This invention relates generally to curable monomers and compositions. More specifically, although not exclusively, this invention relates to curable monomers and their use in compositions, for example, to form biocompatible materials.

A biomaterial is a substance that has been engineered to take a form which, alone or as part of a complex system, is used to direct, by control of interactions with components of living systems, the course of a therapeutic or diagnostic procedure. Such materials may be formed from or comprise, for example, metallic, polymeric, ceramic, and/or composite materials. Such materials may be engineered to function in a specific application, e.g. as scaffolds for tissue engineering applications of soft or hard tissue, for drug delivery purposes, for implants or bone replacements, and so on.

The design of biomaterials for use in the regeneration of hard tissue, such as bone, requires consideration of several different factors such as in situ biocompatibility and the likelihood or potential for degradability into non-toxic biproducts. The biomaterial must also provide the necessary biomechanics required during the healing process.

Bone cement, which consists essentially of polymethylmethacrylate, has been widely and successfully used to anchor artificial joint replacement. It has been proposed to use this material as a bone stabilisation system in the case of extremity fractures, wherein the material is injected and photocured using a light tube. However, the curing process may take up to 10 minutes with an increase in temperature of up to 70°C, which is detrimental to the surrounding tissue (Vegt P et al. Med Devices (Aucki) 2014;7:453-461. Zani BG et al. J Biomed Mater Res B Appl Biomater. 2016; 104(2):291-299).

US2008/039854 discloses the use of an cationic epoxy and a cationic photo-initiator which is curable under UV illumination as a bone reinforcing mixture. The epoxy may be fortified with carbon nanotubes to increase the elastic modulus thereof. The cationic photoinitiator systems are based on sulfonium or iodonium salts leading to high thermal excursions of 62°C and above during UV light curing. Moreover, iodonium salts in general are toxic in neat form.

WO2012/138732 A1 describes the use of biocompatible polycaprolactone fumarate formulations for use as scaffolds for tissue engineering applications to support nerve regeneration. It is suggested that the polycaprolactone fumarate copolymer is crosslinkable via the alkene of the fumarate unit as well as being physically crosslinkable. Scaffolds were fabricated by photocuring using Irgacure 819 as a photoinitiator under UV light at 315 to 380 nm. The mechanical properties were suitable for use in nerve regeneration but were unsuitable for hard tissue applications.

Dental composite resins for use, for example in filling cavities, are known. These resins are cured in situ and harden to form the solid filling. A well-known dental composite is bis-GMA (bisphenol A-glycidyl methacrylate), which forms a crosslinked polymer. Other dimethacrylate monomers are also known such as UDMA (urethane dimethacrylate). These are often used with triethylene glycol dimethacrylate (TEGDMA) in various mixing ratios. However, these polymers are known to release undesirable biproducts such as bisphenol A.

It is therefore a first non-exclusive object of the invention to provide a biomaterial, which is biocompatible, biodegradable, and does not produce toxic degradation products, which also provides the necessary biomechanics required during the healing process of a hard tissue, e.g. bone.

It is a further non-exclusive object of the invention to provide a biomaterial that may be used as an alternative to a metallic implant, said biomaterial having the flexibility to fit any defect with minimal surgical intervention.

Accordingly, a first aspect of the invention provides a polycaprolactone copolymer suitable for use as a polymer precursor in a resin composition, the polycaprolactone copolymer comprising a backbone chain, the backbone chain comprising one or more diol units, one or more units of a dicarboxylic acid or a dicarboxylate or a derivative thereof for example comprising a 1 ,2-alkene (e.g. one or more fumarate units), and further comprising one or more acrylate units.

The backbone chain comprises one or more diol units, one or more units of a dicarboxylic acid or a dicarboxylate or a derivative thereof (e.g. one or more fumarate units), and further comprising one or more acrylate units. That is, the backbone chain is formed from or fabricated from a reaction mixture comprising one or more diol units, one or more units of a dicarboxylic acid or a dicarboxylate or a derivative thereof (e.g. one or more fumarate units), and further comprising one or more acrylate units. In other words, the backbone chain is the reaction product of a mixture containing one or more diol units, one or more units of a dicarboxylic acid or a dicarboxylate or a derivative thereof (e.g. one or more fumarate units), and further comprising one or more acrylate units, and for example other reagents may be present.

In embodiments, the one or more diol units may be one or more of propanediol, butanediol (e.g. 1 ,4-butanediol), pentanediol, hexanediol, octanediol, decanediol and isomers thereof, e.g. 2-methyl-1 , 3-propanediol, 2-butyl-2-ethyl-1 , 3-propanediol, and/or 3-methyl-1 ,3- butanediol. In embodiments, the one or more diol units may comprise an aromatic diol. In embodiments, the one or more diol units may comprise between 1 to 10 carbon atoms, and/or may further comprise one or more heteroatoms, e.g. selected from an oxygen atom, a nitrogen atom, a sulphur atom. In embodiments, the one or more diol units may be one or more of diethylene glycol, dipropylene glycol, or dibutylene glycol.

The polycaprolactone may comprise one or more blocks of polymerised hexanoate monomers, said blocks comprising between 1 and 4 hexanoate monomers, e.g. 1 , 2, 3, or 4 hexanoate monomers.

The polycaprolactone copolymer may comprise at least two diol unit located in the backbone chain, a unit of a dicarboxylic acid or a dicarboxylate or a derivative thereof (e.g. a fumarate unit) located in the backbone chain, a first acrylate unit located at a first terminus of the backbone chain, and a second acrylate unit located at a second terminus of the backbone chain.

We use the term“acrylate” to encompass any suitable acrylic acid or enoic acid, wherein the hydrogen atom of the CH group of the alkene may be substituted with an alkyl group, e.g. comprising 1 , 2, 3, 4, or 5 carbon atoms, for example a CH ₃ , C2H5, C ₃ H7, C4H ₉ , or C5H11 group. In embodiments, one some or all of the one or more acrylate units may comprise or consist of one or more methacrylate units (i.e. wherein the alkyl group is CH ₃ ). In embodiments, one some or all of the one or more acrylate units may comprise or consist or be formed from one or more 2-ethylacrylic acid units. In embodiments, one some or all of the one or more acrylate units may comprise or consist or be formed from one or more 2- propylacrylic acid units, or 2-butylacrylate acid units, or 2-pentylacrylate acid units. In embodiments, the polycaprolactone copolymer may have the following structure:

wherein m is a number between 1.00 and 2.00, e.g. 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60, 1.65, 1.70, 1.75, 1.80, 1.85, 1.90, 1.95, or 2.00; and n is a number between 1.0 and 1.10, e.g. 1.01 , 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09 or 1.10.

In embodiments, m is 1.35.

In embodiments, the molecular weight of the polycaprolactone copolymer as measured using ¹ H NMR (CDCh) may be between 900 g/mol and 1 100 g/mol, e.g. between 950 g/mol and 1050 g/mol, or between 975 g/mol and 1050 g/mol. For example, the molecular weight as measured using ¹ H NMR (CDCh) may be 1000 g/mol or 1045 g/mol.

In embodiments, the molecular weight Mn of the polycaprolactone copolymer (measured using GPC (gel permeation chromatography) using THF (tetrahydrofuran) as an eluent and polystyrene as a standard) may be between 1000 to 2000 g/mol, e.g. between 1200 to 1800 g/mol.

In embodiments, the molecular weight Mw of the polycaprolactone copolymer (measured using GPC (gel permeation chromatography) using THF (tetrahydrofuran) as an eluent and polystyrene as a standard) may be between 2000 to 3500 g/mol, e.g. between 2200 to 3200 g/mol.

In embodiments, the polydispersity index Ip of the polycaprolactone copolymer (measured using GPC (gel permeation chromatography) using THF (tetrahydrofuran) as an eluent and polystyrene as a standard) may be between 1.5 to 2.0, e.g. 1.60 to 1.90, or 1.70 to 1.80.

The copolymer backbone may have a ABAC structure.

In embodiments, the polycaprolactone copolymer may be prepared by reacting a diol (e.g. 1 ,4- butanediol) with caprolactone to produce an intermediate, the intermediate may be further reacted with fumaric acid or fumaryl chloride to produce a further intermediate, the further intermediate may be further reacted with an acrylate, e.g. methacrylic acid and/or methacryloyl chloride.

In embodiments, the polycaprolactone copolymer may be prepared by reacting 1 equivalent of 1 ,4-butanediol with 2m or more mole equivalents (e.g. between 2 and 4 mole equivalents) of caprolactone to produce an intermediate, the intermediate is further reacted with 0.5 mole equivalents of fumaric acid or fumaryl chloride to produce an intermediate, the intermediate is further reacted with 2 mole equivalents of an acrylate, e.g. methacrylic acid and/or methacryloyl chloride.

In embodiments, the polycaprolactone copolymer may be prepared by reacting 1 equivalent of caprolactonediol (CAS number: 31831-53-5) with 0.5 mole equivalents of fumaric acid or fumaryl chloride to produce an intermediate, the intermediate is further reacted with 2 mole equivalents of methacrylic acid and/or methacryloyl chloride.

