), 70 % dry weight. Moreover, ceramic scaffolds cannot be expected to be appropriate for the growth of soft tissues (e.g. heart muscle tissue) considering that these tissues possess different cellular receptors and mechanical property requirements. Synthetic polymers are an attractive alternative and versatile in their applications to the growth of most tissues.

Several synthetic polymers have been proposed in the preparation of scaffolds. Aliphatic polyesters such as polyglycolic acid (PGA), polylactic acid (PLLA), their copolymers (e.g. PLGA) and polycaprolactone (PCL) are the most commonly used polymers for tissue engineering scaffold applications. The degradation products of these polymers (glycolic acid and lactic acid) are present in the human body and are removed by natural metabolic pathways. Hydrophilic synthetic materials intended for biomedical applications have been disclosed which have improved properties when compared to conventional materials when it comes to physico-chemical properties.

Examples of such material are the cross-linked polyurethane-based hydrogels as disclosed in e.g. US-A-3 903 232, US-A-3 961 629, US-A-4 550 126 and EP-A-0 335 669. However, these materials are biodurable and not

biodegradable or bioresorbable.

The lack of biodegradability makes such materials less suitable for application in vivo. In a recent articles on polymer scaffolds, the fundamental differences between biodegradable polymers and biostable polymers (or non-degradable polymers are discussed (Shoichet, Polymers 2010, 43(2), 581-591).

WO-A-2004/062704 describes a biodegradable absorbent foam that comprises a phase-separated polymer consisting of an amorphous segment and a crystalline segment, wherein the amorphous segment comprises a

hydrophilic segment.

Although promising candidates for biodegradable scaffolds useful in tissue engineering have been described in the art, there remains room for improvement. For instance, although polyurethane are promising candidates for scaffolds due to their high level of biocompatibility, their hydrophobic nature limits cell adhesion and thereby the application in cell seeded constructs. In addition, it would be desirable to provide an activated scaffold, for instance, in order to functionalise the scaffold with desirable compounds, such as suitable growth factors for cells. Preferably, such activation or functionalisation would be achieved homogeneously throughout the scaffold.

Objective of the invention is therefore to provide a biodegradable porous polymer network with a desirable degree of hydrophilicity.

Another objective of the invention is to provide a biodegradable porous hydrophilic polymer network which has good mechanical properties, such as stiffness. Further objective of the invention is to provide a process for making a biodegradable porous polymer network more hydrophilic by generating free hydrophilic groups.

Yet a further objective of the invention is to provide a biodegradable porous polymer network that can be function alised with biological molecules.

The inventors surprisingly found that one or more of the above objectives can, at least in part, be met by a biodegradable porous polymer network that comprises specifically defined free functional groups.

Accordingly, in a first aspect the invention is directed to a biodegradable porous polymer network, comprising a phase-separated biodegradable polymer, said polymer comprising an amorphous segment and a crystalline segment, at least said amorphous segment comprising a hydrophilic segment, wherein the scaffold comprises free groups of the following structure

O

I I H ._| H 2

Y C N R N R _; wherein

Y is selected from CH2 or NH,

R¹ is selected from the groups consisting of an optionally substituted linear or branched C1-C18 alkyl or alkenyl group, an optionally substituted C3-C8 cycloalkyl group, an optionally substituted aryl group, and an optionally substituted C7-C10 aralkyl group, and

R² is hydrogen, or comprises a biological compound.

The inventors found that such a polymer network has

advantageously has an increased degree of hydrophilicity in comparison with the same polymer network wherein the specifically defined free functional groups are absent. Surprisingly, good mechanical properties of the polymer network can be maintained despite the insertion of the free functional groups, in particular good stiffness.

The free functional groups can be readily introduced by aminolysis. Although aminolysis of non-biodegradable polyurethane is known, it is surprising that this mechanism can be successfully applied to the essentially different biodegradable (or bioresorbable) polymers of the invention. Moreover, it was found that after modification the functional groups are unexpectedly distributed substantially homogeneous over the entire porous polymer network. Furthermore, non-biodegradable polyurethanes have the

disadvantage of being normally based on aromatic isocyanates, which give rise to harmful by-products.

In accordance with the invention, the amorphous segment must comprise a hydrophilic segment. This amorphous segment, also called the amorphous phase, is amorphous when wet (viz. in the wet state) despite the fact that it may comprise a crystalline polyether. This means that, in the dry state, said crystalline polyether may provide the amorphous phase of the polymer with partially crystalline properties.

The phase-separated character of the polymer on which the porous polymer network of the invention is based, provides the material with desirable characteristics. Without wishing to be bound by any theory, the inventors believe that phase-separation of the various soft (amorphous) and hard (crystalline) segments attributes to the specific mechanical properties of the polymer network, such as its resilience. This is advantageous, for instance, for scaffold applications.

The presence of a hydrophilic segment or group in the amorphous phase of the polymer from which the polymer network of the invention is comprised further provides the polymer network with desirable characteristics for scaffold applications, such as the capacity of absorbing aqueous liquids and being readily biodegradable.

The polymer network of the invention can comprise a relatively large amount of free groups with the structure— Y— C(=0)— NH— R¹— NH— R². Preferably, the biodegradable porous polymer network comprises an amount of 2.5 mmol or less of free groups of structure— Y— C(=0)— NH— R¹— NH— R² per gram of total porous polymer network, preferably 2.0 mmol per gram of total porous polymer network, more preferably 1.0 mmol or less per gram of total porous polymer network, such as 0.5 mmol or less per gram of total porous polymer network, 0.2 mmol or less per gram of total porous polymer network, or 0.1 mmol or less per gram of total porous polymer network. The lower limit of free groups with the structure— Y— C(=0)— NH— R¹— NH— R² is not critical, as long as free groups are present. The amount of free amines can, for instance, be determined via staining with e.g. ninhydrin and absorbance spectroscopy.

In a highly preferred embodiment of the invention, these free groups are present both on the outer surface of the polymer network, as well as within the bulk of the polymer network (i.e. on the inner surface of the porous polymer network). More preferably, the free groups are distributed

substantially homogeneous throughout the porous polymer network. As used herein, the term "distributed substantially homogeneous" is meant to be understood that the relative difference in the amount of free groups

— Y— C(=0)— NH— R¹— NH— R² at two different locations of 1 mm³ in the porous polymer network is not more than 10 % in relative terms, preferably not more than 5 % in relative terms. The level of homogeneity of the distribution of the free groups can suitably be determined by confocal microscopy, which allows detection in three dimensions.

