Recent developments in 3D printable composite materials

Abstract

3D printing technology is now frequently employed in many areas of research and development. However, a relatively narrow range of 3D printable materials with a limited spectrum of physico-chemical properties still restricts the true potential of this potentially disruptive technology. There is rapidly increasing interest in the improvement and diversification of properties of generic printing materials via the introduction of fillers with unique properties, and/or by blending materials exhibiting different properties to generate high performance composites. 3D printed composites have already been utilised in a wide range of applications, including biomedical, mechanical, electrical, thermal and optically enhanced products. The increasing popularity of 3D printed composites can be attributed to the ability to fabricate complex geometries, low cost production, and other advantages associated with rapid prototyping. This review covers all the recent reports in which the properties of generic 3D printable materials have been modified either by adding nanoparticles, fibers, other polymers, or by a chemical reaction for fabrication of composites with enhanced biomaterial, mechanical, electrical, thermal, optical and other properties.

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Introduction

Additive fabrication technologies, commonly referred to as 3D printers, have seen an exponential increase in use in recent years, as a versatile and low cost technology for rapid casting/prototyping.¹ Applications of this technology within a diverse range of industries continue to grow, exploiting the expanding capabilities of advanced 3D printers to produce a range of materials, together with complex monolithic structures and geometries, with micrometre resolution for some applications.^2–4 In comparison to traditional fabrication approaches, in which a complex structure is manufactured via assembly of individual parts, 3D printing allows layer by layer (bottom to top) printing of predefined slices of the designed and desired object,⁵ resulting in rapid manufacturing of complex objects, practically unhindered by design complexity, thus providing substantial liberty in new and untested geometric designs.

However, current manufacturing using 3D printing is overwhelmingly based upon single material printing, typically with a limited range of commercial and often proprietary resins. The obvious issue here is this relatively limited range of materials compatible with printer technologies (and commercial printers) and hence, limited variation in the physical and chemical properties of printed objects.⁶ This issue is being addressed somewhat with modern multi-material 3D printers, which provide for some control of material composition and properties, and which can provide for composite printing through multiple printer heads, providing the capability to print blended and/or layered composite materials, and from which objects with features of variable and gradient functionality can be produced.⁷

Running parallel to these technological developments, there has also been rapidly growing interest in the pre-blending of materials and/or the inclusion of ‘fillers’ into printable resins, which deliver distinct physicochemical properties into the resultant materials, thus producing 3D printed composites which exhibit unique characteristics and capabilities.^8–15 Indeed, in the area of composite design and production, 3D printing represents a technology with immense potential, again providing low cost, simple and rapid prototyping advantages over traditional methods for fabrication of composite materials and objects.¹⁶

In the wider fields of material and chemical sciences, the impact of 3D printing, as witnessed by the proliferation of review articles on the subject, is very obvious. In the area of 3D printed composite materials this is not yet the case. In 2010 Kumar and Kruth published their review on the variety of 3D printed composites for biomedical, mechanical, electrical, thermal, magnetic and optical applications.¹⁴ However, over the subsequent 6 years there has been considerable developments and achievements in this fast growing area, and hence the focus of this current review is to summarise and critique these recent advances. Herein, we present and discuss the latest progress in the development of 3D printable composite materials, their fab-rication methods, applications, advantages and limitations. A detailed overview of printers, matrices and additives used for the development of 3D printable composite materials provided in Tables 1, and 2 describes properties and applications of the resultant 3D printed composites.

Table 1 A summary of 3D printers or printing type, matrix and fillers used for the development of 3D printable composite materials