A further aspect of the invention provides a method of making a polycaprolactone copolymer, the method comprising reacting 1 mole equivalent of a diol (e.g. 1 ,4-butanediol) with 2m mole equivalents (e.g. between 2 and 4 mole equivalents) of caprolactone to produce a first intermediate, reacting the first intermediate with 0.5 mole equivalents of fumaric acid or fumaryl chloride to produce a second intermediate, and reacting the second intermediate with 2 mole equivalents of an acrylic acid or acryloyl chloride (e.g. methacrylic acid/methacryloyl chloride).

A further aspect of the invention provides a method of making a polycaprolactone copolymer, the method comprising reacting 1 mole equivalent of caprolactonediol (CAS number: 31831-53-5) with 0.5 mole equivalents of fumaric acid or fumaryl to produce an intermediate, and reacting the intermediate with 2 mole equivalents of methacrylic acid/methacryloyl chloride.

A yet further aspect of the invention provides a polylactic acid copolymer suitable for use as a polymer precursor in a resin composition, the polylactic acid copolymer comprising a backbone chain, the backbone chain comprising one or more diol (e.g. butanediol) units and further comprising one or more acrylate units. The backbone chain comprises one or more diol (e.g. butanediol) units and further comprising one or more acrylate units. That is, the backbone chain is formed from or fabricated from a reaction mixture comprising one or more diol (e.g. butanediol) units and further comprising one or more acrylate units. In other words, the backbone chain is the reaction product of a mixture containing one or more diol (e.g. butanediol) units and further comprising one or more acrylate units, and for example other reagents may be present.

In embodiments, the polylactic acid copolymer may comprise at least one diol unit (e.g. butanediol unit) located in the backbone chain, a first acrylate unit located at a first terminus of the backbone chain, and/or a second acrylate unit located at a second terminus of the backbone chain.

In embodiments, the one or more acrylate units may comprise or consist of one or more methacrylate units.

The polylactic acid copolymer may comprise blocks of repeating units of lactic acid (e.g. between 1 to 10, 2 to 9, 3 to 8, 4 to 7, 5 to 6 repeating units of polymerised lactic acid (for example, 4 to 5 repeating units of polymerised lactic acid, e.g. 4.00, 4.10, 4.20, 4.30, 4.40, 4.50, 4.60, 4.70, 4.80, 4.90, or 5.00). In embodiments, at least one (e.g. two or more) of the one or more acrylate unit may be adjacent or bonded directly to a lactic acid unit or a block of repeating units of lactic acid (e.g. with no other monomer unit therebetween). In embodiments, the oxygen atom of the alcohol group of a lactic acid unit may form the non carbonyl oxygen of the acrylate unit.

In embodiments, the polylactic acid copolymer may have the following structure:

wherein n is a number between 1.00 to 10.00, for example, between 2.00 to 9.00, or between 3.00 to 8.00, or between 4.00 to 7.00, or between 5.00 to 6.00. In embodiments, n is a number between 4.00 and 5.00, e.g. 4.00, 4.10, 4.20, 4.30, 4.40, 4.50, 4.60, 4.70, 4.80, 4.90, or 5.00.

In embodiments, n may be between any one of 4.00, 4.10, 4.20, 4.30, 4.40, 4.50, 4.60, 4.70, 4.80 or 4.90 to any one of 4.10, 4.20, 4.30, 4.40, 4.50, 4.60, 4.70, 4.80, 4.90, or 5.00. In embodiments wherein the polylactic acid copolymer comprises blocks of lactic acid subunits wherein the number of subunits is between 4.00 and 5.00 (i.e. n is a number between 4.00 and 5.00), advantageously, this provides a copolymer with optimal physical properties, including improved handleability.

The polylactic acid copolymer may be prepared by reacting a diol (e.g. 1 ,4-butane-diol) with of lactide (e.g. L-lactide), to produce an intermediate, the intermediate is further reacted with an acrylic acid or an acryloyl chloride (e.g. methacrylic acid and/or methacryloyl chloride).

The polylactic acid copolymer may be prepared by reacting 1 mole equivalent of a diol (e.g. 1 ,4-butane-diol) with n mole equivalents or more of lactide, e.g. L-lactide, to produce an intermediate, the intermediate may be further reacted an acrylate, e.g. with 2 mole equivalents of methacrylic acid/methacryloyl chloride.

The copolymer backbone may have an AB structure.

In embodiments, the molecular weight of the polylactic acid copolymer as measured using ¹ H NMR (CDCI ₃ ) may be between 600 g/mol and 1300 g/mol, e.g. between 700 g/mol and 1200 g/mol, or between 800 g/mol and 1100 g/mol, or between 900 g/mol and 1000 g/mol. For example, the molecular weight as measured using ¹ H NMR (CDCI ₃ ) may be 917 g/mol.

In embodiments, the molecular weight Mn of the polylactic acid copolymer (measured using GPC (gel permeation chromatography) using THF (tetrahydrofuran) as an eluent and polystyrene as a standard) may be between 500 to 1500 g/mol, e.g. between 800 to 1200 g/mol, e.g. 1015 g/mol.

In embodiments, the molecular weight Mw of the polylactic copolymer (measured using GPC (gel permeation chromatography) using THF (tetrahydrofuran) as an eluent and polystyrene as a standard) may be between 500 to 1500 g/mol, e.g. between 700 to 1300 g/mol, e.g. 1290 g/mol. In embodiments, the polydispersity index Ip of the polylactic acid copolymer (measured using GPC (gel permeation chromatography) using THF (tetrahydrofuran) as an eluent and polystyrene as a standard) may be between 1.0 to 1.5, e.g. 1.10 to 1.40, or 1.20 to 1.30.

In embodiments, the polylactic acid copolymer is optically active and has an enantiomeric excess, e.g. of between 90 to 100 % ee, for example, greater than 95% ee. In embodiments, the polylactic acid copolymer is optically pure, or enantiomerically pure (or at least substantially enantiomerically pure, i.e. >90% ee or >95% ee) .

It has been surprisingly found that the use of an enantiomerically pure polylactic acid copolymer in a resin composition is able to form crosslinked polymers with advantageous properties. It has been found that polylactic acid copolymers fabricated using enantiomerically pure L-lactide, are semi-crystalline. In contrast, racemic polylactic acid (fabricated from a mixture of D- and L- lactide monomers) comprises an amorphous phase. Advantageously, the crystalline phase of polylactic acid, fabricated using L-lactide only, has a greater biostability and life span. Without wishing to be bound by any particular theory, the inventors believe that this is due to resistance to hydrolytic degradation. This is advantageous when an extended period of time is required for tissue regeneration, e.g. hard tissue regeneration (for example, polylactic acid influenced cell response with higher adhesion patterns on semi-crystalline substrates as reported by A. J. Salgado et al. in Materials Science Forum, Vols. 514-516, pp. 1020-1024, 2006) whilst the mechanical properties of the biomaterial, e.g. implant, remains uncompromised.

A yet further aspect of the invention provides a method of making a polylactic acid copolymer, the method comprising reacting 1 mole equivalent of a diol (e.g. 1 ,4-butane- diol) with n mole equivalents or more of lactide, e.g. L-lactide, to produce an intermediate; reacting the intermediate with 2 mole equivalents of an acrylate, e.g. methacrylic acid/methacryloyl chloride to produce the polylactic acid copolymer.

A yet further aspect of the invention provides a resin precursor composition, the resin precursor composition comprising:

a polycaprolactone copolymer suitable for use as a polymer precursor in a resin composition, the polycaprolactone copolymer comprising a backbone chain, the backbone chain comprising one or more diol (e.g. butanediol) units, one or more units of a dicarboxylic acid or a dicarboxylate or a derivative thereof (e.g. one or more fumarate units), and further comprising one or more acrylate units (e.g. methacrylate units); and/or

a polylactic acid copolymer suitable for use as a polymer precursor in a resin composition, the polylactic acid copolymer comprising a backbone chain, the backbone chain comprising one or more diol (e.g. butanediol) units and further comprising one or more acrylate units (e.g. methacrylate units).

A yet further aspect of the invention provides a cross-linked polymer fabricated using a resin composition, the resin composition comprising a precursor resin, the precursor resin comprising:

a polycaprolactone copolymer suitable for use as a polymer precursor in a resin composition, the polycaprolactone copolymer comprising a backbone chain, the backbone chain comprising one or more diol (e.g. butanediol) units, one or more units of a dicarboxylic acid or a dicarboxylate or a derivative thereof (e.g. one or more fumarate units), and further comprising one or more acrylate units (e.g. methacrylate units); and/or

a polylactic acid copolymer suitable for use as a polymer precursor in a resin composition, the polylactic acid copolymer comprising a backbone chain, the backbone chain comprising one or more diol (e.g. butanediol) units and further comprising one or more acrylate units (e.g. methacrylate units).

A yet further aspect of the invention provides a scaffold for use in the replacement and/or regeneration of tissue, e.g. hard tissue such as bone, the scaffold comprising and/or being formed from the cross-linked polymer of the invention.