The biodegradable phase-separated polymer comprised in the polymer network of the invention is preferably based on a biodegradable phase-separated polymer of the following formula (I), which is subsequently aminolysed:

-[R-Q ¹ [-R'-Z !-[R"-Z²-R'-Z3]_p-R"-Z4]_q-R'-Q²]_n- (I), wherein

R is selected from one or more aliphatic polyesters, polyetheresters, polyethers, polyanhydrides and/or polycarbonates, and at least one R comprises a hydrophilic segment, R' and R" are independently C2-C8 alkylene, optionally substituted with C 1-C 10 alkyl or C1-C 10 alkyl groups substituted with protected S, N, P or O moieties and/or comprising S, N, P or O (e.g. ether, ester, carbonate and/or anhydride groups) in the alkylene chain,

Z^!-Z⁴ are independently amide (-NH-C(=0)-), urea (-NH-C(=0)-NH-) or urethane (-NH-C(=0)-0-),

Q¹ and Q² are independently urea (— NH— C(=0)— NH— ),

urethane(-NH-C(=0)-0-), amide (-NH-C(=0)-), carbonate

(_0-C(=0)-0-), ester (-C(=0)-0-), or anhydride (-C(=0)-0-C(=0)-), n is an integer from 5-500, and p and q are independent 0 or 1, provided that when q is 0, R is a mixture of at least one crystalline polyester, polyetherester or polyanhydride segment and at least one amorphous aliphatic polyester, polyether, polyanhydride and/or polycarbonate segment.

Polymers of general formula (I), and processes for the preparation thereof, are known from WO-A-2004/062704, the content of which is herewith completely incorporated by reference.

The O containing moieties in the alkylene chain, if present, are preferably hydrophilic groups, in particular ether groups, since such

hydrophilic groups can provide a reduced degradation time to the polymer, which may be desirable for the polymer's use in implants.

The simplest form of such polymer is the case when q = 0. In this case, the polymer may be represented by the following formula (II),

-[R-Q^!-R'-Q²]_n- (II)

The amorphous segment is comprised in the— R— part of the polymer according to formula (I). In case q = 1, the

Q¹[-R'-Zi-[R"-Z2-R'-Z3]_p-R"-Z4]_q-R'-Q2 part of the polymer according to formula (I) represents the crystalline segment. In this particular case the amorphous and crystalline segments are alternating, thus providing the hard segment with a uniform block-length.

R may represent a mixture of two or more different types of aliphatic polyesters, polyetheresters, polyethers, polyanhydrides and/or polycarbonates, which mixture comprises both amorphous and crystalline types, so that both are comprised in a scaffold of the invention. In the case that a mixture of amorphous and crystalline types of R segments are provided in a polymer according to the formula (I), at least one hydrophilic segment is provided in at least one amorphous R segment.

Q¹ and Q² may be selected from amide, urea, urethane ester, carbonate or anhydride groups, whereas Z¹ through Z⁴ can be chosen from amide, urea or urethane groups. In the preparing the biodegradable

phase-separated polymer used in the polymer network of the invention, one or more of Q¹, Q², Z¹, Z², Z³, and Z⁴ can be attacked by the diamine via aminolysis to provide free groups of the following structure

O

II H _Λ

Y— C— N— R -NH₂

wherein Y and R¹ have the same meaning as defined in claim 1. In the context of this application, the biodegradable porous polymer network having such free amine groups is considered to be "activated". The free amine groups can optionally be further reacted with one of more moieties that comprise a biological compound, thereby providing a biodegradable porous polymer network that is functionalised. In the latter case, the biodegradable porous polymer network of the invention will comprise free groups of the following structure

O

II H _Λ H ₉

Y— C— N— R— N— R

wherein Y and R¹ have the same meaning as defined in claim 1, and wherein R² comprises a biological compound.

In accordance with the invention, the free groups of the structure

O

II H _Λ H ₉

Y— C— N— R— N— R

can have an ahphatic R¹ moiety, an aliphatic R¹ moiety, or an R¹ moiety which comprises both aliphatic and aromatic parts. Preferably, R¹ is selected from the group consisting of an optionally substituted linear or branched Ci-Cis alkyl group, an optionally substituted C3-C8 cycloalkyl group, and an optionally substituted aryl group (such as an optionally substituted phenyl group), and an optionally substituted C7-C10 aralkyl group. More preferably, R¹ is selected from the group consisting of an optionally substituted linear or branched Ci-Cis alkyl group, and an optionally substituted C3-C8 cycloalkyl group.

The group R' in— Z²— R'— Z³— may be the same or different (or similar) to R' in -Qⁱ-R'-Zⁱ- or -Z4-R'-Q2_.

The hydrophilic segment (which is comprised in R in formulas (I) and (II) above) can suitably be an ether segment, such as a polyether segment derivable from such polyether compounds as polyethylene glycol,

polypropylene glycol or polybutylene glycol. Also, a hydrophilic segment may be derived from polypeptide, poly(vinyl alcohol), poly(vinyl pyrrohdone) or poly(hydroxylmethyl methacrylate). A hydrophilic segment is preferably a polyether.

The term "biodegradable" as used in this application is meant to refer to the ability of a polymer to be acted upon biochemically in general by living cells or organisms or part of these systems, including hydrolysis, and to degrade and disintegrate into chemical or biochemical products.

The term "bioresorbable" as used in this application is meant to refer to the ability of being completely metabolised by the human or animal body.

The term "polymer network" as used in this application is meant to refer to a plurality of polymeric strands held together by any of a variety of means, such as covalently bonded cross-linking units, long range attractive forces, hydrogen bonds, entanglement of the molecular chains, etc. The polymeric strands can be a single polymer or a blend, and the polymer network may contain other substances within the bulk of the network or on the surface of the network. A porous polymeric network typically provides sufficient structure for a degree of rigidity, and has spaces between the polymeric strands which may be occupied by other components. Preferably, the polymer network of the invention is a network of interconnected pores.

The term "phase-separated polymer" as used in this application is meant to refer to a polymer comprising soft (amorphous) segments, as well as hard (crystalline) segments, the hard segment having a phase transition temperature of at least mammalian body temperatures (which is generally 37 °C for humans) and the phase-separated morphology being manifest when the polymer network prepared from such a polymer is applied in the human or animal body for a sufficient period of time. Also, the polymer placed under temperature conditions comparable to the human or animal body exhibits said phase-separated morphology. A phase-separated polymer is characterised by the presence of at least two immiscible or partly miscible phases with a different morphology at normal environmental conditions. Within one material a rubber phase and a crystalline phase (at a temperature above the glass transition temperature of the amorphous phase and below the melting temperature of the crystalline phase) may be present or a glassy and a crystalline phase (at a temperature below the glass transition temperature of the amorphous phase). Also, at least two amorphous phases can be present at a temperature between the two phase transitions, e.g. one glassy and one rubbery phase. At a temperature above the highest phase transition which is either a melting temperature or a glass transition temperature, the liquid and rubbery or the two rubbery phases, respectively, can form a phase mixed morphology or they can still be immiscible. The presence of immiscible liquid and/or rubbery phases usually results in a polymer with phase-separated morphology without the initial desired mechanical properties at normal environmental conditions.

