WO2016181402A1

WO2016181402A1 - Processing shape memory thermoset polymers into complex 3d shapes

Info

Publication number: WO2016181402A1
Application number: PCT/IL2016/050509
Authority: WO
Inventors: Shlomo Magdassi; Daniel Cohn; Matthew Zarek; Michael Layani; Ido COOPERSTEIN
Original assignee: Yissum Research Development Company Of The Hebrew University Of Jerusalem Ltd
Priority date: 2015-05-14
Filing date: 2016-05-15
Publication date: 2016-11-17

Abstract

The invention provides a process for forming a polymeric 3D object, by printing a photo-polymerizable material, in the melt, or composition comprising same, into an object having a memorizable first shape, which is deformable into a metastable shape, revertible back to said first shape at a temperature above a triggering temperature of the polymer.

Description

PROCESSING SHAPE MEMORY THERMOSET POLYMERS INTO

COMPLEX 3D SHAPES

TECHNOLOGICAL FIELD

The invention generally provides processes for the construction of 3D objects from shape memory polymers.

BACKGROUND

Recent work in additive manufacturing has pushed the fabrication of inanimate materials beyond stationary objects to those inherently capable of dynamic behavior. This concept is manifested by 4D printing [1], self-evolving structures, self-assembled machines [2] and active composite materials [3]. These implementations exploit an environmentally-sensitive component within an assembly to drive a macroscopic transformation over time. Many applications, including soft robotics, autonomous machines and medical devices would gain from such active materials.

Present approaches at the frontier of active materials, are based on embedding sections composed of responsive materials in the 3D structure. For example by using a multi-material inkjet printer [1,3], objects were printed in which the mechanism of movement is based on the precise placement of a hydrophilic material or shape memory composite that articulates the response of the structure while immersed in water or heated.

WO2015/112915 [4] discloses biodegradable stent prosthesis formed as tubular bodies, comprising a biodegradable polymeric material. The prosthesis are expandable from crimped configurations to expanded configurations.

US2013/331792 [5] discloses devices partially made of shape memory materials which upon increasing temperature by insertion into a substrate, can deploy by reverting back to a shape memory annealed form.

WO2016/019078 [6] teaches 3D printing of biopolymer-based inks for manufacturing a broad range of products with desirable properties.

WO2014/204634 [7] teaches synthetic bone grafting materials or tissue engineering scaffolds with desired structural and biological properties and methods of their applications in vivo. WO2013/044078 [8] discloses compositions of hydroxyapatite and block copolymers, methods of their preparation, and uses thereof, wherein the co-polymers have degradable hydrophobic blocks and hydrophilic blocks for stable interfacing with hydroxyapatite, resulting in stable polymer-hydroxyapatite suspensions.

However, many of the technologies available in the art depend mainly on association of the shape memory polymers with joints or composites of metal and are also costly and at times highly complex to carry out.

PUBLICATIONS

[1] Tibbits, S. (2014). 4D Printing: Multi-Material Shape Change. Architectural Design, 84(1), 116-121.

[2] Felton, S., Tolley, M., Demaine, E., Rus, D., & Wood, R. (2014). A method for building self-folding machines. Science, 345(6191), 644-646.

[3] Ge, Qi, H. Jerry Qi, and Martin L. Dunn. "Active materials by four-dimension printing. " Applied Physics Letters 103.13 (2013): 131901

[4] WO2015/112915

[5] US2013/331792

[6] WO2016/019078

[7] WO2014/204634

[8] WO2013/044078

GENERAL DESCRIPTION

The inventors have developed and herein propose an alternative method, without necessitating costly hardware and the rigmarole of computer simulations, which offers freedom of design to transform almost any printable 3D structure into one with programmable dynamic behavior. The process of the invention is based on photo-curing semicrystalline macromonomers, at their molten state, with a digital light-processing stereolithography printer (depicted pictorially in Fig. 1A).

This use of semicrystalline macromonomers enabled the printing of several models (shown in Figs. 1B-D), which exhibited shape memory behavior. The digital models were downloaded from the web as STL file types from popular free and open source CAD hosting sites or generated by commercial CAD software (Autodesk Inventor) and modified with native printer software. At room temperature, the printed structures were rigid, while above the melting temperature (T_m), the structures were pliant and elastomeric. In such states, any deformation applied to the structure can be fixated by cooling below the T_m temperature. Reheating the structure to a temperature above their melting temperature led to a recovery of the original printed shape.

Thus, in a first aspect, the invention provides a process for the construction of a 3D object, the object being formed of a polymeric material at a temperature above the melting temperature (T_m) of a photo-polymerizable material (being a precursor of said polymeric material), providing an object having a shape which may be deformed into a metastable form, wherein the deformation may be achievable at a triggering temperature, e.g., being a temperature below a melting point of the cured 3D object.

The invention further provides a process for the construction of a 3D object, the object being constructed of a photocurable or photopolymerizable material having a melting temperature, T_m, in its uncured form, wherein upon curing the object possesses a first shape (namely a memorizable static shape) at a temperature below the triggering temperature, is susceptible to deformation to a second shape while at a temperature above the triggering temperature and kept at said second shape when cooled below its triggering temperature, said object being stable in said second shape until reheated above the triggering temperature and permitted to revert to the memorized static shape.