It has been found that a resin composition comprising a polylactic acid copolymer blended with a polycaprolactone copolymer according to the invention may be cured, e.g. photocured, to provide a cross-linked polymer. The resin composition may be used as a biomaterial with comparatively advantageous properties. It has been found that the polylactic acid polymer precursor is brittle, whereas the polycaprolactone polymer precursor is more flexible. Consequently, the mechanical properties of the cross-linked polymer may be tuned by adjusting the proportions of the polymer precursors within the resin composition. Advantageously, the resin composition of the invention may be tuned to provide a cross-linked polymer with optimal properties for different applications, e.g. different applications of biomaterials. Advantageously, the at least one acrylate moiety of both the polylactic acid polymer precursor and the polycaprolactone polymer precursor enables the polymer precursors of the precursor resin to crosslink with one another to form a highly crosslinked polymer network. The composition may be photocurable. In embodiments the acrylate moieties react upon irradiation with an appropriate light source. Advantageously, the resin composition of the invention may be photo-polymerised using conventional light curing technologies. Preferably, the resin composition may be photo-polymerised using visible or UV light.

Advantageously, as the resin composition polymerises only a slight temperature rise is experienced. The temperature rise may be less than 35°C, say less than 30, 29, 28, 27, 26, 25, 24, 23, 22, 21 , 20°C. The low exothermicity is beneficial when the composition is used in vivo as it limits or avoids causing damage (e.g. thermal damage) to adjacent or proximate cells.

A polylactic acid copolymer refers to a polymer chain comprising at least one block of repeating subunits of lactic acid, e.g. between 1 and 10 subunits, or 2 to 8 subunits, or 3 to 7 subunits, or 4 to 6 subunits (i.e. monomers) of lactic acid, for example, formed from lactide. For example, the polylactic acid copolymer may comprise a polymer chain comprising two blocks of repeating subunits of lactic acid, e.g. between 4 and 5 subunits (i.e. monomers) of lactic acid, such that there is a total of 8 to 10 subunits (i.e. monomers) of lactic acid per polymer chain. The polylactic acid copolymer may comprise a polymer chain comprising more than two blocks of repeating subunits of lactic acid e.g. between 4 and 5 subunits (i.e. monomers) of lactic acid. The polylactic acid polymer precursor according to the invention may further comprise additional monomers and/or blocks of repeating subunits of monomers that are different to lactic acid within the polymer chain.

A polycaprolactone copolymer refers to a polymer chain comprising at least one of repeating subunits of hexanoate, e.g. between 2 and 4 subunits (i.e. monomers) of hexanoate, for example, formed from caprolactone. The polycaprolactone polymer precursor according to the invention may further comprise additional monomers and/or blocks of repeating subunits of monomers that are different to the hexanoate subunits within the polymer chain. In embodiments, the polylactic acid copolymer may be present in more than 0 and less than 100 wt.% of the precursor resin, and the polycaprolactone copolymer may be present in more than 0 and less than 100 wt.% of the precursor resin. For example, the polylactic acid copolymer may be present in the composition in a quantity of between greater than or equal to any one of 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 90 wt. % to less than or equal to any one of 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, 30, 25, 20, 15, or 10 wt.% of the precursor resin. The polycaprolactone copolymer may be present in the composition in a quantity of between greater than or equal to any one of 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 90 wt. % to less than or equal to any one of 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, 30, 25, 20, 15, or 10 wt.% of the precursor resin.

In embodiments, the polylactic acid copolymer may be present in greater than or equal to 50 wt.% and less or equal to than 90 wt.% of the precursor resin, and the polycaprolactone copolymer may be present in less than or equal to 50 wt.% and greater than or equal to 10 wt.% of the precursor resin

In embodiments, the precursor resin consists of the polylactic acid copolymer and the polycaprolactone copolymer.

In embodiments, the at least one acrylate moiety of either or both of the polylactic acid copolymer and/or the polycaprolactone copolymer comprises a terminal acrylate moiety.

In embodiments, the polylactic acid copolymer and/or the polycaprolactone copolymer may comprise a first terminal acrylate moiety and a second terminal acrylate moiety, located at opposite terminal ends of the backbone chain.

The polycaprolactone copolymer and the polylactic acid copolymer comprise one or more butanediol units, e.g. 1 ,4-butanediol units. Advantageously, the use of a butanediol unit within the polylactic acid copolymer and/or the polycaprolactone copolymer provides non toxic degradation products in respect of this monomer. In mammals, 1 ,4-butanediol metabolises to gamma-butanoic acid (GABA) and succinic acid, which are neurotransmitter and a key intermediate in the TCA cycle, respectively. The polycaprolactone copolymer comprises one or more units of a dicarboxylic acid or a dicarboxylate or a derivative thereof (e.g. one or more fumarate units). Advantageously, the unit of a dicarboxylic acid or a dicarboxylate or a derivative thereof (e.g. one or more fumarate units) may comprise a 1 ,2-alkene. In embodiments, the dicarboxylic acid or derivative thereof may be selected from maleic acid or a derivative thereof. In embodiments, the dicarboxylic acid or derivative thereof may be selected from fumaric acid or a derivative thereof (e.g. fumaryl chloride).

The 1 ,2-alkene may be usable to cross-link with another molecule of the polycaprolactone copolymer (e.g. via the 1 ,2-alkene, for example, via the alkene of a or the fumarate unit; and/or at least one acrylate unit) and/or the polylactic acid copolymer (e.g. the at least one acrylate unit). This cross-linking action may be in addition to the acrylate-acrylate crosslinking of the polymer precursors within the resin composition of the invention. More advantageously, in embodiments, there is no need for a crosslinker to be added to cure, e.g. photocure, and/or crosslink the resin composition according to the invention. Even more advantageously, fumarate is biocompatible because it is converted to fumaric acid, which is consumed by cells in the citric acid cycle.

In embodiments, the resin composition may be photocurable. In embodiments, the resin composition may further comprise a photoinitiator and/or a co-initiator. The photoinitiator may be a blue light photoinitiator, e.g. camphorquinone (CQ), titanocene compound or derivative. In embodiments, the photoinitiator is camphorquinone with an absorption band between 400 to 500 nm. The co-initiator may be dimethylamino-ethyl dimethacarylate (DMAEMA). In embodiments, the co-initiator may be N-phenyl glycine (NPG) and/or benzodioxole and derivatives thereof.

Advantageously, the one or more acrylate moiety of the polylactic acid copolymer and the one or more acrylate moiety of the polycaprolactone copolymer may be cross-linked by photopolymerisation using conventional UV light and/or blue light photoinitiator chemistry. It has been found that photocurable compositions according to the invention are able to photopolymerise in less than 60 seconds using a CQ/amine (e.g. dimethylamino-ethyl dimethacarylate) photoinitiator/co-initiator system, with the final degree of conversion of monomers to a polymer matrix of 94-95%. For comparison, conventional dental resins such as 2,2 bis[4-2(2-hydroxy-3-methacroyloxypropoxy) phenyl] propane (Bis-GMA) and 20 wt.% triethyleneglycol dimethacrylate (TEGDMA) with the same binary photoinitiation system reaches 78-80% conversion.

In embodiments, The photo-initiator may be present in up to 2 w/w%, preferably up to 1w/w%, most preferably less than 0.9, 0.8, 0.7, 0.6, 0.5 w/w%. In embodiments, The co initiator may be present in less than 2 w/w%, say less than 1.5 w/w%, for example, less than 1.4, 1.3, 1.2, 1.1 , 1.0 w/w%. In embodiments, the photoinitiator and the co-initiator may be present in a ratio of 1 :2 wt.% of the total system, e.g. 0.4 wt.% photoinitiator and 0.8 wt.% co-initiator, for example, 0.4 wt.% camphorquinone (CQ) and 0.8 wt.% dimethylamino- ethyl dimethacarylate (DMAEMA).

In embodiments, the composition further comprises a filler. The filler may comprise a single material or a plurality of materials. The filler may be or may comprise a ceramic filler. The filler may comprise a metal pyrophosphate and/or a metal orthophosphate. The filler may comprise a calcium phosphate-based filler such as calcium pyrophosphate and/or calcium orthophosphate. In embodiments, the filler may comprise an apatite, for example hydroxyapatite. In embodiments, the filler may comprise one or more of brushite, a/b tricalcium phosphate (TCP), calcium pyrophosphate (amorphous and crystalline), ), e.g. b- calcium pyrophosphate, and/or silicon based fillers such as bioactive glass.

Resin compositions comprising a filler according to the invention may be cured, e.g. photocured, to provide resin-based composite materials. These may find application as, for example, dental fillers. Without wishing to be bound by any particular theory, the inventors believe that ceramic-based fillers, e.g. calcium pyrophosphate, may improve biocompatibility and/or mechanical properties of the resin-based composite material.