The term "amorphous" as used in this application is meant to refer to segments present in the polymer of the invention with at least one glass transition temperature below mammalian body temperatures, and may also refer to a combination of an amorphous and crystalline segment which is completely amorphous at mammalian body temperatures. For example, polyethylene glycol (PEG) in a pre-polymer may be crystalline in pure form, but may be amorphous when comprised in the R segment of a polyurethane of the formula (I) or (II). Longer PEG segments may also be partly crystalline when comprised in the R segment of a polyurethane of the formula (I) or (II), but will become amorphous ("dissolves") when placed in contact with water. Therefore, such longer PEG segments are part of the soft segment of the phase separated polymer of the formulas (I) or (II), whereas the hard segment should remain crystalline in nature to provide sufficient support to a foam in the wet and packed state for, at least, a certain period of time.

The term "crystalline" as used in this application is meant to refer to segments, present in the polymer of the invention, that are crystalline at mammalian body temperatures, i.e. that have a melting temperature above mammalian body temperatures.

The term "hydrophilic segment" as used in this application is meant to refer to a segment comprising at least one, preferably at least two, more preferably at least three hydrophilic groups such as can be provided for instance by C— O— C, or ether, linkages. A hydrophilic segment may thus be provided by a polyether segment. A hydrophilic segment may also be provided by polypeptide, poly(vinyl alcohol), poly(vinyl pyrrolidone) or

poly(hydroxylmethyl methacrylate). A hydrophilic segment is preferably derived form polyalkylene glycol, such as polyethylene glycol, polypropylene glycol, or polybutylene glycol. The preferred hydrophilic segment is a polyethylene glycol (PEG) segment.

The term "segment" as used in this application is meant to refer to a polymeric structure of any length. In the art of polymer technology a long polymeric structure is often referred to as a block, whereas a short polymeric structure is often referred to as a segment. Both these conventional meanings are understood to be comprised in the term "segment" as used herein. A biodegradable porous polymer network according to the invention can comprise a polymer wherein urethane, urea or amide bonds are provided. These bonds constitute part of the crystalline segment of the polymer. Since these hard, crystalline segments are chemically incompatible with amorphous segments, phase separation in the polymer occurs. The hard segments crystallise and form strong hydrogen bonds with other hard segments resulting into physical cross-links.

Furthermore, the biodegradability of the material can suitably be accomplished by the provision of enzymatically cleavable or hydrolysable bonds. For the material to be biodegradable, several types of polymers known to the art may thus be comprised in the polymer. Such biodegradable polymers may include polymers with one or more ester, anhydride and/or carbonate hydrolysable moieties, optionally combined with ether moieties. Such groups are very suitable provided in the R element according to the formula (I) or (II) for a polymer for use in a scaffold of the invention, although the ether or ester moieties may also be comprised in the R' and/or R" elements of the crystalline segment. In the case that q is zero in polymers of formula (I) or in the case that there are no hydrogen-bond forming groups present in the copolymer, e.g. in polymers other than those of formula (I), i.e. such as in those of the formula (II), phase-separation of crystalline hard segment and amorphous soft segments is provided by incompatible polyether, polyester, polyanhydride and/or polycarbonate groups, at least one phase being crystalline, comprised for example through R in formula (I) or otherwise.

It is believed that the polymers used in the polymer network of the invention can degrade by the hydrolysis and/or enzymatic mechanism of ester, carbonate, anhydride, urethane, urea or amide linkages. The rate of

degradation and other properties can be regulated by choosing the content and combination of these moieties in the polymer.

Examples of synthetic biodegradable polymers that can be applied in the manufacturing of the porous polymer network of the invention are those based on polyesters, polyhydroxyacids, polylactones, polyetheresters, polycarbonates, polydioxanones, polyanhydrides, polyurethanes,

polyester(ether)urethanes, polyurethane urea, polyamides, polyesteramides, poly-orthoesters, polyaminoacids, polyphosphonates and polyphosphazenes. The polymeric material may also be composed of mixtures of above components either as different building blocks of the copolymer or cross-linked polymer or as a blend of two or more (co)polymers.

For providing the porous polymer network with absorbent characteristics, it has furthermore been found that the polymers used for the preparation of the polymer network of the invention can be improved

considerably by combination of the polymer with hydrophilic polymers or groups. This means that the above mentioned polymers are chemically combined with these hydrophilic groups, e.g. by incorporating hydrophilic polymers in the backbone or side-chains of the resulting polymers. Also, a polymer network of the invention may comprise physical blends of

biodegradable and hydrophilic polymers. Hydrophilic polymers or groups may be based on polyethers, polypeptides, poly(vinyl alcohol), poly(vinyl

pyrrolidone) or poly(hydroxylmethyl methacrylate) (poly-HEMA). The preferred hydrophilic polymer is a polyether, viz. a polymer or segment comprising at least one— C— O— C— group, because these compounds are easy to handle in chemical synthesis reactions. Moreover, these compounds are generally regarded as safe (GRAS). The preferred polyether is polyethylene glycol. The hydrophilic groups are part of the soft segment where they will increase the degradation rate of the ester, carbonate or anhydride groups under the conditions were the scaffold of the invention is to be applied, and may additionally be part of the hard segment.

In particular, the absorption capacity (amount of water uptake and rate thereof) and degradation behaviour can thus be controlled by

incorporating during synthesis a suitable quantity of these hydrophilic polymers or groups. It is thus also possible to incorporate hydrophilic groups into the hard segment to increase the solubility and/or rate of degradation of the hard segment and thus shorten the time needed for complete degradation or resorption of the polymer, however, care should be taken that the hard segment provides the phase-separated polymer with sufficient resilience, even when wet.

From the above it is clear that, by proper selection of the soft and hard segments the period of time for biodegradation by enzymes and fluids of the human or animal body can be controlled, as well as the extent to which the material is degraded. Complete biodegradation will result in fragments that are small enough to be metabolised by the body. A polymer network according to the invention may suitably comprise polymeric materials that are not completely bioresorbable, but only biodegradable to an extent that allows clearance, in smaller or larger fragments, from the cavity where they were applied.