As used herein, the constructed object, once cured, may be deformable into a "metastable" form or a shape which is different from the shape of the object when constructed. The metastable form, while temporary to the extend desired by the manufacturer or user and for the particular utility, is a stable configuration for as long as the object is maintained at a temperature below a triggering temperature of the cured 3D object. The "triggering temperature" refers to a temperature of the photocured 3D object, at which temperature deformation of the 3D object becomes possible to the extent that the deformed metastable form may be maintained as such (fixed) once the temperature is reduced to a temperature below the triggering temperature. In other words, to deform an object from its memorizable shape, the temperature of the object must be raised above the triggering temperature of the polymer from which the object is formed. At that temperature deformation can be achieved. Once the temperature of the deformed object is reduced back to a temperature below the triggering temperature, the object becomes fixated at the deformed shape. This is a metastable shape which may be one of many possible deformed shapes. The triggering temperature varies fro each polymer or polymer composition and may be easily identifiable. One versed in the art would know how to determine the triggering temperature of the object. For example, in some embodiments, depending inter alia on the polymer or polymer composition, the temperature can be identified by the fact that the object will turn transparent above the triggering temperature and become translucent at a temperature below the triggering temperature.

In some embodiments, the melting temperature T_m of the photo-polymerizable material (being a precursor of the cured polymeric material) is the same or similar to the triggering temperature of the cured 3D object. In other embodiments, the melting temperature T_m of the photo-polymerizable material (being a precursor of the cured polymeric material) is lower than the triggering temperature of the cured 3D object. In other words, in such embodiments, deforming the cured 3D object may be at a temperature which is the same, lower or higher as compared to the Tm temperature of the photo-polymerizable precursors.

The process of the invention involves forming a 3D object having an initial shape or form. This shape or form is a "memorizable shape" to which any deformed shape will revert to once the deformed object is heated to a temperature above the triggering temperature.

The process of the invention further comprises forming a 3D object by printing, e.g., inkjetting, a photo-polymerizable material (being a precursor of the cured polymeric material) at a temperature above the T_m of the photo-polymerizable material, and curing said 3D object, the 3D object thus obtained having a memorizable first shape; permitting said cured 3D object at a first shape to deform into a metastable deformed shape at a temperature above the object triggering temperature. As long as the temperature is maintained above the triggering temperature, the object will revert back to its memorizable shape. Once the temperature is lowered below the triggering temperature, the metastable deformed shape may be maintained. Thus, the process of the invention may additionally and optionally comprise permitting said 3D object to revert to said memorized first shape from a metastable deformed shape at a temperature above its triggering temperature.

As used herein, the process for making an object of the invention, e.g., printing, makes use of photo-polymerizable materials (polymer precursors such as prepolymers, polymers of low molecular weight that may be further polymerized, oligomers, monomers and generally any photo-polymerizable material) for the construction of a shape memory polymer, which is a semicrystalline thermoplastic polymer that retains its semi-crystallinity after photo-polymerization.

The photo-polymerizable material may be a solid material or a composition of one or more materials, comprising optionally a binder, at least one ethylenically unsaturated compound (being a precursor to the shape memory polymer), and further optionally a photoinitiator.

The ethylenically unsaturated compound may be selected amongst organic materials having one or more carbon-carbon double bonds. In some embodiments, the polymerizable precursors are selected amongst polyglycolic acid, polycaprolactone, polylactic acid, polydioxanone, polypentadeca lactone and other members of the poly(hydroxyl acid) polymer family.

In other embodiments, the precursors are selected to be readable with isocyanates to form polyurethanes and then functionalized with photo-polymerizable end groups.

In further embodiments, the precursors are selected amongst such having star or graft architectures. For example, the precursor may be a pentaerythritol or any derivative thereof that can be used as an initiator for polycaprolactone star architectures.

In some embodiments, the precursor is selected amongst modified polycaprolactone with photo-polymerizable functional groups. Such modified polycaprolactones may include a variety of polycaprolactone polymers available for example from Sigma Aldrich or Perstrop CAPA™ polymers polycaprolactone polymers.

Further, these polycaprolactone or any other polymer precursor may be modified with a double bond displaying functional group, such as isocyanatoethyl methacrylate.

In some embodiments, the degree of methacrylation of the precursor can be tuned to tailor the degree of crystallinity of the precursor, since the shape memory property requires that after photo-polymerization the resulting polymer is, remains or becomes semicrystalline.

In some embodiments, a low degree of methacrylation, such as 50%, can allow a relatively low molecular weight semicrystalline precursor to maintain its crystallinity after polymerization. If the precursor is fully methacrylated, then it may not maintain crystalline phases after polymerization, and therefore would not be a shape memory material. For example, the polycaprolactone 4,000 presented in the examples, has a shape memory effect by methacrylation degree below 60%.

The at least one photo-polymerizable material or precursor is a thermoset material and may be composed of at least one photopolymerizable precursor that has a semicrystalline phase having a broad melting temperature (T_m). In some embodiments, the at least one photo-polymerizable material or precursor is two or more such materials, e.g., multiple precursors, each optionally having a distinct melting temperature. Such a blend permits obtaining a 3D printed object with multiple structural transitions. Thus, any 3D object formed according to the invention may have one or more metastable forms or shapes and may be transformable into each of said forms and shapes depending on the particular triggering temperature.