Advantageously, it has been found that resin-based composites comprising a filler, e.g. a ceramic filler such as calcium pyrophosphate, are usable for biomaterials for regenerative applications, such as in-vivo bone remodelling (Grover et al. Biomaterials, Volume 34, Issue 28, September 2013, Pages 6631-6637). Wthout wishing to be bound by any particular theory, the inventors believe that ceramic fillers, and specifically calcium-phosphate fillers such as calcium pyrophosphate are usable to stimulate new bone growth and to act as a conduit for new bone ingression into a defect (i.e. to be osteo-inductive and osteo- conductive). The resin composition comprising a filler according to the invention may be applied or injected followed by curing to form a resin-based composite biomaterial with stability during the entire healing procedure. More advantageously, the resin-based composite biomaterial may degrade with resorption and excretion from the system without further surgical intervention.

In embodiments, the filler may be present in the resin composition in greater than or equal to 5 wt.% and less than or equal to 10 wt.% of the resin composition. It is important to note that there may be a refractive index mismatch between the filler and the precursor resin comprising the polylactic acid polymer precursor and the polycaprolactone polymer precursor. In certain embodiments, the filler may prevent light from penetrating the composition, which may prevent deep curing. The amount of filler in the composition may be tuned such that there is a balance between the mechanical properties of the resulting resin-based composite and the photocuring depth.

A yet further aspect of the invention provides a method of making a cross-linked polymer and/or a scaffold according to the invention, the method comprising:

a. providing a resin precursor composition, the resin precursor composition comprising:

a polycaprolactone copolymer suitable for use as a polymer precursor in a resin composition, the polycaprolactone copolymer comprising a backbone chain, the backbone chain comprising one or more diol (e.g. butanediol) units, one or more units of a dicarboxylic acid or a dicarboxylate or a derivative thereof (e.g. one or more fumarate units), and further comprising one or more acrylate units; and/or a polylactic acid copolymer suitable for use as a polymer precursor in a resin composition, the polylactic acid copolymer comprising a backbone chain, the backbone chain comprising one or more diol (e.g. butanediol) units and further comprising one or more acrylate units; and

b. exposing the resin composition to light, e.g. UV light and/or blue light, to initiate cross-linking of the polylactic acid copolymer and/or the polycaprolactone copolymer.

The method may further comprise step c. comprising mixing the polylactic acid copolymer, the polycaprolactone copolymer, and/or a or the photoinitiator and/or a or the filler, to form the resin composition. Preferably step c occurs before step b.

The filler may comprise a ceramic filler, e.g. a calcium pyrophosphate filler. Advantageously, the resin composition according to the invention may be used for the development of an injectable, for example photo-curable, and/or biodegradable biomaterials, e.g. for use in hard tissue restoration such as upper and lower extremity fractures. Advantageously, the resin composition according to the invention is a‘flowable’ biomaterial, which has the flexibility and versatility to conform to any cavity. The resin composition may be photo-cured in-situ with minimal surgical intervention

The potential application of the resin composition and resulting crosslinked materials find a wide range of applications in, for example, biomedical applications such as implants, surgical sutures, restorative dentistry, drug delivery, and tissue engineering. Moreover, the resin composition according to the invention may be utilised in additive manufacture such as 3D-printing to fabricate bespoke objects for use in a wide range of applications. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. For the avoidance of doubt, the terms“may”,“and/or”,“e.g.”, “for example” and any similar term as used herein should be interpreted as non-limiting such that any feature so-described need not be present. Indeed, any combination of optional features is expressly envisaged without departing from the scope of the invention, whether or not these are expressly claimed. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

The invention will now be exemplified by way of the following non-limiting Examples with reference to the accompanying drawings in which:

Figure 1 is a synthetic route to Monomer 1 , a first embodiment of the invention; Figure 2 is a synthetic route to Monomer 2, a second embodiment of the invention; Figure 3A is a schematic experimental set-up for measuring the curing kinetics by FTNIRS of the formulations F1 to F5 according to the invention;

Figure 3B is a schematic experimental set-up for measuring the curing kinetics by ATR of the formulations F1 to F5 according to the invention;

Figure 4A is a graph showing the real-time photopolymerisation reaction of formulations F1 to F5 at a sample thickness of 0.5 mm;

Figure 4B is a graph showing the real-time photopolymerisation reaction of formulations F1 to F5 at a sample thickness of 3 mm;

Figure 5 is a graph showing the dynamic reaction temperature profile during photopolymerisation;

Figure 6A is a graph showing the real-time ATR measurements of the photocuring reaction of formulations F1 to F5 at a sample thickness of 0.5 mm;

Figure 6B is a graph showing the real-time ATR measurements of the photocuring reaction of formulations F1 to F5 at a sample thickness of 3 mm;

Figure 6C is a graph showing the real-time ATR measurements of the photocuring reaction of formulations F1 to F5 at a sample thickness of 6 mm;

Figure 7A is a graph showing real-time ATR measurements of the photocuring of Composite 1 at different curing depths;

Figure 7B is a graph showing real-time ATR measurements of the photocuring of Composite 2 at different curing depths;

Figure 8 is a schematic representation of micro-tensile testing using a split ASTM D638 Type V sample mould;

Figure 9 is a graph showing the mass loss (%) vs. time of formulations according to the invention;

Figure 10 shows two graph showing cytocompatibility results of formulations according to the invention;

Figure 11 is shows two graph showing cytocompatibility results of formulations according to the invention;

Figure 12 is a series of SEM images showing soas-2 cells following 24 hours of seeding onto photocured formulations according to the invention. 1. Synthesis of the Monomers according to Examples of the Invention Synthesis of Monomer 1

Referring first to Figure 1 , there is shown a synthetic route 1 to Monomer 1 according to an embodiment of the invention. There is shown Intermediate 11 and Monomer 1. Monomer 1 was synthesised in a two-step process as follows using 1 ,4-butanediol (CAS: 110-63-4), L- lactide (CAS: 451 1-42-6), stannous 2-ethylhexanoate (CAS: 301-10-0), methacryloyl chloride (CAS: 920-46-7).

Step 1 : In a first round bottom flask L-Lactide (20g, 139 mmol, 6.26eq) was dissolved in dry toluene (80g). The mixture was purged with argon under magnetic stirring and heated to 90°C for 1 h until full solubilization. In a second round bottom flask, a solution of 50 wt.% stannous octoate (8.99g, 22.2 mmol, 0.5 eq Sn(oct)/OH) in dry toluene (9g) was added drop-wise at room temperature and under argon to a solution of 1 ,4-butanediol (2g, 22.2 mmol, 1 eq) in dry toluene (85g). This mixture was then added to the first round bottom flask and stirred at 90°C for 20h. The extent of reaction was evaluated by ¹ H NMR. ¹ H analysis revealed 98% of conversion. The reaction mixture was allowed to cool to room temperature. Methanol (2.13g, 67 mmol, 3 eq) was added (to provide the hydroxyl group at the end of chain) and stirred 1 h at room temperature. A white precipitate was removed by filtration and the toluene was evaporated. The crude was dissolved in dichloromethane (300ml_) and washed using potassium carbonate 0.5M (200ml_). A white gel was removed by filtration over Celite(RTM) and sand. The organic layer was washed five time more using sodium hydrogen carbonate 1 M (200ml_) until no precipitate remained. The organic layer was washed twice with deionized water, dried over Na2SC>4 and filtered. Finally, the product was recovered after evaporation of the solvents under reduced pressure to obtain Intermediate 11 a slightly yellow viscous oil (17.7g, yield: 80%).

Step 2: Intermediate 11 was co-evaporated twice with tetrahydrofuran to remove all traces of water. In a two neck round bottom flask, Intermediate 1 1 (17.7g, 37.3 mmol, 1 eq) and triethylamine (4.91 g, 49 mmol, 1.3 eq) were dissolved in dry tetrahydrofuran (85g). The mixture was cooled at 0°C and freshly distilled methacryloyl chloride (4.29g, 41 mmol, 1.1 eq) was added drop-wise. The mixture was stirred at room temperature overnight. The extent of reaction was evaluated by ¹ H NMR. ¹ H analysis revealed 100% conversion. The triethylamine salt was removed by filtration and the tetrahydrofuran was evaporated. The crude product was dissolved in dichloromethane (300ml_) and washed using HCI 0.5M (100mL) and twice using water (100ml_). The organic layer was dried over Na2SC>4 and filtered to obtain Monomer 1 as a colourless liquid (16g, yield: 80%).

Synthesis of Monomer 2

Referring now to Figure 2, there is shown a synthetic route 2 to Monomer 2 according to an embodiment of the invention. There is shown caprolactonediol 21 , Intermediate 22, and Monomer 2. Monomer 2 was synthesised in a two-step process as follows using CAPA2043 (CAS: 31831-53-5), fumaryl chloride (627-63-4), and methacryloyl chloride 920-46-7. Step 1 : Caprolactonediol 21 (CAPA2043) was co-evaporated twice with tetrahydrofuran to remove all trace of water. In a two neck round bottom flask, a solution of fumaryl chloride (3.82g, 25 mmol, 0.5 eq) in chloroform (40g) was added drop-wise at room temperature to a mixture of caprolactonediol 21 (CAPA2043, 20g, 50 mmol, 1 eq) and potassium carbonate (15.2g, 110 mmol, 2.2 eq) in chloroform (80g). The mixture was stirred at room temperature for 4h. The extent of reaction was evaluated by ¹ H NMR. ¹ H NMR analysis revealed 100% of conversion. Potassium carbonate was removed by filtration. The crude was washed using HCI 0.5M (150ml_) and twice using water (150ml_), then dried with Na2SC>4, and filtered. Finally, the product was recovered after evaporation of the solvents under reduced pressure to obtain Intermediate 22 as a colorless oil (18g, yield: 82%).