If the polymer network of the invention is applied in the human or animal body and is left in place without the intention of ever being removed thereof (such as in the form of a scaffold), the degradation products have to be metabolised by the body. Therefore, polymeric material from which a polymer network of the invention is prepared is preferably chosen such that it is completely bio-absorbable (bioresorbable). Application of such a bio-absorbable scaffold in surgical intervention has the advantage that the material does not necessarily have to be removed after surgery, but that it can be left in place.

The polymer network of the invention has the advantage that it typically disintegrates in a period of time of several days, or at maximum several weeks. This reduces the incidence of complications induced by the removal of haemostats and increases patient's convenience. According to the invention, a material is provided having superior mechanical properties, including excellent elasticity and support to the surrounding tissue, which is important in stanching the flow of blood and/or keeping the tissue in its position. Yet the material is capable of disintegrating rapidly, followed by clearance from a body cavity were it is applied. This combination of features cannot be arrived at by using conventional biodegradable materials of animal derived origin.

The polymer network of the invention may have a density of 0.01-0.2 g/cm³, preferably of 0.02-0.07 g/cm³. Furthermore, a polymer network of the invention may have a porosity in the range of from 85 to 99 %, preferably in the range of from 90 to 99 %, such as in the range of 92 to 98 %, or in the range of from 95 % to 98 %. A polymer network of the invention has sufficient fluid absorption capacity at body temperature.

The fluid absorption capacity is mainly determined by the capillary absorption of water into the pores, due to the presence of the hydrophilic nature of the polymer and the pore geometry. The amount of water absorbed in a highly porous polymer network is almost equal for a range of porosities, since the total pore volume of the polymer network is hardly affected. This means that the capacity measured in grams of water per gram polymer is dependent on the density of the polymer network: e.g. doubling of the density from 0.01 g/cm³ to 0.02 g/cm³ will give half the absorption capacity (g/g). Therefore, the absorption capacity is measured as the amount of water (g) absorbed per volume (cm³), which is preferably 0.5-0.99 g/cm³, more preferably 0.75-0.97 g/cm³. For example, a hydrophilic polyurethane polymer network with a density of 0.04 g/cm³ and having a porosity of 96.4 % can have an absorption capacity of 0.8 g of water per cm³. This is similar to a capacity of 20 grams of water per gram of polymer material.

A polymer network of the present invention has mechanical properties such as a sufficient resilience or elasticity, which are maintained under "wet" conditions, i.e. when the polymer network is in contact with bodily fluids, including e.g. purulent material. A polymer network of the invention with a porosity in the range of 95-98 % preferably has a Young's modulus of 10 kPa or more, such as in the range of 10-20 kPa. A polymer network of the invention with a porosity in the range of 88-95 % preferably has a Young's modulus of 18 kPa or more, such as in the range of 18-40 kPa. A polymer network in the invention with a porosity in the range of 80-88 % preferably has a Young's modulus of 22 kPa or more, such as in the range of 22-40 kPa.

A polymer network of the invention is hydrophilic, viz. shows a good wettability. A good wettability may be defined as having a water contact angle (for water droplets) that is substantially lower than 80°, preferably lower than 40°, more preferably substantially zero degrees. In an embodiment, the polymer network of the invention has a water contact angle of 75° or less, preferably of 70° or less, such as in the range of from 2° to 70°, or in the range of 2° to 50°.

A phase-separated morphology results in a polymer having at least two phase transitions in one polymer as indicated by two melting

temperatures, two glass transition temperatures or one melting point and one glass transition temperature.

It was found that the above-mentioned requirements can be suitably obtained when the biodegradable polymer in the polymer network is based on a phase-separated synthetic polymer comprising— C(=0)— O— groups in the backbone of the polymer. Preferably, the polymer is a polyurethane

(-NH-C(=0)-0-), polyester (-C(=0)-0-), polyanhydride

(— C(=0)— O— C(=0)— ) or polycarbonate (— O— C(=0)— O— ) based polymer, viz. a polymer wherein a nitrogen atom (polyurethane based), carbon atom (polyester or polyanhydride based) or oxygen atom (polycarbonate) is connected to the C-atom of said— C(=0)— O— groups together with either an aliphatic carbon atom next to the O-atom (polyurethane, polyester and polycarbonate) or a carbonyl group (polyanhydride).

The backbone of the polymer is preferably formed of a copolymer, which comprises two or more different units, at least one selected from the urethane, urea or amide moieties, and at least one selected from the group of ester, anhydride or carbonate moieties combined with an ether moiety. A very suitable copolymer for application as a hydrophilic

biodegradable foam is a poly ether (ester)urethane.

In preferred embodiment, the polymer networks comprising phase-separated polyesters, polyanhydrides and combinations thereof with polycarbonate and polyether groups may be either random or block copolymers in which a block can contain one or more of the above mentioned moieties. Preferably, block copolymers are used, in particular multi-block segmented copolymers in which both a crystalline and an amorphous phase are present. Physical blends of a phase-separated polymer with another phase-separated or a single-phase amorphous (co)polymer may be used in formation of polymer networks with intermediate properties. By varying the combination of polymers, the properties of the resulting polymer network can be tuned such as rate of degradation, hydrophilic and mechanical properties. This is highly advantages for scaffold applications For example, a scaffold of a blend of a polyester urethane and a co-polyester with a similar composition as the soft segment of the polyurethane gives properties intermediate of those of the two components, due to the compatibility of the polymers. Furthermore,

poly(ether)ester urethanes with different soft segment composition, the soft segments being either compatible or not, and with the same type of hard segment may be mixed and produced into a scaffold with intermediate properties.

High molecular weights are not required to obtain a polymer with good initial mechanical properties. Preferred intrinsic viscosities lie between 0.5 and 4 dl/g, depending on the type of polymer that is used. For instance, for certain polyurethanes, an intrinsic viscosity of 0.6 dl/g can still yield a highly porous polymer network with good mechanical properties. Phase-separated polyurethanes according to formula (I) with molecular weights of the pre-polymer of 2000 g/mol may have an initial elastic modulus varying from 30-120 MPa and a tensile strength of 10-45 MPa. The elongation at break varies from 500-1200 % (measured on polymeric films). Alternatively, synthetic polymers may be used based on polyamides (viz. polymers containing— NH— C(=0)— units in the backbone) or polyurea (viz. polymers containing— NH— C(=0)— NH— units in the backbone).

Combinations of urethane, urea and/or amide linkages in the above mentioned structures are also possible. A very suitable copolymer for application in a hydrophilic biodegradable foam is a poly ether (ester) urethane.