In some embodiments, the photo-polymerizable precursors are selected to have a melting temperature enabling printing of the 3D structure in the molten state. Thus, in such embodiments, the precursors are selected to have a melting temperature above room temperature (rt, 25-30°C). In some embodiments, the melt temperature is above 30°C, above 40°C, above 50°C, above 60°C, above 70°C, above 80°C, above 90°C, above 100°C, above 110°C, above 120°C, above 130°C, above 140°C, above 150°C, above 160°C or above 170°C.

In some embodiments, the photo-polymerizable precursors are selected to have a melting temperature between rt and 170°C, between rt and 160°C, between rt and 150°C, between rt and 140°C, between rt and 130°C, between rt and 120°C, between rt and 110°C, between rt and 100°C, between rt and 90°C, between rt and 80°C, between rt and 70°C, between rt and 60°C, between rt and 50°C or between rt and 40°C.

In some embodiments, the photo-polymerizable precursors are selected to have a melting temperature between 30 and 170°C, between 30 and 160°C, between 30 and 150°C, between 30 and 140°C, between 30 and 130°C, between 30 and 120°C, between 30 and 110°C, between 30 and 100°C, between 30 and 90°C, between 30 and 80°C, between 30 and 70°C, between 30 and 60°C, between 30 and 50°C or between 30 and 40°C.

In some embodiments, the photo-polymerizable precursors are selected to have a melting temperature between 50 and 170°C, between 50 and 160°C, between 50 and 150°C, between 50 and 140°C, between 50 and 130°C, between 50 and 120°C, between 50 and 110°C, between 50 and 100°C, between 50 and 90°C, between 50 and 80°C, between 50 and 70°C or between 50 and 60°C.

In some embodiments, the photo-polymerizable precursors are selected to have a melting temperature between 30 and 50°C.

In some embodiments, the photo-polymerizable precursors are selected to have a melting temperature between 50 and 70°C.

In some embodiments, the photo-polymerizable precursors are selected amongst polycaprolactone polymers. In some embodiments, the photo-polymerizable precursors are polycaprolactone polymers having an average molecular weight of between 8,000 and 25,000Da.

In some embodiments, the photo-polymerizable precursors are polycaprolactone polymers having an average molecular weight of between 8,000 and 24,000Da, between 8,000 and 23,000Da, between 8,000 and 22,000Da, between 8,000 and 21,000Da, between 8,000 and 20,000Da, between 8,000 and 19,000Da, between 8,000 and 18,000Da, between 8,000 and 17,000Da, between 8,000 and 16,000Da, between 8,000 and 15,000Da, between 8,000 and 14,000Da, between 8,000 and 13,000Da, between 8,000 and 12,000Da, between 8,000 and l l,000Da, between 8,000 and 10,000Da, between 8,000 and 9,000Da, between 10,000 and 25,000Da, between 10,000 and 24,000Da, between 10,000 and 23,000Da, between 10,000 and 22,000Da, between 10,000 and 21,000Da, between 10,000 and 20,000Da, between 10,000 and 19,000Da, between 10,000 and 18,000Da, between 10,000 and 17,000Da, between 10,000 and 16,000Da, between 10,000 and 15,000Da, between 10,000 and 14,000Da, between 10,000 and 13,000Da, between 10,000 and 12,000Da or between 10,000 and l l,000Da.

In some embodiments, the average molecular weight is between 10,000 and 16,000Da, between 11,000 and 15,000Da, between 12,000 and 14,000Da or between 13,000 and 14,000 Da.

In some embodiments, the average molecular weight is 14,000 Da.

The photo-polymerizable composition or formulation may further comprise at least one photoinitiator that is sensitive to actinic radiation, encompassing the ultraviolet and visible wavelength regions. In some embodiments, the photoinitiator is selected to be sensitive to a UV source, such as carbon arcs, mercury-vapor arcs, content fluorescent lamps, electron flash units, electron beam units, lasers, and photographic flood lamps. In some embodiments, the photoinitiator is selected to be sensitive to a radiation source emitting UV radiation between 200-1,000 nm.

In some embodiments, the photoinitiator is selected from the group of photoinitiators that cause photopolymerization (radical or ionic) upon irradiation by a UV/IR/MW light source. In some embodiments, the photoinitiator is a combination of two or more such materials.

The photoinitiator, one or more, may be any such material known in the art, for example such materials disclosed in Crivello, J.V., et al. Photoinitiators for Free Radical Cationic and Anionic Photopolymerization. 2nd edition. Edited by BRADLEY, G. London, UK: John Wiley and Sons Ltd, 1998, p. 287-294.