Step 2: Intermediate 22 was co-evaporated with tetrahydrofuran to remove all trace of water. In a two neck round bottom flask, Intermediate 22 (18g, 37.3 mmol, 1 eq) was dissolved in dry dichloromethane (110g). Triethylamine (4.9g, 48.5 mmol, 1.3 eq) was added. The mixture was cooled at 0°C and freshly distilled methacryloyl chloride (4.67g, 45 mmol, 1.2 eq) was added drop-wise. The mixture was stirred at room temperature overnight. The extent of reaction was evaluated by ¹ H NMR. ¹ H analysis revealed 100% of conversion. The organic layer was washed twice using hydrochloric solution 0.5M (200ml_), and twice using water (100ml_), then dried with Na2SC>4, and filtered. Finally, the product was recovered after evaporation of the solvents under reduced pressure to obtain Monomer 2 as a viscous oil (17.9g, yield: 86%). 2. Formulations 1 to 7 of Resin Composition according to Examples of the Invention

Referring now to Table 1 , there is shown the composition of the formulations F1 to F7 according to embodiments of the invention. Formulation F8 was also used, which had the composition 20:80 Monomer 1 to Monomer 2.

Density measurements were performed using a helium pycnometer at room temperature (20°C). Viscosity measurements were performed on a Discovery Hybrid Rheometer HR-1 by TA Instruments (Brusselsesteenweg 500, 1731 Asse, Belgium). Measurements were performed with a shear rate sweep of 1-2000 (1/s), using 20 mm cone-plate with cone angle of 2 degrees (991437). The plate temperature was adjusted accordingly (25 °C and/or 37 °C).

Comparative Example 1 CE1 was prepared as a comparative formulation using 80 wt.% 2,2 bis[4-2(2-hydroxy-3-methacroyloxypropoxy) phenyl] propane (Bis-GMA) and 20 wt.% triethyleneglycol di methacrylate (TEGDMA).

3. Photocuring of the Formulations 1 to 5 of Resin Composition

The formulations F1 to F5, and CE1 , were photocured using camphorquinone (CQ) as a blue light photoinitiator present as 0.4 wt.% of the total formulation, and dimethylamino- ethyl dimethacarylate (DMAEMA) as a co-initiator present as 0.8 wt.% of the total formulation. The real-time degree of conversion (DC) and rate of polymerisation of the formulations F1 to F5 were measured using Fourier transformed infra-red spectroscopy (FTIRS), both in near IR (Transmission mode) and mid-IR range (attenuated total reflectance (ATR) mode). The extent of real time curing and post curing polymerisation was determined (semi) quantitatively by comparing the amount of remaining double bonds to the initial amount with respect to time during polymerisation.

Referring now to Figure 3A, there is shown a schematic experimental set-up 3 for measuring the real-time degree of conversion (DC) and curing kinetics of the formulations F1 to F5 using FTNIR (Fourier transformed near infra-red spectroscopy).

There is shown a light source 31 , a thermocouple 32, and the sample 33 of formulation F1 to F5, or CE1.

For the light source 31 , the inventors use a SPECTRA X light engine (RTM) manufactured by Lumencor (RTM) of 14940 NW Greenbrier Parkway, Beaverton, OR 97006 USA, which was calibrated to deliver a light intensity of -1000 mW/cm ² .

The thermocouple 32 was used to take reaction temperature measurements during the photopolymerisation.

Referring now to Figure 3B, there is shown a schematic experimental set-up 3 for measuring the real-time degree of photopolymerisation of the formulations F1 to F5 using mid-range IR (ATR).

There is shown a light source 3T, a thermocouple 32’, and the sample 33’ of formulation F1 to F5, or CE1. i. Near-IR Range (Transmission Mode)

Real time photocuring measurements of formulations F1 to F5 in the near-infrared range were determined by monitoring the transmission of vinyl-C-H absorbance (intensity) at 6170 cm ^-1 through different sample thicknesses (0.5mm and 3mm) for 300 seconds. This included irradiation polymerisation when photocuring was proceeding for 120 seconds, and post irradiation, also known as dark curing, when the curing light was switched off. The progress of post irradiation polymerisation was monitored for up to 170 seconds, although it is known to evolve up to 24 hours after the initial photocuring session.

The degree of conversion was calculated using the equation below:

Final absorbance _{6170 cm} -

Conversion (%) = 1— x 100

Initial absorbance _{6170 cm} -

Referring first to Figure 4A, there is shown a graph 4A showing the real-time photopolymerisation reaction of formulations F1 to F5 at a sample thickness of 0.5 mm.

Referring also to Figure 4B, there is shown a graph 4B showing the real-time photopolymerisation reaction of formulations F1 to F5 at a sample thickness of 3 mm.

Referring also to Tables 2 and 3, there is shown mean values for the degree of conversion of each formulation (F1 to F5, CE1) immediately after irradiation (final DC) and post irradiation for a further 170 seconds (dark curing) for a sample thickness of 0.5mm (Table 2) and 3mm (Table 3). The reaction temperature increase values in Table 3 were measured using the equipment shown in Figure 3A at 25°C.

It is shown that all formulations reached plateau within the first 50 seconds of irradiation. However, there is a gradual decline in degree of conversion both immediately after irradiation (Final DC) and post-irradiation (Dark Curing) from F1 to F5 (as the proportion of Monomer 1 in each formulation increases) for both sample thicknesses of 0.5mm and 6mm.

Without wishing to be bound by any particular theory, the inventors believe that this trend may be linked to the high viscosity of Monomer 1 (comprising a polylactic acid butanediol copolymer). The increasing viscosity in formulations F1 to F5 limits the ability of amine- derived radicals (from the co-initiators) to diffuse through the growing 3D matrix to reach the carbon double bonds in order to initiate a chain reaction for further polymerisation to occur. Instead, the radicals become‘trapped’ or immobilised in the matrix due to vitrification. This may also happen to other active species such as incomplete double bonds and/or photoinitiator molecules, which may limit the degree of conversion in some formulations. ii. M id- 1 R range (attenuated total reflectance (ATR) mode)

Real time photocuring measurements of formulations F1 to F5 in the mid-infrared range were determined using attenuated total reflectance (ATR) device composed of a horizontal multiple-reflection diamond crystal with 45° mirror angle. Static scans of pre- and post curing of all formulations (F1 to F5 and CE1) were performed to obtain a baseline and establish an isosbestic point as an internal reference.

The degree of conversion (DC) was calculated using the following equation:

100

Resin (cured) was measured using the absorption wavenumber 1637cm ¹ corresponding to C=C stretching vibration. Resin (uncured) was measured using the absorption wavenumber 1453cm ¹ corresponding to a methylene group (CH2) because it remained unchanged during polymerisation. Referring now to Figure 5, there is shown a graph 5 showing the dynamic reaction temperature profile during photopolymerisation of Formulations of the invention and Comparative Examples. This data was measured using the set-up shown in Figure 3B. Figure 5 shows data for the Comparative Example“BT2” (a commercial dental resin mixture comprising BisGMA/TEGDMA in an 80:20 ratio), Formulation F4 with 5 wt.% calcium pyrophosphate of the total composition (C1), Formulation F4 with 10 wt.% calcium pyrophosphate of the total composition (C2), and Comparative Example BT2 with 10 wt.% calcium pyrophosphate of the total composition (BT2 C1).

A temperature control ATR platform (Pike Technologies, 6125 Cottonwood Dr, Fitchburg, Wi 53719, USA) was utilised and calibrated to deliver a platform temperature of physiological temperature at 37°C (± 0.5 °C). The increase in reaction temperature was measured using the thermocouple 32’ shown in Figure 3B. After the initial reaction measurements, temperature change was monitored for further 250 seconds with another dose of irradiation for 120 seconds. This measurement was used to correct the data by subtracting it from the initial temperature rise in order to achieve‘true’ reaction temperature rise during polymerisation.

It is shown that there is a significant difference in reaction temperature increase between CE1 and the formulations according to the invention (F1 to F5) at physiological temperature. The high reaction temperatures of CE1 would be detrimental to local cellular tissues.

Referring now to Figure 6A, there is shown a graph 6A showing the real-time ATR measurements of the photocuring reaction of formulations F1 to F5 at a sample thickness of 0.5 mm.

Referring also to Figure 6B, there is shown a graph 6B showing the real-time ATR measurements of the photocuring reaction of formulations F1 to F5 at a sample thickness of 3 mm.