The phase-separated polymers can be semi-crystalline

homopolymers, block copolymers or multi-block segmented copolymers. At least one phase has preferably a transition temperature higher than 37 °C. The segment or block with the highest transition temperature is referred to as the "hard" block, while the segment or block with the lowest transition

temperature is referred to as the "soft" block. The hard block may consist of urethane, urea, amide, polyester or poly-anhydride groups, preferably with a phase transition from a crystalline to liquid state, or a combination of these elements. The soft block preferably comprises an amorphous polyester, polyanhydride or polycarbonate with a glass transition temperature of 37 °C or below. Such a temperature makes a scaffold very suitable for use in the human body.

The pliability, compressibility and elasticity of the polymer network can be controlled by selecting the ratio between hard and soft blocks as well as their composition in the polymer. The content and composition of the hard block contributes to the initial strength of the polymer network in the wet and dry condition. Therefore, the content and composition of the hard block must be chosen such that sufficient initial strength of the polymer network in the wet and dry condition is obtained. In order to produce a polymer network of which the structure is maintained after wetting, the hard blocks preferably has a less hydrophilic character than the soft blocks. In order to achieve a faster dissolution of the polymer and rapid loss of materials properties, which is in some cases advantageous, a more hydrophilic hard block may be selected. In accordance with the invention, the amorphous segment can comprise one or more selected from polyesters, polyetheresters, polyethers, polyanhydrides and/or polycarbonates. Preferably, the amorphous segment comprises a polyester derived from lactide (D, L or D/L) and ε-caprolactone. More preferably, the amorphous segment comprises a polyester derived from lactide (D, L or D/L) and ε-caprolactone, and has a number average molecular weight in the range of from 1000 to 4000 g/mol. The amorphous polyester can comprise about 25 wt.% of lactide, about 25 wt.% of ε-caprolactone and about 50 wt.% of polyethylene glycol.

The amorphous segment can suitably comprise polyethylene glycol in an content of 1-80 wt.% based on total weight of the amorphous segment, more preferably 5-60 wt.%, even more preferably 20-50 wt.%, such as about 50 wt.%.

The crystalline segment preferably comprises a polyurethane block. Such a polyurethane block can be obtained by reaction of a diisocyanate and a diol. A suitable diisocyanate is for example 1,4-butanediisocyanate. A suitable diol is for example 1,4-butanediol. In an embodiment the crystalline segment is derived from 1,4-butanediisocyanate and 1,4-butanediol building blocks (and optional further building blocks). It was found that the mechanical properties obtained with smaller crystalline segments are inferior to the mechanical properties obtained with larger crystalline segments. Therefore, the crystalline segment preferably comprises three diisocyanate building blocks or more, such as in the range of 3-9 diisocyanate building blocks. The number average molecular weight of a polyurethane crystalline segment can be 300 g/mol or more, such as 400 g/mol or more, or 500 g/mol or more.

The crystalline segment can comprise a polyester block. A suitably polyester block can suitably comprise poly(s-caprolactone), poly(glycolic acid), poly(trimethylene carbonate) or combinations thereof. The number average molecular weight of a polyester crystalline segment can be 1000 g/mol or more, such as 1200 g/mol or more, or 1500 g/mol or more. In formulas (I) and (II) above, R is preferably selected from one or more aliphatic polyesters, polyetheresters, polyethers, polyanhydrides and/or polycarbonates, and at least one R comprises a hydrophilic segment.

R' and R" are independently C2-C8 alkylene, optionally substituted with C1-C 10 alkyl or C1-C 10 alkyl groups substituted with protected S, N, P or O moieties and/or comprising S, N, P or O in the alkylene chain. When no hydrophilic segment is present in the part of the polymer that is associated with the aliphatic polyether, polyester, polyanhydride and/or polycarbonate, a suitable biodegradable and hydrophilic polymer may be provided by selecting at least one R element to be a polyether. Alternatively, the hydrophilic segment may also be comprised in the R' or R" element, although this is not preferred. A hydrophilic segment is always present in the soft segment.

The R element may suitably comprise an amorphous polyester, obtained, for instance, by ring opening polymerisation of cyclic lactones such as lactide (L, D or L/D), glycolide, ε-caprolactone, δ-valerolactone, trimethylene carbonate, tetramethylene carbonate, l,5-dioxepane-2-one or para-dioxanone. These polyester pre-polymers preferably contain hydroxyl end-groups obtained by using 1,4-butanediol or polyethylene glycol as an initiator.

R' is preferably C2-C8 alkylene, optionally substituted with C1-C10 alkyl or C1-C 10 alkyl groups substituted with protected S, N, P or O moieties and/or comprising S, N, P or O in the alkylene chain. R' is preferably derived from a diisocyanate of the formula 0=C=N— R'— N=C=0 (formula IV), such as alkane diisocyanate, preferably 1,4-butanediisocyanate (BDI).

R" is preferably C2-C8 alkylene, optionally substituted with C1-C10 alkyl or C1-C 10 alkyl groups substituted with protected S, N, P or O moieties and/or comprising S, N, P or O in the alkylene chain.

Z^!-Z⁴ may be urea, amide or urethane, preferably urethane. In that case, the polymer of formula (I) is a polyurethane.

Preferably, the hard segments have a uniform block length. This means that within one polymer according to formula (I), the values for p and q are constant. A uniform block length also implies very good phase-separation and can be obtained by different chain-extending methods.

In a further aspect, the invention is directed to a process for preparing a biodegradable porous polymer network, comprising

- aminolysing a polymer network with a diamine, thereby providing the polymer network with free amine groups, wherein the polymer network comprises a phase-separated biodegradable polymer, said polymer comprising an amorphous segment and a crystalline segment, at least said amorphous segment comprising a hydrophilic segment, wherein said polymer comprises one or more selected from a urethane linkage and an ester linkage, and

optionally reacting at least part of the free amine groups with one or more moieties that comprise a biological compound.

The inventors found that desirable biodegradable porous polymer networks can advantageously be prepared in a simple method by aminolysing an existing polymer network using a diamine. The existing polymer network may be in the form of a scaffold or film. It was found that this method can yield a porous polymer network which is activated both on the outer surface, as well as within the bulk of the polymer network. If the optional step of reacting the activated porous polymer network with one or more moieties that comprise a biological compound is performed, the method of the invention can yield a porous polymer network which is functionalised both on the other surface, as well as within the bulk of the polymer network.

In a suitable embodiment, the diamine used for aminolysing the existing polymer network can be represented by the formula H2N-R¹-NH2. R¹ may be an aliphatic moiety or an aromatic moiety, or it may comprise both aliphatic and aromatic parts. R¹ can, for instance, be selected from the groups consisting of an optionally substituted linear or branched Ci-Cis alkyl or alkenyl group, an optionally substituted C3-C8 cycloalkyl group, an optionally substituted aryl group (such as an optionally substituted phenyl group), and an optionally substituted C7-C10 aralkyl group. Preferably, R¹ is selected from the group consisting of an optionally substituted linear or branched C1-C18 alkyl group, and an optionally substituted C3-C8 cycloalkyl group.