Non-limiting examples of suitable photoinitiators one or more of benzophenone and substituted benzophenones, 1-hydroxycyclohexyl phenyl ketone, thioxanthones such as isopropylthioxanthone, 2-hydroxy-2-methyl-l phenylpropan-l-one, 2-benzyl-2- dimethylamino-(4-morpholinophenyl) butan-l-one, benzil dimethylketal, bis (2,6- dimethylbenzoyl)-2,4,4 trimethylpentylphosphine oxide, 2,4,6 trimethylbenzoyl diphenylphosphine oxide, 2,4,6-trimethoxybenzoyldiphenylphosphine oxide, 2-methyl- l-(methylthio) phenyl] -2-morpholinopropan-l -one, 2,2-dimethoxy-l, 2 diphenylethan- 1-one, 5,7-diiodo-3- butoxy-6-fluorone, diphenylphosiphine oxides, pheylglyoxylates, a-hydoxyketones, difunctional a hydoxyketones, benzoin ethers, acetophernones, benzoyl oximes, acyl phosphine oxide, Michler's ketone, thixanthone, anthroguionone, benzophenone, methyl diethanol amine, and 2-N-butoxyethyl-4-(dimethylamino) benozoate.

Other suitable commercial photoinitiators may be selected from Irgacure™ 184, Irgacure™ 500, Irgacure™ 369, Irgacure™ 1700, jrgacure™ 651, Irgacure™ 1000, Irgacure 1300, Irgacure ^V1 1870, Darocur 1173, Darocur 2959, Darocur 4265 and Darocur™ ITX available from BASF AG, Lucerin™ TPO available from BASF AG, Esacure™ KT046, Esacure™ KIP 150, Esacure™ KT37 and Esacure™ EDB available from LAMBERTI, H-Nu™ 470 and H-Nu™ 470X available from SPECTRA GROUP Ltd.

A printed object according to the invention is composed of a polymeric material which is a "shape memory" polymer, capable of retaining one or more temporary shapes. The transition from a temporary shape to a permanent shape (namely from one or more metastable shape(s) to the memorizable shape and vice versa) is induced by a temperature change.

According to some embodiments, the 3D object is fully and wholly made of a shape memory polymer. In some embodiments, the object consists a single shape memory polymer or a blend of such shape memory polymers. In some embodiments, the object is free of any metal-based or metallic materials. In some embodiments, the object comprises one or more elements or features or segments, each of which consists a shape memory polymer and at least one of which is manufactured according to the present invention.

The invention further provides a photosensitive ink for printing a 3D structure or object with a shape memory effect, the method for printing the structure or object comprises digital light processing printer (DLP).

In some embodiments, the ink comprises monomers, oligomers, photoinitiators, dyes and inhibitors.

In some embodiments the photopolymerizable precursor is referred to as a monomer, oligomer and/or polymer.

In some embodiments, the 3D printing by photopolymerization can be performed at temperatures below that of the melting point, by adding a solvent or a reactive diluent.

In some embodiments, the photosensitive printable ink comprises high molecular weight (meth)acrylated polymers.

The invention further provides a formulation for printing a 3D object, the formulation comprising at least one shape memory polymer resin (a photopolymerizable material), at least one photoinitiator, and optionally at least one inhibitor and optionally at least one dye.

In some embodiments, the formulation permits the construction of a 3D object, as defined, by printing. In some embodiments, the process of forming the object is 3D printing. The printing process may be any 3D printing method which is based on photopolymerization. Such a process can be performed by inkjet printing followed by UV polymerization, e.g., by Polyjet technology. The ink properties such as viscosity and surface tension should be tailored to the printing technology.

In some embodiments, the printing process is digital light processing (DLP). The printing process comprises printing a photo-polymerizable material or composition when in the melt. Thus, the photo-polymerizable material or composition is thermally treated at a temperature sufficient to cause the photo-polymerizable material or composition to liquefy, i.e., soften or melt or flow or reduce in viscosity, and thus be printed. The thermal treatment to the melt precedes photo-irradiation for achieving polymerization and/or crosslinking of the photo-polymerizable material.

Thus, the invention provides a process for forming a polymeric 3D object by a photopolymerizable process. The process comprising printing a photo-polymerizable material in the melt, or composition comprising same, said printing being optionally DLP, to form an uncured 3D object, and photo-irradiating said object to cause polymerization of said 3D object and obtain a polymerized 3D object with a memorizable first shape, the first shape being deformable into a metastable (second) shape and revert back to said first shape at a temperature above the triggering temperature of the polymer. In other words, the polymerized (cured) 3D object has a memorizable first shape which is transformable into a deformed, metastable or otherwise second shape, which is different from the first shape, at a temperature above the triggering temperature of the polymer, whereupon cooling to a temperature below the triggering temperature said second shape may be maintained and reversed to said memorizable first shape at a temperature above or at the triggering temperature.

Curing or polymerization of the at least one photopolymerizable material is achievable under UV irradiation. As the 3D object may be achievable by printing, inkjet printing, by a layer-by-layer method, each printed layer may be irradiated prior to placing of the subsequent layer. Alternatively, the object may be irradiated after at least a portion of the object has been formed or after the complete object has been formed.

In some embodiments, the process of the invention comprises pre-processing, object production and simultaneous or after object-production curing.

The pre-processing involves, for example, utilization of a 3D CAD file/software for setting photopolymer placement on a substrate. The production of the 3D object involves jetting droplets of the melt photopolymerizable material onto the substrate. The layers accumulate to create the 3D object. In some embodiments, each jetted droplet or layer is instantly UV-cured. Where the object is self-standing, it may be freed from the substrate to afford the 3D object in its memorizable form. In some embodiments, the invention provides a process for the construction of a 3D object, the object being formed of a polymer having a triggering temperature, the process comprising: constructing a 3D object of at least one photopolymerizable precursor of said polymer at a temperature above a melting temperature (T_m) of the photopolymerizable precursor, said 3D object being curable by irradiation into a memorizable shape or form; said memorizable shape or form being deformable into one or more metastable shapes or forms at a temperature above the triggering temperature of the cured polymer.