Referring also to Figure 6C, there is shown a graph 6C showing the real-time ATR measurements of the photocuring reaction of formulations F1 to F5 at a sample thickness of 6 mm.

Referring also to Tables 4, 5, and 6, there is shown mean values for the degree of conversion of each formulation (F1 to F5, CE1) immediately after irradiation (final DC) and post-irradiation for a further 170 seconds (dark curing) for a sample thickness of 0.5mm (Table 4), 3mm (Table 5), and 6mm (Table 6) at 25°C.

Referring also to Tables 7, 8, and 9, there is shown mean values for the degree of conversion of each formulation (F1 to F5, CE1) immediately after irradiation (final DC) and post-irradiation for a further 170 seconds (dark curing) for a sample thickness of 0.5mm (Table 7), 3mm (Table 8), and 6mm (Table 9) at 37°C. The Reaction Temperature (T _max ) shown in Table 8 was recorded using the experimental set-up shown in Figure 3B at 37°C.

Table 8 Extent of polymerisation at 3 m thickness at 37°C

It is shown that the maximum degree of conversion of formulations F1 to F5 into a polymer matrix drops slightly at high curing depth (6 mm) for formulations containing higher amounts of Monomer 1. Without wishing to be bound by theory, the inventors believe that this effect may be attributed to the highly viscous nature of Monomer 1 (comprising a copolymer of polylactic acid and butanol). Nonetheless, the final DC values of formulations F1 to F5 are well above the 35-77% threshold for conventional dental resins, as specified in “Biocompatibility of Dental Materials” (Schmalz, Gottfried, Arenholt Bindslev, Dorthe, Springer, 2009).

4. Fabrication of Resin-based Composites, according to Examples of the Invention

Resin-based composites were developed comprising the resin composition formulations F1 to F5, and further comprising a filler, the filler comprising bioactive crystalline calcium pyrophosphate.

Calcium pyrophosphate was made according to an established protocol described in W02008006204. The star-shaped pyrophosphate crystals were milled using a zirconia ball miller followed by a series of sieving to obtain 40-63 pm size filler particles. This minimised the void and porosity in the resulting resin-based composite. It was found that a relatively low pyrophosphate content, e.g. 5 to 10 wt.%, was the optimal content for providing good mechanical properties without compromising photo polymerisation. This enables light to be delivered to achieve deep curing (>2mm). It was also found that the resin composition that provided the best tensile and compressive properties, in addition to the best photopolymerisation measurements, and the best handleability and injectability, was formulation F4.

Composite 1 (C1): A resin-based composite was fabricated using resin composition formulation F4 and 5 wt.% calcium pyrophosphate of the total composition. The mixture was photocured to form Composite 1.

Composite 2 (C2): A resin-based composite was fabricated using resin composition formulation F4 and 10 wt.% calcium pyrophosphate of the total composition. The mixture was photocured to form Composite 2.

Referring now to Figure 7A and 7B, there is shown a graph showing real-time ATR measurements of the photocuring of Composite 1 (Figure 7 A) and Composite 2 (Figure 7B) at different curing depths (0.5mm, 3mm, and 6mm). The ATR measurements were taken during irradiation (up to 120 seconds) and post-irradiation (up to 300 seconds from the beginning of curing).

Referring also to Tables 10, 11 , and 12, there is shown mean values for the degree of conversion of each composite (C1 and C2) immediately after irradiation (final DC) and post- irradiation for a further 170 seconds (dark curing) for a sample thickness of 0.5mm (Table 10), 3mm (Table 11), and 6mm (Table 12) at 25°C.

Referring also to Tables 13, 14, and 15, there is shown mean values for the degree of conversion of each composite (C1 and C2) immediately after irradiation (final DC) and post irradiation for a further 170 seconds (dark curing) for a sample thickness of 0.5mm (Table 13), 3mm (Table 14), and 6mm (Table 15) at 37°C.

It is shown that the degree of conversion (DC) reached a plateau within the first 100 seconds of irradiation for all three depths (0.5mm, 3mm, and 6 mm), which suggests that 120 seconds of initial irradiation at -1000 mW/cm ² should suffice in achieving adequate DC. Similarly, dark curing following irradiation was also shown to be more apparent when compared to photocuring the neat resin composition formulation F4 alone (see Tables 4 to 6). Without wishing to be bound by any theory, the inventors believe that this effect may be attributed to the formation of micro-voids within the resin matrix, which allow active radicals and pendent double bonds to continue diffuse through the network without any restriction, even after the vitrification stage. It is also shown that post-irradiation polymerisation at 3 m depth (dark curing) is relatively higher in comparison to the neat resin composition formulation F4 alone (see Tables 4 to 6). However, the degree of conversion reduced significantly (by more than 10%) at 6 mm curing depth. Without wishing to be bound by any particular theory, the inventors believe that the reduction in DC during irradiation is associated with inadequate delivery of light at deeper layers due to light scattering as a result of refractive index mismatch between polymer and calcium pyrophosphate particles. This further explains reduction in reaction kinetics for 6 mm thick samples, with DC reaching maximum at much later stage during irradiation.

5. Mechanical Testing

i. Tensile Testing

Referring now to Figure 8, there is shown a schematic representation of micro-tensile testing using a split ASTM D638 Type V sample mould with 3.81 mm gauge length and 1.65 mm width.

ESPE Visio Beta Vario Light Unit (3M) was used to ensure uniform polymerisation and avoid overlapping (due to the size of the sample) caused by spot curing. This light unit is equipped with four fluorescent tubes to deliver 400mW/cm ² at a wavelength of 400-500nm. Samples were irradiated for several minutes to achieve ideal condition (complete curing).

The resin-based composites, e.g. C1 and C2, are designed to target and provide stability for hard tissue fracture. Therefore, the mechanical properties of neat polymer systems at physiological strain rates were assessed.

Referring now to Tables 16 and 17, there is shown the mean values of E-modulus, tensile strength, and ultimate strength, for each of the formulations F1 to F5, F8, CE1 , and composites C1 and C2, according to Examples of the invention.

The tests were conducted until breaking using strain rates of 1 s ¹ (Table 16) and 0.05 (Table 17). A strain rate of 0.05 s ¹ corresponds to the maximum strain rate during sprinting and downhill running, whereas a strain rate of 1 s ¹ represents the critical strain rate of a human femur beyond which the energy absorption capacity begins to decline resulting in failure.

The elastic modulus (the ratio of stress over strain) was determined by a line of regression at two points in the elastic region. T ensile strength was determined at 0.2% strain tolerance, which is a commonly used tolerance point in industry (C.T.F. ROSS, 2 - Stress and Strain, Editor(s): C.T.F. ROSS, In Woodhead Publishing Series in Civil and Structural Engineering, Mechanics of Solids, Woodhead Publishing, 1999, Pages 54-87, ISBN 9781898563679). Ultimate strength corresponds to the strength of a material just before breaking/necking.

It is known that Monomer 1 , comprising a copolymer of polylactic acid and butanediol, is inherently more brittle than Monomer 2. Therefore, increasing the proportion of Monomer 1 from F1 to F5 improves the overall modulus and tensile strength.

As shown in Table 16, the amount of stress applied at a given deformation increased drastically from about 6 GPa (for F1) to almost 18 GPa (for F4).

It is shown that the formulations and composites according to the invention are particularly suitable for use in bone scaffolds. The elastic modulus of human cortical bone in the longitudinal direction is reported to be in the range of 4-23 GPa (~18 (compression)), although this is dependent on various factors such as age, mineral content, porosity, and anatomical location. In addition, trabecular bone has an ultimate strength of around 50 MPa and 8 MPa under compression and tensile respectively. Its elastic modulus has been reported to be 400 MPa longitudinally (Hart et. al.; J Musculoskelet. Neuronal. Interact. 2017; 17(3): 114-139). ii. Compression Testing

Referring now to Tables 18 and 19, there is shown the mean values of E-modulus, yield strength, and ultimate compressive strength, for each of the formulations F1 to F5, F8, CE1 , and composites C1 and C2, according to Examples of the invention.

Compression tests were conducted on the samples of Composite 1 and Composite 2 comprising 6x12 mm cylinder blocks prepared using a stainless-steel split mould. The samples were irradiated from top and bottom for 120 seconds to ensure that deeper layers (cylinder mid-point) would have receive sufficient (dual) light dose to achieve maximum polymerisation.

It is shown that the E-modulus, yield strength, and ultimate strength under compression increase as the amount of Monomer 1 is increased within the formulation. Without wishing to be bound by any particular theory, the inventors believe that the decline in elastic modulus is due to shearing and settling, during the early onset of the test. It is known that settling is a common problem encountered during compression testing, which results in sudden increase in deformation without any increase in stress. iii. Degradation Studies

It is known that there are two main enzymes that are capable of degrading polylactic acid and polycaprolactone.

A degradation study was conducted on the hydrolytic and enzymatic degradation of photocured Formulation F4 of the invention and also on Comparative Examples. The aim of this study was to evaluate the effects of hydrolytic degradation on mass loss, water content, and mechanical properties of these materials.