Examples of suitable diamine compounds include ethylenediamine, 1,2-propanediamine, 1,3-propanediamine, 1,4-butanediamine,

1,5-pentanediamine, 1,6-hexanediamine, 1,8-octanediamine,

1,9-nonanediamine, 1, 10-decanediamine, 1, 12-dodecanediamine,

1, 11-tridecanediamine, 1, 13-tridecanediamine, 1, 14-tetradecanediamine, 1, 16-hexadecanediamine, 1, 18-octadecanediamine,

2, 2-dimethyl- 1,3-propanediamine, 2-methyl- 1,5-pentanediamine,

2-methyl- 1,8-octanediamine, 2,2,4-trimethylhexamethylenediamine,

2,4,4-trimethylhexamethylenediamine, 5-methyl- 1,9-nonanediamine,

2-butyl-2-ethyl- 1,5-pentanediamine, 3-methylhexamethylenediamine, cyclohexanediamine, l,3-bis(aminomethyl)cyclohexane, isophoronediamine, norbornanedimethylamine, 4,4'-diaminodicyclohexylmethane,

2,2- (4, 4'- diaminodicyclohexyl)prop ane,

3,3'-dimethyl-4,4'-diaminodicyclohexylmethane, l,3-bis(aminomethyl)benzene, m-xylylenediamine, o-phenylenediamine

4,4'-methylene-bis(2-ethyl-6-methylaniline), and combinations thereof.

In a preferred embodiment of the process of the invention, the polymer network that is aminolysed comprises a polymer according to general formula (I),

-[R-Q![-R'-Zi- [R"-Z2-R'-Z¾-R"-Z4]_q-R'-Q2]_n- (I), wherein

R is selected from one or more aliphatic polyesters, polyetheresters, polyethers, polyanhydrides and/or polycarbonates, and at least one R comprises a hydrophilic segment, R' and R" are independently C2-C8 alkylene, optionally substituted with C1-C10 alkyl or C1-C10 alkyl groups substituted with protected S, N, P or O moieties and/or comprising S, N, P or O (e.g. ether, ester, carbonate and/or anhydride groups) in the alkylene chain, Z¹, Z², Z³, and Z⁴ are independently amide, urea or urethane,

Q¹ and Q² are independently urea, urethane, amide, carbonate, ester or anhydride,

n is an integer from 5-500, and p and q are independent 0 or 1, provided that when q is 0, R is a mixture of at least one crystalline polyester, polyetherester or polyanhydride segment and at least one amorphous aliphatic polyester, polyether, polyanhydride and/or polycarbonate segment.

In the process of the invention it is preferred that the free amine groups that are provided in the polymer network by aminolysis with the diamine are comprised in free groups of the following structure

O

II H _Λ

Y— C— N— R -NH₂

wherein Y is selected from CH2 and NH, and

R¹ is selected from the groups consisting of an optionally substituted linear or branched C1-C18 alkyl or alkenyl group, an optionally substituted C3-C8 cycloalkyl group, an optionally substituted aryl group (such as an optionally substituted phenyl group), and an optionally substituted C7-C 10 aralkyl group.

The extent of aminolysis can vary depending on the desired properties of the resulting porous polymer network. These desired properties can differ depending on the intended application. Suitably, the polymer network is aminolysed to an extent that the product has 2.5 mmol of the free amine groups per gram of aminolysed polymer, preferably 2.0 mmol of the free amine groups per gram of aminolysed polymer, more preferably 1.0 mmol or less per gram of aminolysed polymer, such as 0.5 mmol or less per gram of aminolysed polymer, 0.2 mmol or less per gram of aminolysed polymer, or 0.1 mmol or less per gram of aminolysed polymer. The free amino groups are preferably comprised in free groups of the structure as defined above. The extent of aminolysis can be controlled by the amount of diamine used in the aminolysis step of the process of the invention. It is also possible to control the extent of aminolysis by the time period of performing the aminolysis step. It is suitable to perform the aminolysis step in the presence of an alcohol, for instance, by using the alcohol as a solvent for the diamine. The type of alcohol is not typically limiting. Suitable examples of alcohols that may be used include isopropanol, 2-butanol, i-butanol, 2-pentanol, 3-pentanol, cyclobutanol, and cyclopentanol. In a preferred embodiment, the aminolysis step is carried out in the presence of a secondary alcohol, such as in the presence of isopropanol.

The aminolysis step can suitably be performed by immersing the polymer network in a solution of the diamine. Such a solution may have a concentration of diamine, for instance, in the range of 1-25 wt.%. The aminolysis step can advantageously be performed at room temperature, i.e. without additional heating. The time period of the aminolysis step can vary, but typically is in the range of from 5 minutes to 480 minutes, such as in the range of from 10 to 240 minutes, from 15 minutes to 120 minutes, or from 20 minutes to 90 minutes.

Suitably, the aminolysed polymer network can be subjected to one or more washing steps, such as with water (preferably demineralised water). The aminolysed polymer network may further be subjected to freeze drying.

In the optional step the activated biodegradable porous polymer network can be functionalised by reacting free amine groups with one or more moieties that comprise a biological compound. The one or more moieties that comprise a biological compound suitably comprise a carboxyl group. Such a carboxyl group can suitably react with a free amine group to form an amide bond. The moiety can itself be the biological compound, or it can contain the biological compound. A wide variety of biological compounds can be used. Some examples of suitable biological compounds include nucleic acids, lipids, proteins and free amino acids, carbohydrates, and connective tissue. Preferred examples include heparin, collagen, fibrin, hyaluronic acid, albumin, elastin, hormones, and growth factors. Heparin is a particularly interesting biological compound, since it has binding sites for growth factors thereby allowing the provision of a scaffold based on a biodegradable porous polymer network according to the invention that comprises growth factors for cells. In addition, heparin can protect growth factors from early degradation and attached growth factors remain bioactive.

The one or more moieties that comprise a biological compound may be reacted with the free amine groups through a suitable coupling agent.

Examples of such coupling agents are well-known in the art and, for instance, include N-hydroxysuccinimide, N-hydroxysulphosuccinimide and carbodiimide coupling agents (such as l-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N,N'-dicyclohexyl-carbodiimide).

Advantageously, the one or more moieties that comprise a biological compound can be coupled to the polymer network at room temperature.

Suitably, the polymer network can be subjected to one or more washing step after having performed this optional coupling step so as to remove unbound moieties. Advantageously, the process of the invention yields a biodegradable porous polymer network wherein one or more biological compounds are coupled to the polymer network via covalent linkage.