In some embodiments, the 3D object is constructed by printing.

In some embodiments, the object is constructed from two or more different photopolymerizable precursors, each having a different and distinct melting temperature.

In some embodiments, the invention provides a process for constructing a 3D object, the process comprising forming a 3D object of a polymeric material at a temperature above melting temperature (T_m) of at least one photo-polymerizable material, being a precursor of said polymeric material, by printing said at least one photo-polymerizable material and affecting curing thereof, permitting construction of the 3D object in a memorizable form, said form being transformable into one or more deformed metastable forms at or above a triggering temperature of the cured 3D object.

In some embodiments, the invention provides a process comprising constructing a 3D object of a photopolymerizable material, said material having a melting temperature, T_m, wherein upon curing of said material the object possesses a memorizable shape and one or more deformed shapes, the memorizable and deformed shapes may be transformable one to the other at a temperature above a triggering temperature of the cured polymer.

In some embodiments, the memorizable shape is susceptible to deformation to a deformed shape while at a temperature above the triggering temperature and maintained at said deformed shape when cooled below its triggering temperature.

In some embodiments, said object is stable in said deformed shape until reheated to a temperature above the triggering temperature and permitted to revert back to the memorized static shape. In some embodiments, the at least one photo-polymerizable material is selected amongst semicrystalline thermoplastic polymers that retain their semi-crystallinity after photo-polymerization.

In some embodiments, the at least one photo-polymerizable material is a solid material or a composition of one or more materials.

In some embodiments, the at least one photo-polymerizable material is at least one ethylenically unsaturated compound.

In some embodiments, the at least one ethylenically unsaturated compound is selected amongst organic materials having one or more carbon-carbon double bonds.

In some embodiments, the at least one photo-polymerizable material is selected amongst polyglycolic acid, polycaprolactone, polylactic acid, polydioxanone, polypentadeca lactone and poly(hydroxyl acid) polymers.

In some embodiments, the at least one photo-polymerizable material is selected amongst modified polycaprolactone with photo-polymerizable functional groups.

In some embodiments, the at least one photo-polymerizable material is selected to have a melting temperature enabling printing of the 3D structure in the molten state.

In some embodiments, the at least one photo-polymerizable material is selected to have a melting temperature above room temperature (rt, 25-30 °C).

In some embodiments, the temperature is above 30°C, above 40 °C, above 50°C, above 60°C, above 70°C, above 80°C, above 90°C, above 100°C, above 110°C, above 120°C, above 130°C, above 140°C, above 150°C, above 160°C or above 170°C.

In some embodiments, the temperature is between rt and 170°C, between rt and 160°C, between rt and 150°C, between rt and 140°C, between rt and 130°C, between rt and 120°C, between rt and 110°C, between rt and 100°C, between rt and 90°C, between rt and 80°C, between rt and 70°C, between rt and 60°C, between rt and 50°C or between rt and 40°C.

In some embodiments, the at least one photo-polymerizable material is selected amongst polycaprolactone polymers.

In some embodiments, the polycaprolactone polymers have an average molecular weight of between 8,000 and 25,000Da.

In some embodiments, the 3D object is wholly made of a shape memory polymer. In some embodiments, the object consists a single shape memory polymer or a blend of shape memory polymers.

In some embodiments, the object is free of any metal-based or metallic materials.

In some embodiments, the construction of the 3D object is achieved by inkjet printing.

In some embodiments, said printing is digital light processing (DLP).

In some embodiments, the invention further provides a process for forming a polymeric 3D object, the process comprising printing at least one photo-polymerizable material, in the melt, or composition comprising at least one photo-polymerizable material, said printing being optionally DLP, to form an uncured 3D object, and photo- irradiating said object to cause polymerization of said 3D having a memorizable first shape, the first shape being deformable into a metastable (second) shape and is revertible back to said first shape at a temperature above a triggering temperature of the polymer.

In some embodiments, the photo-polymerizable material is a polycaprolactone with an average molecular weight of between 8,000 and 25,000Da.

In some embodiments, photo-irradiation is carried out simultaneous with said printing.

In some embodiments, the melt temperature is above 30°C, above 40 °C, above 50°C, above 60°C, above 70°C, above 80°C, above 90°C, above 100°C, above 110°C, above 120°C, above 130°C, above 140°C, above 150°C, above 160°C or above 170°C.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figs. 1A-D: Stereolithography with a molten macromethacrylate can impart shape memory to nearly any object.

Fig. 1A is a schematic depiction of the printing process with a heated reservoir. Fig. IB shows a model of a cardiovascular stent having a length of 3 cm, strut thicknesses of 600 microns, and open cells of 2.5 mm by 2.5 mm, reverting to its original shape at 70°C, prepared and operable according to the invention. Fig. 1C shows another object- an Eiffel Tower model, 6 cm tall reverting to its original shape at 70°C, prepared and operable according to the present invention. Fig. ID shows another object- a bird with a 3 cm wing span reverting to its original shape at 70°C, prepared and operable according to the invention.