The degradation study was carried out using identical cylindrical blocks (6x12mm) of the formulation F4. The blocks were prepared using F4 blends, which were either filled with beta-calcium pyrophosphate (b-q8 ₂ R ₂ q ₇ , b-CaPP) or were not filled (unfilled). The b-CaPP was made in accordance with the protocol of W02008006204.

The study was run for a total of 8 weeks (time points at 1 , 2, 4, 6 and 8 weeks, wherein n = 3 per timepoint) per treatment/control group.

For enzymatic degradation, Lipase (EC 3.1.1.3) from Rhizopus orzyae (Protein content: 50 U/mg) was dissolved in a sterile Dulbecco’s Phosphate Buffered Saline (DPBS), supplemented with penicillin-streptomycin (100U/mL - 100pg/mL) to minimise any possible risk of microbial infection. Enzyme-mediated degradation was explored at two different lipase concentrations (LP1 and LP2). LP1 (14 mg/L) corresponds to the physiological concentration of lipase in serum of healthy volunteers, whereas LP2 (140 mg/L) is the 10- fold of LP1 to simulate accelerated degradation. Comparative Examples were also prepared comprising a BT2 formulation. The BT2 formulation is a commercial dental resin mixture comprising BisGMA/TEGDMA in an 80:20 ratio. Filled (10 wt.% b-CaPP) and unfilled BT2 blend were used for comparison purposes throughout this study. The filled BT2 blend comprised 10 wt.% of b-CaPP (beta-calcium pyrophosphate, b-q82R2q7).

To measure mass loss (%) at each predefined timepoint, the blocks were removed from the DPBS media, rinsed with distilled water then dried in an oven to a constant value. Once dried, mass loss as a result of any hydrolytic degradation was calculated using Equation 1 , wherein Wo _ry and Wmi _tiai correspond to dry and initial mass (baseline) of specimen before immersion.

Mass loss ( 100 (1)

Referring now to Figure 10, there is shown a graph 10 showing the mass loss (%) of photocured neat and b-CaPP-filled system as a result of hydrolytic degradation over time in DPBS medium at 37 °C.

The graph 10 shows the degradation of the following materials: Formulation F4 (1 1); Formulation F4 LP1 (12); Formulation F4 LP2 (13); BT2 LP2 (14); cured Formulation F4 + 10wt.% b-CaPP (15); cured Formulation F4 + 10wt.% b-03RR-I_R2 (16); cured BT2 + 10wt.% b-08RR-I_R2 (17).

It is shown that all sample underwent hydrolytic degradation. The largest loss of mass of almost 1 % was observed for the F4 composite degraded in the presence of LP2 in DPBS, at 37 °C (Formulation F4 + 10wt.% b-03RR-I_R2 (16)). It is shown that both b-CaPP-filled and unfilled BT2 systems (14 and 17 respectively) appeared to be less susceptible to hydrolytic degradation, when compared with F4 based systems (13 and 16 respectively). iv. Biological Studies

The in-vitro cytocompatibility of human osteoblast-like cells (Saos-2 cells) towards Monomer 1 , Monomer 2, and Formulation F4 was tested. This was achieved by assessing cellular activities such as cell metabolism proliferation, following indirect (unpolymerized/photocured extracts) exposure to each component in liquid phase. The study further includes observing cell adhesion of Soas-2 cells on unconditioned photocured specimens.

a. Preparation of Uncured Copolymers and Resin Compositions Extract solutions of neat monomers and respective components of their unpolymerized formulations were prepared in Dulbecco’s modified Eagle’s medium (DMEM) (Biosera, Heathfield, UK) supplemented with 10% foetal bovine serum (FBS) (Biosera, Heathfield, UK), L-glutamine (100 pg/mL) and penicillin-streptomycin (100 U/mL - 100 pg/mL). Extracts were prepared using 0.1g unpolymerized monomer or formulation per ml_ of supplemented DMEM. Once prepared, solutions were aged in Bijou vials in an incubator (37 °C under 5% CO2 humidified atmosphere) for‘24 hours’ and 7 days’ time period, separately. As a precautionary measure, media extracts were sterile filtered using a 0.22 pm membrane filter with additional spotting on an agar plate (48 hours at 37 °C in a 5% CO2 humidified atmosphere) to rule out any possible risk of infection that may arise during cell treatment.

Comparative Examples were also prepared comprising Bis-GMA, TEGDMA and BT2. b. Preparation of Cured Resin Compositions

Extract solutions of cured systems of Formulation F4 were prepared by photocuring thin disk specimens (inner diameter 10, thickness 0.5 mm) (n = 3) for 120 seconds, using CG/DMAEMA (1 :2 wt.%) and Spectra X as Pl/Co-I and LCU that was calibrated to deliver 1000 ± 0.20 mW/cm ² of irradiance, respectively. Once prepared, specimens were cotton swabbed with 70% ethanol and immersed in supplemented DMEM. Media volumes were adjusted to 0.1 g/ml_ accordingly, before incubating for separate periods of time (24 hours and 7 days) at 37 °C under 5% CO2 humidified atmosphere. Following the pre-defined ageing period, extracted media were sterile filtered and spotted on an agar plate as precautionary measures, prior to cell treatment.

Comparative Examples were also prepared comprising filled and unfilled photocured BT2. c. Preparation of Cell Culture

Human osteoblast-like cell line, Saos-2, were sourced commercially (ATCC HTB-85, USA) and thawed. Cells were then cultivated in a T75 (75 cm ² ) flask (Thermo Fisher Scientific, Denmark) within supplemented DMEM media and incubated under standard conditions (37 °C with 5% CO2 in a humidified atmosphere). DMEM was replenished every 48 hours. Upon reaching near confluency, cells were sub-cultured using a protocol for passaging adherent cells which involved using trypsin-EDTA (0.25% (w/v) - 1 mM/L) for cell detachment. To avoid gene drifts, cells from 25-28 ^th passages were only used for cytocompatibility and surface attachment studies. d. In-vitro Cytocompatibility Studies

The cytotoxicity of newly synthesised Monomers 1 and 2, and their corresponding formulations, and the compositions of the Comparative Examples, were determined via indirect contact method, using alamarBlue (AB) assay. AB is a fluorometric/calorimetric bioassay designed to detect metabolic activity based on an oxidation-reduction (REDOX) growth indicator, known as resazurin. Innate cellular metabolic activity results in the chemical reduction of resazurin (Non-fluorescent, blue) into resorufin (Fluorescent, red). While continuous growth of (viable) cells maintain a reduced environment, inhibition in cell metabolic activity and subsequent growth leads to an oxidized environment. This served to determine cytocompatibility based on change in the absorbance measurement from oxidised form (Resazurin, 600 nm) to reduced AB form (Resorufin, 570 nm), following treatment with media extracts.

Upon reaching near confluency in a T75 flask, soas-2 cells were seeded in 96 flat bottom black well plates (Costar, Corning Corporation, USA) at cell seeding density of 20 cell/cm ² (per well) and incubated at 37 °C with 5% CO2 in a humidified atmosphere, for 24 hours. Post 24 hours, old media was aspirated, and wells were washed three times with DPBS before treating with 50 pl_ of collected media extracts (24 hours/ 7 days). Additional 50 mI_ of freshly supplemented DMEM was added to give a total volume of 100 mI_ with a media extract to fresh DMEM ratio of 1 : 1 , per well. Cells were incubated for further 24 hours under standard conditions, before performing alamarBlue assay. This involved aspirating the old media and washing wells with DPBS before adding supplemented DMEM containing 10% working solution (0.15 mg/ml_ of DPBS) of AB dye. Cells were incubated, under standard conditions, for 4 hours before measuring absorbances at 570 nm, using Spark plate reader (Tecan Group Ltd., Mannedorf, Switzerland). Using absorbance measurements at 600 nm as reference, reduction of AB (Resorufin) was calculated as percentage difference between treated and untreated cells (Control groups). e. Saos-2-Cells Attachment: SEM Imaging

SEM imaging of saos2 cells attachment on photocured disk specimens was achieved using thin disk specimens. Without any pre-conditioning, cured disk specimens were disinfected with 70% ethanol and placed inside wells of a transparent sterile 24 well plate (Nunclon Delta, Thermo Fisher Scientific, Denmark). Thermanox® coverslips (174950) (Thermo Scientific, Denmark) were used as positive controls. Once in place, saos-2 cells were then sub-cultured onto photopolymerised disks at cell seeding density of 20,000 cells/cm ² , in a supplemented DMEM. Following few gentle orbital shakes, cells were incubated for 24 hours at 37°C with 5% CO2 in a humidified atmosphere.

Post 24 hours, old media were aspirated, and wells containing disks, were gently rinsed 3 times with DPBS to remove any excess media and/or debris. Cells were then fixed by immersing disk specimens in 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer (Agar Scientific Ltd., Essex, UK), for 10 minutes. Specimens were then gradually dehydrated through a graded series of ethanol solutions (20 %, 30 %, 50% and 70%). To avoid cured specimens being dissolved by ethanol, dehydration at each serial ethanol concentration was no more than 2 minutes. Equally, dehydration stopped at 70% ethanol concentration solution. Used ethanol solution was discarded and specimens were critically dried by hexamethyldisilazane (HDMS) for 2-3 minutes, before air drying overnight in the fume cupboard.