In a further aspect, the invention is directed to a biodegradable porous polymer network obtainable by the process of the invention. The biodegradable porous polymer network may be in the form of a scaffold or film.

In yet a further aspect, the invention is directed to the use of a biodegradable porous polymer network as a scaffold for tissue engineering. In particular, the biodegradable porous polymer network of the invention finds use in musculoskeletal tissue engineering.

The invention will now be further explained by means of the following examples, which are not intended to limit the scope of the invention in any way. Examples

Scaffolds were obtained from Polyganics BV, the Netherlands) consisting of PDLLA/PCL (50/50) and butanediisocyanate. Porous scaffolds consisting of 5 % polymer and 95 % porosity were produced by solvent leaching. Films of the polymer were also produced.

Aminolysis of scaffolds was performed by immersing samples in 5 %

1,6-hexanediamine (Merck, NJ USA) in isopropanol for 60 minutes at room temperature. After stringent washing with demineralised water samples were freeze dried over night.

From the porous scaffolds the amount of free amines were quantified via 1 M ninhydrin staining and absorbance spectroscopy at 535 nm (n = 5).

Subsequently, scaffolds were prepared for cress-linking with heparin. Scaffolds were saturated with 50 mM 2-morpholinoethane sulphonic acid buffer (MES, pH 5.0). The reaction was prepared via standard carbodiimide coupling. A total of 0.25 % heparin sulphate (w/v) (Organon, Oss, the Netherlands) was added to 33 mM l-ethyl-3-dimethyl aminopropyl carbodiimide (EDC) and 6 mM

N-hydroxysuccinimide (NHS) in MES buffer. 1 ml of reaction mix

(EDC/NHS/heparin in MES) was added per 8 mg of scaffold.

After 4 hours of incubation at room temperature, scaffolds were washed: 2 x 60 minutes with 0.1 M Na₂HPO₄, 2 x 60 minutes with 1 M NaCl, 2 x 60 minutes with 2 M NaCl, and 6 x 60 minutes with demineralised water. Finally, samples were freeze dried overnight. Native, aminolysed and heparin loaded scaffolds were compared with the following techniques.

Immunofluorescent staining: Cryosections of 5 μηι were blocked with 1 % BSA/PBS and subsequently incubated with the following antibodies: HS4C3, P5D4, and GtaMIgALEXA₄88. Sections were analysed with fluorescent microscopy. Immunofluorescence staining showed heparin binding to the activated scaffold only (see + + group in figure 1). Without diamine activation (- +) no heparin was detected (figure 1). These results were similar in both porous scaffolds and films. Furthermore, heparin staining was visible homogeneous throughout the complete porous scaffold.

Hydrophilicity test: Polymer films were coated with heparin as described above. After coating the sessile droplet method was used to calculate the hydrophilicity of the surface. A water droplet was placed on the polymer film and a icture was made. Contact angle was determined by the angle between the polymer film and the droplet (n = 5).

Aminolysed polyurethane films, and aminolysed with heparin coupling showed a significant decreased water contact angle compared to native scaffolds (figure 2).

Mechanical analyse: Porous scaffolds, 8 mm diameter and 5 mm in height, with and without diamine activation and heparin coating were analysed under physiological conditions (PBS, 37 °C). Mechanical properties were analysed with a BOSE ElectroForce™ BioDynamic™ with orthopaedic chamber, equipped with a 22N load-cell. Young's moduli were determined over 50 % compression at 1 mm/min (n = 6).

After 50 % compression, aminolysed polyurethane scaffolds shows a significant decrease in Young's modulus of about 50 % compared to native scaffolds.

Heparin coupling had no further effect on the mechanical stiffness (figure 3).

The surface of the polyurethane scaffold was successfully aminolysed and coupled with heparin. The heparin coating was present throughout the complete scaffold (both inner and outer surface). Coating of the polyurethane with heparin results in an increased hydrophilic surface. The surface activation results in a significant reduction in Young's modulus.

Claims

1. Biodegradable porous polymer network, comprising a

phase-separated biodegradable polymer, said polymer comprising an amorphous segment and a crystalline segment, at least said amorphous segment comprising a hydrophilic segment, wherein the polymer network comprises free groups of the following structure

O

II H ._| H 2

Y C N R N R _} wherein

Y is selected from CH2 or NH,

R¹ is selected from the groups consisting of an optionally substituted linear or branched C1-C 18 alkyl or alkenyl group, an optionally substituted C3-C8 cycloalkyl group, an optionally substituted aryl group, and an optionally substituted C7-C 10 aralkyl group, and

R² is hydrogen, or comprises a biological compound.

2. Biodegradable porous polymer network according to claim 1, wherein said porous polymer network is in the form of a scaffold or a film.

3. Biodegradable porous polymer network according to claim 1 or 2, wherein the biological active moiety comprises one or more selected from the group consisting of nucleic acids, lipids, proteins and free amino acids, carbohydrates, and connective tissue.

4. Biodegradable porous polymer network according to any one of claims 1-3, wherein the biological active moiety comprises one or more selected from heparin, collagen, fibrin, hyaluronic acid, albumin, elastin, hormones, and growth factors.

5. Biodegradable porous polymer network according to any one of claims 1-4, wherein the polymer network has a porosity in the range of from 85 to 99 %.

6. Biodegradable porous polymer network according to any one of claims 1-5, wherein the polymer network has a porosity in the range of from 90 to 99 %.

7. Biodegradable porous polymer network according to any one of claims 1-6, wherein the polymer network has a porosity in the range of from 92 to 98 %.

8. Biodegradable porous polymer network according to any one of claims 1-7, wherein the polymer network has a porosity in the range of from 95 % to 98 %.

9. Biodegradable porous polymer network according to any one of claims 1-8, wherein free groups of structure

are present on the outer surface of the polymer network, as well as on the inner surface of the porous polymer network.

10. Biodegradable porous polymer network according to any one of claims 1-9, wherein free groups of structure

are distributed substantially homogeneous throughout the porous polymer network.

11. Biodegradable porous polymer network according to any one of claims 1-10, wherein the polymer network has a water contact angle of 75° or less.

12. Biodegradable porous polymer network according to any one of claims 1-11, wherein the polymer network has a water contact angle of 70° or less.

13. Biodegradable porous polymer network according to any one of claims 1-10, wherein the polymer network has a water contact angle of from 60° to 70°.

14. Biodegradable porous polymer network according to any one of claims 1-13, wherein the polymer network has a Young's modulus of 10 kPa or more.