Figs. 2A-C: Dependence of thermal and mechanical properties of the printed objects on the degree of methacrylation.

Fig. 2A depicts degree of crystallinity of the macromonomer precursor (circles) and degree of crystallinity of printed objects (diamonds) as a function of the degree of methacrylation.

Fig. 2B shows tensile moduli of printed dog-bones below (diamonds) and above (circles) the triggering temperature.

Fig. 2C shows strain at break of printed dog-bones above the triggering temperature.

Figs. 3A-C: Fabrication of a shape memory temperature sensor.

Fig. 3A shows a 3D printed construct (top) programmed into its temporary state to enable inkjet printing on a 2D surface (bottom).

Fig. 3B shows silver conductive ink printed (OmniJetlOO inkjet printer) on the shape memory construct before sintering at room temperature.

Fig. 3C shows fabricated temperature sensor in its "off" state (top) and its "on" state (bottom).

Fig. 4: provides a photo of a shape memory object printed with a dye and without a dye. It is clearly noticeable that a printed pattern with a dye has, in some instances, better resolution.

DETAILED DESCRIPTION OF EMBODIMENTS

To demonstrate the scope of the process of the invention, various model thermoset shape memory vascular stents were manufactured. Vascular stents are a challenge to print because of the intricate strut design comprising sub-millimeter thicknesses and the large number of voids. As Fig. IB shows, a stent with strut thicknesses about 600 microns could be manufactured by the method of the invention. Earlier attempts to implement shape memory stents struggled with the conventional fabrication methods, a fact which might explain the lack of progress over the last decade. In addition, a scaled Eiffel Tower (Fig. 1C) and a bird (Fig. ID) model were printed, which were repeatedly cycled through different temporary shapes. All the constructs ultimately recovered their permanent shape upon heating.

Transparency was another mutable property of these printed structures. At room temperature the printed structures were opaque due to the presence of crystallites; when the crystallites melt, the objects became transparent to a level dependent on the thickness of the printed part.

The shape memory thermosets in this study were based on polycaprolactone (PCL) (number average molecular weight 10,000), a semicrystalline polymer with a melting temperature about 55°C. A semicrystalline polymer is defined as any polymer that has crystalline domains. In contrast, standard (meth)acrylate-based resins have a low molecular weight so they are liquid at ambient temperature. Methacrylate groups were covalently linked to the chain ends of the PCL macrodiol via a simple alcohol- isocyanate reaction. In comparison to typical low molecular weight resins, the presence of a high-molecular weight non-methacrylated PCL fraction did not function as a plasticizer but rather as reinforcing filler when the material was beneath the T_m. Therefore, a series of macromethacrylates were prepared with various degrees of methacrylation (determined by NMR) to map the mechanical properties. The methacrylated macromonomer had a molecular weight higher than conventional photo- curable resins, and, therefore, was heated above its melting temperature to form a viscous melt (-30 Pa- s). The 3D printing was performed by layer-by-layer UV curing (Picoplus39, Asiga) within a customized heating bath.

The photopolymerization occurred at the bottom of the reservoir, curtailing the deleterious effect of molecular oxygen inhibition. A photoinitiator, an inhibitor to prevent premature cross-linking, and a dye to control the cure depth (see Fig. 4) were added to the molten macromethacrylates. The build times were dependent on the size of the models and the layer thicknesses; for example it took 44 minutes to print a 1 cm cube with a 100 micron layer thickness.

The unreacted macromonomer was partially removed from the voids of the printed object by immersion in warm isopropyl alcohol under sonication. Final curing was performed by additional UV exposure in the Asiga flash unit for 30 seconds.

Thermally-induced shape memory behavior is contingent on two molecular requirements: chemical or physical cross-links to set the permanent three-dimensional shape, and a T_m or a glass transition temperature, to control the molecular switching segments which fix the temporary shape. To set the temporary shape, the material was deformed while heating above the transition temperature (T^_s) of the molecular switching segment. The temporary shape was obtained when T < T_tran_s- The material recovered its permanent shape upon T > T_trans- The mutually-constrained requirement to cure the material as a liquid, in the melt or in solution, with a 3D shape is the main reason shape memory thermoset structures have been confined solely to spartan geometries. With the approach presented here, the shape memory structure can be manipulated in three dimensions.

To evaluate the mechanical, thermal and shape memory properties of the printed objects, dogbone specimens were designed in Google SketchUp and 3D printed. The degree of methacrylation of the macromethacrylates directly affected the cross-linking density and, therefore, the degree of crystallinity (determined by differential scanning calorimetry) of the thermosets. Macromethacrylates with a degree of methacrylation less than 40% could not be successfully photo-cured. The cross-linking of the macromethacrylates hindered the formation of crystallites (Fig. 2A), thus resulting in a lower and broader melting transition compared to the non-cross-linked thermoplastic material. Fig. 2A shows that the degree of crystallinity of the macromethacrylates prior to UV radiation is about 45%, whereas the UV cured polymer has a decreasing crystalline fraction when the degree of methacrylation increases. This is also reflected in the degree of swelling in a solvent which decreases with the increase of the degree of methacrylation, in line with Flory' s swelling theory. Fig. 2B presents the dependence of the tensile moduli of the printed thermosets below and above T_m, and the strain at break above the T_m on the degree of methacrylation (Fig. 2C). The implication is that by modulating the methacrylate content, a spectrum of mechanical properties is obtainable.