Once fixed, disks were carefully removed and mounted on aluminium stubs covered with an adhesive carbon tab (Diameter: 12 mm) (Agar Scientific, Essex, UK). To improve conductivity, disk specimens were copper taped at one side and sputter coated (Emitech K550X, Quorum Technologies, Kent, United Kingdom), with two 15 nm gold layers, before mounting them onto the EVO MA10 SEM chamber (Carl Zeiss AG, Germany). Cell attachment images were captured using secondary electron mode, with electron beam operating at accelerating voltage of 20 kV under high vacuum. f. Statistical Analysis

Statistical analysis of AB reduction (%) by saos-2 cells following treatment, were performed using GraphPad Prism 5.03 (GraphPad Software Inc., San Diego, CA, USA). One-way ANOVA and post-hoc Bonferroni multiple comparison tests were carried out to establish statistical significances at 95% (P £ 0.05) confidence interval between groups per media extract treatment (24 hours/7-day old). g. Results

Referring now to Figure 1 1 , there is shown two graphs (a) and (b), which show the AB Reduction(%) (resazurin into resorufin (%)) as a function of metabolic activity of saos-2 cells treated with media extracts of neat monomers and their corresponding formulation components of the invention, as well as the Comparative Examples. The graphs show the reduction of AB (Resazurin) into resorufin (%) by soas-2 cells treated with 24 hours (graph a) and 7 day old media extracts (graph b) of unpolymerized monomers and their corresponding formulation components, for 24 hours. The dashed lines represent untreated groups (Control). Unpolymerized F4, BT2 and their respective loaded formulations, contained CQ (0.4 wt.%) and DMAEMA (0.8 wt.%) as photoinitiator and co initiator, respectively. Columns and errors bars represent mean and standard error of mean (SEM) in percent, respectively. Asterisks (* p < 0.05, ** p < 0.01 , *** p < 0.005, **** p < 0.001) indicate statistically significant differences between groups, based on post-hoc Bonferroni multiple comparison tests following one-way ANOVA.

The graphs show the cytocompatibility of Monomer 1 (PLLA-DM), Monomer 2 (PCF-DM), Formulation F4 (F4), Formulation F4 + b-CaPP (F4+ b-CaPP), Bis-GMA (BisGMA), TEGDMA (TEGDMA), BT2 (BT2), BT2+ b-CaPP (BT2+ b-CaPP), and b-CaPP. Graph (a) and Graph (b) show the cytocompatibility of soas-2 cells treated with two different media extracts per group: low concentration (24 hour-aged extracts), and graph (b) high concentration (7 day-aged media extracts), for both unpolymerized and fully cured samples. The time period of 24 hours and 7 days refers to the ageing period of supplemented DM EM with a given monomer or blend or formulation.

Statistical analysis revealed the variation in relative metabolic activity of soas2 cells to be statistically significant (P £ 0.05). Treatment with neat Monomer 1 , Monomer 2, Formulation F4, and its corresponding cured composite (Formulation F4 + b-CaPP (F4+ b-CaPP)) exhibited improved cytocompatibility. Metabolic activity of soas2-cell lines appeared to have increased following treatment with both media extracts (24hours/7 days). Exposure to 24 hours media extract at a ratio of 1 : 1 (v/v) to supplemented DMEM led to an increase of

8.11 , 14.93, 13.46, and 14.01 % in resorufin production, when compared with untreated group (Control) respectively. While resorufin production (%) largely remained the same, treatment with 7-day old collected media extracts led to an increase of 10.86, 19.86, 9.94,

4.12, when compared with control, respectively.

Statistical analysis revealed that the group treated with 7-day old media extracts of Monomer 2 (PCF-DM) to be highly significant (P £ 0.05), when compared with control. Similarly, treatment with 24 hours media extract of b-CaPP (10 wt.%) improved resorufin production by 6.33%, with respect to control. Improvement in soas-2 metabolic activity by b-CaPP can also be seen in cells treated with filled BT2 composite media extract (see graph a), where a relative increase of 5.79% in resorufin production was observed. However, treatment with 7day old media extracts of b-CaPP and filled BT2 (graph b), exhibited significant cytotoxicity (P £ 0.05), with resorufin production reducing to 76.46 and 48.56%, when compared with control respectively.

For both media extracts dataset (24 hours/7day old), cells treated with TEGDMA extracts exhibited the highest decline in resorufin amount. Production reduced to 66.92 and 33.59 %, when compared with untreated control groups (P £ 0.05). Cytotoxic effects of TEGDMA can also be seen in BT2 blend containing. Though decline in resorufin production (%) was statistically significant (P £ 0.05) only at group treated with 7-day old BT2 media extract, when compared with control via post-hoc comparison test following statistical analysis using one-way ANOVA. Meanwhile, cells treated with both BisGMA media extracts (24 hours/ 7- day old), exhibited relatively less cytotoxicity with relative resorufin production declining to 93.87 and 94.01 %. Both of which were not statistically significant (P > 0.05), when compared with respective untreated control groups.

It is worth noting that the presence of CQ/DMAEMA as Pl/Co-I in unpolymerized F4 and BT2 based systems did not affect AB reduction (%) as absorbance values remains the same when compared with respective F4 and BT2 containing no CQ/DMAEMA mixture, (P > 0.05).

Referring now to Figure 12, there is shown two graphs (a) and (b), which show the AB reduction % (Resorufin) as a function of soas-2 cell lines metabolic activity following exposure to photocured filled and unfilled F4 and BT2 media extracts for 24 hours under standard incubation conditions.

The graphs show the reduction of AB (Resazurin) into resorufin (%) by soas-2 cells treated with 24 hours (graph a) and 7-day old (graph b) media extracts of photocured formulations, for 24 hours. Filled systems were incorporated with 10 wt.% b-CaPP as filler. Dashed lines represent untreated groups (Control). Columns and errors bars represent mean and standard error of mean (SEM) in percent, respectively.

Statistical analysis showed that variation in metabolic activities are not statistically significant (P > 0.05). Nonetheless, treatment with both filled and unfilled photocured F4 media extracts appeared to have improved cytocompatibility. Resorufin production increased by 12.64 and 15.65% following treatment with 24 hours media extracts, and 15.48 and 19.10% with following 7-day old collected media extracts, when compared with control groups respectively. Similarly, cells exposed to photocured filled BT2 media extracts, also exhibited higher AB reduction (%). Resorufin production increased by 14.64% and 13.64% for 24 hours and 7-day old collected media extracts when compared with respective control groups.

On the other hand, cells exposed to photocured unfilled BT2 media extracts exhibited lower cytocompatibility with relative resorufin production of 97.26 and 93.63% following treatment with 24 hours and 7-day old media extracts, respectively. h. Soas-2 cells Attachment

Referring now to Figure 13, there is shown a series of SEM images of soas-2 cells following 24 hours of seeding on surfaces of photocured disk specimens of unfilled and filled (10 wt.%) composites of F4 and BT2, that were not preconditioned in DMEM. Thermanox® coverslips were used as a positive control. Cells appear to have attached with morphology generally associated with soas-2 cell lines (Polygonal). On both Thermanox and unfilled BT2 surfaces, cell attachments had taken place in small isolated clusters with most cells appear as round, typically associated with initial phases of substrate adhesion. The incorporation of b-CaPP filler in BT2, appears to have increased the size of clusters with adhesive structures, characterised by leading edges (lamellipodia), beginning to develop. Signs of cellular growth and extension (elongation) can be clearly seen as well. Similar expanding cell structures can also be observed on filled F4 surfaces, except for larger clusters with majority of cells appear to have completed the spreading phase and undergoing proliferation (cell division). The most pronounced cell adhesion and proliferation of soas-2 cells, was observed on unfilled F4, where under same incubation period, substrate surfaces were completely covered with cells. Significant increase in adhesive structures such as cytoplasmic projections (filopodia) extending beyond lamellipodia, can be clearly realised. Similarly, cell growth and proliferation on top of the underlying layer(s) is an indicative of osteoblast favouring colonisation.

Therefore, it has been shown in the cytocompatibility studies, and in the SEM images, that the monomers, formulations, and corresponding crosslinked or cured polymers of the invention exhibit improved cytocompatibility and reduced cytotoxicity than those materials of the prior art. There is greater cell growth observed when using the polymers of the invention. This result is surprising because no pre-conditioning in DMEM was required. Although the materials of the invention appear to degrade slightly more readily under hydrolytic and enzymatic conditions, it has been surprisingly shown that the degradation products exhibit greater cytocompatibility (and less cytotoxicity) in comparison to the materials of the prior art. Therefore, the polymers of the invention are usable as improved dental composite resins.

It will be appreciated by those skilled in the art that several variations to the aforementioned embodiments are envisaged without departing from the scope of the invention.

It will also be appreciated by those skilled in the art that any number of combinations of the aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.