15. Biodegradable porous polymer network according to any one of claims 1-14, wherein the polymer network has Young's modulus of

10 kPa or more in case the biodegradable porous polymer network has a porosity in the range of 95-98 %;

18 kPa or more in case the biodegradable porous polymer network has a porosity in the range of 88-95 %; and

22 kPa or more in case the biodegradable porous polymer network has a porosity in the range of 80-88 %.

16. Biodegradable porous polymer network according to any one of claims 1-15, wherein the polymer network has Young's modulus

in the range of 10-25 kPa in case the biodegradable porous polymer network has a porosity in the range of 95-98 %; in the range of 18-40 kPa in case the biodegradable porous polymer network has a porosity in the range of 88-95 %; and

in the range of 22-40 kPa in case the biodegradable porous polymer network has a porosity in the range of 80-88 %.

17. Biodegradable porous polymer network according to any one of claims 1-16, wherein said hydrophilic segment is derived from polyether, polypeptide, poly(vinyl alcohol), polyvinylpyrrolidone) or poly(hydroxymethyl methacrylate), preferably polyether.

18. Biodegradable porous polymer network according to any one of claims 1-17, wherein said amorphous segment comprises one or more selected from polyesters, polyetheresters, polyethers, poly anhydrides and/or

polycarbonates.

19. Biodegradable porous polymer network according to any one of claims 1-18, wherein said amorphous segment comprises a polyester derived from lactide and ε-caprolactone.

20. Biodegradable porous polymer network according to any one of claims 1-19, wherein said amorphous segment comprises a polyester derived from lactide and ε-caprolactone with a number average molecular weight of between 1000 and 4000 g/mol.

21. Biodegradable porous polymer network according to any one of claims 1-20, wherein said amorphous segment comprises polyethylene glycol in a content of 1-80 wt.%.

22. Biodegradable porous polymer network according to any one of claims 1-21, wherein said amorphous segment comprises polyethylene glycol in a content of 5-60 wt.%.

23. Biodegradable porous polymer network according to any one of claims 1-22, wherein said amorphous segment comprises polyethylene glycol in a content of 20-50 wt.%.

24. Biodegradable porous polymer network according to any one of claims 1-23, wherein said amorphous segment comprises polyethylene glycol in a content of about 50 wt.%.

25. Biodegradable porous polymer network according to any one of claims 1-24, wherein said crystalline segment comprises one or more selected from urethane, urea, amide, polyester and poly anhydride groups.

26. Process for preparing a biodegradable porous polymer network, preferably according to any one of claims 1-25, comprising

aminolysing a polymer network with a diamine, thereby providing the polymer network with free amine groups, wherein the polymer network comprises a phase-separated biodegradable polymer, said polymer comprising an amorphous segment and a crystalline segment, at least said amorphous segment comprising a hydrophilic segment, wherein said polymer comprises one or more selected from a urethane linkage and an ester linkage, and

27. Process according to claim 26, wherein the diamine is represented by the formula H2N-R¹-NH2, and wherein R¹ is selected from the groups consisting of an optionally substituted linear or branched Ci-Cis alkyl or alkenyl group, an optionally substituted C3-C8 cycloalkyl group, an optionally substituted aryl group, and an optionally substituted C7-C10 aralkyl group.

28. Process according to claim 26 or 27, wherein the polymer network that is aminolysed comprises a polymer according to formula (I),

-[R-Q![-R'-Zi- [R"-Z2-R'-Z¾-R"-Z4]_q-R'-Q2]_n- (I), wherein

R is selected from one or more aliphatic polyesters, polyetheresters, polyethers, polyanhydrides and/or polycarbonates, and at least one R comprises a hydrophilic segment, R' and R" are independently C2-C8 alkylene, optionally substituted with C1-C10 alkyl or C1-C10 alkyl groups substituted with protected S, N, P or O moieties and/or comprising S, N, P or O (e.g. ether, ester, carbonate and/or anhydride groups) in the alkylene chain,

Z^!-Z⁴ are independently amide, urea or urethane,

n is an integer from 5-500, and p and q are independent 0 or 1, provided that when q is 0, R is a mixture of at least one crystalhne polyester, polyetherester or polyanhydride segment and at least one amorphous aliphatic polyester, polyether, polyanhydride and/or polycarbonate segment.

29. Process according to any one of claims 26-28, wherein said free amine groups are comprised in free groups of the structure,

O

II H _Λ

Y— C— N— R -NH₂

wherein Y is selected from CH2 and NH, and

R¹ is selected from the groups consisting of an optionally substituted linear or branched Ci-Cis alkyl or alkenyl group, an optionally substituted C3-C8 cycloalkyl group, an optionally substituted aryl group, and an optionally substituted C7-C 10 aralkyl group.

30. Process according to any one of claims 26-29, wherein said polymer network is aminolysed to an extent that the product has 2.5 mmol or less of the free amine groups per gram of aminolysed polymer.

31. Process according to any one of claims 26-30, wherein said polymer network is aminolysed to an extent that the product has 2.0 mmol or less of the free amine groups per gram of aminolysed polymer.

32. Process according to any one of claims 26-31, wherein said polymer network is aminolysed to an extent that the product has 1.0 mmol or less of the free amine groups per gram of aminolysed polymer.

33. Process according to any one of claims 26-32, wherein said polymer network is aminolysed to an extent that the product has 0.5 mmol or less of the free amine groups per gram of aminolysed polymer.

34. Process according to any one of claims 26-33, wherein said polymer network is aminolysed to an extent that the product has 0.2 mmol or less of the free amine groups per gram of aminolysed polymer.

35. Process according to any one of claims 26-29, wherein said polymer network is aminolysed to an extent that the product has 0.1 mmol or less of the free amine groups per gram of aminolysed polymer.

36. Process according to any one of claims 26-35, wherein said polymeric network is aminolysed with the diamine in the presence of an alcohol.

37. Process according to claim 36 wherein said alcohol is selected from the group consisting of isopropanol, 2-butanol, t-butanol, 2-pentanol,

3-pentanol, cyclobutanol, and cyclop entanol.

38. Process according to any one of claims 26-37, wherein the reaction between the free amine groups with the one or more moieties that comprise a biological compound comprises using a coupling agent.

39. Process according to claim 38, wherein said coupling agent is selected from the group consisting of N-hydroxysuccinimide,

N-hydroxysulphosuccinimide, l-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N,N'-dicyclohexyl-carbodiimide.

40. Biodegradable porous polymer network obtainable by the process according to any one of claims 26-39.

41. Biodegradable porous polymer network according to claim 40 in the form of a scaffold or film.

42. Use of a biodegradable porous polymer network according any one of claims 1-25, 40 or 41 as a scaffold for tissue engineering.