The established metrics for shape memory behavior are the strain fixity rate R_f, which describes the capability of the switching segment to fix the transient shape, and the strain recovery rate R_r, which describes the ability of the material to reclaim its permanent shape. In this case, R_f indicates the crystallite shape fixing performance, and R_r indicates the effectiveness of the cross-links in driving the return to the permanent shape. For all the materials R_r was greater than 93% and R_f was greater than 98%, demonstrating their excellent shape memory behavior. To evaluate the feasibility of integrating the printed shape memory material in a thermally responsive device, the inventors fabricated an electric temperature sensor. The device was composed of a 3D shape memory polymer printed object (Fig. 3A), onto which electrical contacts were inkjet printed with silver nanoparticle ink (Fig. 3B). This silver ink was unique due to its ability to undergo a sintering process at room temperature, thus suitable for the temperature sensitive polymers utilized in this study. Fig. 3C illustrates the function of the device: the temporary shape was an open electrical circuit, and when heated above the T_m, the circuit closed and a light went on. It should be noted that the use of a shape memory material has an additional advantage because a 3D object can be temporarily turned into a flat surface onto which other materials can be added by simple printers, eliminating the need for highly sophisticated and costly printers which enable three-axis movement.

EXAMPLES

Example 1: Synthesis of PCL Macromonomer PCL 1 OK (Sigma- Aldrich) was charged to a round bottom fl ask and dried under vacuum at 120 °C for 2 h. Isocyanatoethyl methacrylate (Tokyo Chemical Industry) was added under nitrogen and reacted for 2 h at 85 °C in dioxane (Bio-Lab Israel), which was previously dried over molecular sieves (Merck). The macromonomer was precipitated with cold petroleum ether (Bio-Lab Israel). The macromonomer was dried in a fume hood overnight.

Resin Additives: The PCL formulations contained 4 wt 2,4,6-trimethylbenzoyl- diphenyl-phosphineoxide as photoinitiator (BASF, Germany), 0.1 wt of vitamin E (Ahava, Israel) to prevent premature crosslinking, and 0.01 wt Orasol Orange G dye (BASF, Germany).

3D Printing Process: The shape memory resin was prepared for printing by first heating the PCL macromethacrylate above its melting temperature (¾60 °C) followed by adding the photoinitiator, inhibitor and dye to the melt. Once the melt is homogenous, it is poured into the custom printer monomer bath, where it is maintained at 90°C during the printing process. The print parameters are listed in Table 1: Build Parameter Value

Slice thickness 0.1 mm

Burn-in layers 1 layer

Separation distance 15.0 mm

Separation velocity 5.0 mm s^"1

Approach velocity 5.0 mm s^"1

Slide velocity 10.0 mm s^"1

Slides per layer 1

Exposure (burning time) 12.0 s

Exposure time 8.0 s

Wait time (after slide) 3.0 s

Wait time (after exposure) 0 s

Wait time (after separation) 3.0 s

Wait time (after approach) 2.0 s

LED wavelength 385 nm

Pixel size 30 μπι

Light intensity 17.5 mV

Water bath temperature (heat source) 90 °C

Table 1 Example 2:

65 grams of methacrylated PCL10K was melted in a water bath under magnetic stirring. After the melt was homogenized, 0.07 g of vitamin E, 2.7 g of TPO (initiator) and 18 mg of Orosol G was added to the melt. The melt was poured into the custom aluminum resin bath within the Asiga PIC039 Printer bay. Standard .STL format models were printed while the resin bath was heated using water circulation to above 90 °C.

Example 3

65 grams of methacrylated PCL10K was melted in a water bath under magnetic stirring. After the melt was homogenized, 0.07 g of vitamin E, 2.7 g of TPO (initiator) and 18 mg of Orosol G was added to the melt. The melt was poured into the custom aluminum resin bath within the Asiga PICO 2 Printer bay. Standard .STL format models were printed while the resin bath was heated using electric Peltier heaters to above 90°C.

Example 4

33 grams of methacrylated PCL10K was melted in a water bath under magnetic stirring. After the melt was homogenized, 0.03 g of vitamin E, and 1.4 g of TPO (initiator) was added to the melt. The melt was poured into the custom aluminum resin bath within the Asiga PICO 2 Printer bay. Standard .STL format models were printed while the resin bath was heated using electric Peltier heaters to above 90°C.

Example 5

33 grams of methacrylated PCL10K was melted in a water bath under magnetic stirring. After the melt was homogenized, 1.4 g of TPO (initiator) and 18 mg of Orosol G was added was added to the melt. The melt was poured into the custom aluminum resin bath within the Asiga PICO 2 Printer bay. Standard .STL format models were printed while the resin bath was heated using electric Peltier heaters to above 90°C.

Example 6

Same as Example 2 but with the following dyes: Toner Magenta (Clariant GmbH), Toner Yellow (Clariant GmbH), and combinations thereof. Example 7

Same as Example 2 but using liquid TPO (BASF) as the photoinitiator.

Example 8

Same as Example 2 but using Irgacure 819 (BAPO) (BASF) as the photoinitiator.

Claims

1. A process for the construction of a 3D object, the object being formed of a polymer having a triggering temperature, the process comprising: constructing a 3D object of at least one photopolymerizable precursor of said polymer at a temperature above a melting temperature (T_m) of the photopolymerizable precursor, said 3D object being curable by irradiation into a memorizable shape or form; said memorizable shape or form being deformable into one or more metastable shapes or forms at a temperature above the triggering temperature of the cured polymer.

2. The process according to claim 1, wherein the 3D object is constructed by printing.

3. The process according to claim 1, wherein the object is constructed from two or more different photoploymerizable precursors, each having a different and distinct melting temperature.

4. A process for constructing a 3D object, the process comprising forming a 3D object of a polymeric material at a temperature above melting temperature (T_m) of at least one photo-polymerizable material, being a precursor of said polymeric material, by printing said at least one photo-polymerizable material and affecting curing thereof, permitting construction of the 3D object in a memorizable form, said form being transformable into one or more deformed metastable forms at or above a triggering temperature of the cured 3D object.

5. A process comprising constructing a 3D object of a photopolymerizable material, said material having a melting temperature, T_m, wherein upon curing of said material the object possesses a memorizable shape and one or more deformed shapes, the memorizable and deformed shapes may be transformable one to the other at a temperature above a triggering temperature of the cured polymer.

6. The process according to claim 5, wherein the memorizable shape is susceptible to deformation to a deformed shape while at a temperature above the triggering temperature and maintained at said deformed shape when cooled below its triggering temperature.

7. The process according to claim 5, wherein said object is stable in said deformed shape until reheated to a temperature above the triggering temperature and permitted to revert back to the memorized static shape.

8. The process according to any one of the preceding claims, wherein the at least one photo-polymerizable material is selected amongst semicrystalline thermoplastic polymers that retain their semi-crystallinity after photo-polymerization.

9. The process according to any one of the preceding claims, wherein the at least one photo-polymerizable material is a solid material or a composition of one or more materials.

10. The process according to any one of the preceding claims, wherein the at least one photo-polymerizable material is at least one ethylenically unsaturated compound.

11. The process according to claim 10, wherein the at least one ethylenically unsaturated compound is selected amongst organic materials having one or more carbon-carbon double bonds.

12. The process according to claim 11, wherein the at least one photo- polymerizable material is selected amongst polyglycolic acid, polycaprolactone, polylactic acid, polydioxanone, polypentadeca lactone and poly(hydroxyl acid) polymers.

13. The process according to claim 11, wherein the at least one photo- polymerizable material is selected amongst modified polycaprolactone with photo- polymerizable functional groups.

14. The process according to any one of claims 1 to 13, wherein the at least one photo-polymerizable material is selected to have a melting temperature enabling printing of the 3D structure in the molten state.

15. The process according to claim 14, wherein the at least one photo- polymerizable material is selected to have a melting temperature above room temperature (rt, 25-30 °C).

16. The process according to claim 15, wherein the temperature is above 30°C, above 40 °C, above 50°C, above 60°C, above 70°C, above 80°C, above 90°C, above 100°C, above 110°C, above 120°C, above 130°C, above 140°C, above 150°C, above 160°C or above 170°C.

17. The process according to claim 15, wherein the temperature is between rt and 170°C, between rt and 160°C, between rt and 150°C, between rt and 140°C, between rt and 130°C, between rt and 120°C, between rt and 110°C, between rt and 100°C, between rt and 90°C, between rt and 80°C, between rt and 70°C, between rt and 60°C, between rt and 50°C or between rt and 40°C.

18. The process according to any one of the preceding claims, wherein the at least one photo-polymerizable material is selected amongst polycaprolactone polymers.

19. The process according to claim 18, wherein the polycaprolactone polymers have an average molecular weight of between 8,000 and 25,000Da.

20. The process according to any one of the preceding claims, wherein the 3D object is wholly made of a shape memory polymer.

21. The process according to claim 20, wherein the object consists a single shape memory polymer or a blend of shape memory polymers.

22. The process according to claim 21, wherein the object is free of any metal-based or metallic materials.

23. The process according to any one of the preceding claims, wherein the construction of the 3D object is achieved by inkjet printing.

24. The process according to claim 23, wherein said printing is digital light processing (DLP).

25. A process for forming a polymeric 3D object, the process comprising printing at least one photo-polymerizable material, in the melt, or composition comprising at least one photo-polymerizable material, said printing being optionally DLP, to form an uncured 3D object, and photo-irradiating said object to cause polymerization of said 3D having a memorizable first shape, the first shape being deformable into a metastable (second) shape and is revertible back to said first shape at a temperature above a triggering temperature of the polymer.

26. The process according to claim 25, wherein the photo-polymerizable material is a polycaprolactone with an average molecular weight of between 8,000 and 25,000Da.

27. The process according to claim 25, wherein photo-irradiation is carried out simultaneous with said printing.

28. The process according to claim 25, wherein the melt temperature is above 30°C, above 40 °C, above 50°C, above 60°C, above 70°C, above 80°C, above 90°C, above 100°C, above 110°C, above 120°C, above 130°C, above 140°C, above 150°C, above 160°C or above 170°C.