Printer/printing type	Matrix	Additives/fillers	Ref.
Inkjet	Bioactive glass	HA (40 wt%)	10
Inkjet	Calcium phosphate	Calcium sulphate	38
Inkjet	HA (60 wt%)	TCP (40 wt%)	17
Inkjet	TCP	SiO2 (0.5 wt%) and ZnO (0.25 wt%)	19
	Calcium silicate	Ce	107
4th 3D Bioplotter™	PCL (38.5 wt%) and bioactive glass (52.3–61.5 wt%)	Fe3O4 (0–9.3 wt%)	12
FDM	PLA	TCP	46
FDM	Polyamide	HA (5 wt%) and zirconium oxide (15 wt%)	47
Inkjet	β-TCP	Mg (∼2.12 wt%)	41
Inkjet	Bioactive glass (60 wt%)	TCP (40 wt%)	37
FDM (mini extrusion)	PCL	TCP (10–30 wt%)	40
Inkjet	Calcium phosphate	Collagen (1.5 wt%)	33
SLS	Calcium phosphate (0–100 wt%)	HA (100–0 wt%)	39
FDM	PCL	Graphene	50
Inkjet	Apatite–wollastonite glass ceramic	HA	108
Inkjet	HA (50–60 wt%)	PVOH (40–50%)	42
FDM	Thermoplastic polymer	Natural fibre	23
Direct print tool	Poly lactic acid	Bioactive glass (50 wt%)	18
Melt electrospinning writing	Hydrogel	PCL microfibers (12% to 28%)	49
SLS	Forsterite (Mg2SiO4) 95, 90, 80, 70, 60 wt%	Nano-58S bioactive glass 5, 10, 20, 30, 40 wt%	22
Inkjet	Epoxy resin	SiC & carbon fibres (31 vol%)	53
—	PCL	Alginate (4 wt%), and phenamil methanesulfonate	109
FDM	ABS	Hydrous magnesium silicate	64
FDM and robocasting	Thermoplastic polymer	Steel (47 vol%) and zirconia (45 vol%)	58
SLA	Photopolymer	Graphene oxide (0.2 wt%)	59
SLA	UV sensitive resin	Magnetised Al2O3 particles	62
SLA	Hard photopolymer (proprietary acrylic-based photopolymer resins)	Soft photopolymer (20% v/v)	2
	Tetracalcium phosphate	β-Tricalcium phosphate or calcium sulphate	35
Inkjet	TCP	SrO (1 wt%) and MgO (1 wt%)	20
SLS	Calcium phosphate	Poly(hydroxybutyrate-co-hydroxyvalerate)	21
SLS	Carbonated HA	Poly(L-lactic acid)	21
Indirect 3D printing	Glass	β-Spodumene, lithium disilicate	43
	PCL	Hydroxyapatite (25 wt%)	45
Inkjet	Vaseline	TiO2 (2.5–10 wt%)	65
FDM	PDMS	ABS (anisotropic material)	68
Laser sintering	Polyamide	Carbon nanofiber (3 wt%)	55
FDM	ABS	CNTs (3 wt%)	13
Inkjet	WC (34 wt%)	ZrC (55 wt%)	60
	TiC	Al–Si alloy	61
Aerosol 3D printing	Poly-lactide-co-glycolide	Nano titania (30 wt%)	34
FDM	ABS	CNTs (3 wt%)	110
SLS	Zirconia (41.5 wt% and 80 wt%)	Alumina	63
Nozzle printing and inkjet printing	Elastomeric fibre	AgNP	66
SLA	3D printable oil-in-water emulsion	AgNP	90
	Paper based substrate	Graphene oxide, graphene hydrogel–polyaniline coating on the substrate	89
Inkjet	CNTs	Gellan & xanthan (0.10 w/v%)	80
Inkjet	Poly(ethylene terephthalate)	PdCu, AgCu	88
Inkjet	Styrene-block-butadiene-block-styrene rubber fibres	Ag nanoparticles	86
Electrospray deposition	Polyvinylidene fluoride	Aluminum nanopowders (50 wt%)	66
Inkjet	Poly(2-methoxyaniline-5-sulfonic acid)	CNTs	78
FDM	PLA	CNTs	82
Robocasting	SiC	Graphene nanoplatelets (20 vol%)	71
FDM	Polycarbonate polymer	W	74
FDM	ABS	Copper and iron (10–50 wt%)	100
	Polylactide	CNTs (5 wt%)	73
Inkjet	Polylactide-co-glycolide	Graphene (60 vol%)	76
SLA	Photo resin	Al2O3	94
SLS	Nylon-12	Carbon black (4 wt%)	85
FDM	ABS	Twisted jute natural plant fibres and carbon fibres	56
SLA	Polyethylene glycol di-acrylate matrix	Barium titanate nanoparticles (10 mass%)	87
FDM	Polymorph thermoplastic	Carbon black (15 wt%)	79
FDM	ABS	Graphene flakes (5.6 wt%)	97
SLA	Photo resin	BST (50–60 wt%)	101
SLA	Epoxy resin	Diamond microparticles (30 w/v%)	99
FDM	PDMS	ABS, glass fibre reinforced polymer	111
FDM	ABS thermoplastic pellets	Carbon fibre powder (5 wt%)	52
Inkjet	Polymer	Quantum dots	106
Inkjet	Polymer	CdSe/ZnS quantum dot	105
Inkjet	Polyaniline, poly(3,4-ethylenedioxythiophene)	Silica	102
FDM	ABS or polypropylene	BaTiO3/Ba0.64Sr0.36TiO3/CaTiO3	93
FDM	ABS	BaTiO3 (70 wt%)	95

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Table 2 A summary of properties and applications of 3D printable composite materials and 3D printed composites

Composite	Composite properties	Application	Ref.
3D printed composites for biomedical applications	Bending strength 80 MPa, Young's modulus 80 GPa, reduced and isotropic shrinkage in the composite	Bone and tissue replacement	10
	Enhanced compressive strength	Tissue engineered bone scaffolds	38
	Porosity 53.1%, bulk density 93.9 g cm^−3, enhanced biocompatibility	Bone substitutes	17
	Enhanced mechanical and biological properties	Scaffolds	19
	Porosity (60%), enhanced compressive strength (13–16 MPa), magnetic heating, bioactivity, and osteogenic ability	Drug delivery and hyperthermia	12
	Enhanced mechanical strength and bone regeneration properties	Bone implants and scaffolds	46
	Tensile strength 22.78 MPa 761, tensile modulus 93 MPa, enhanced thermal stability increased viscosity and rate of crystallisation	Biomedical applications	47
	High bending strength (19.9 MPa), biocompatible, biodegradable	Bone implants	16 and 37
	Biocompatible, high mechanical strength, degradable	Bone implants	33
	Fracture toughness (1.33 MPa m^1/2), compressive strength (18.35 MPa)	Bone scaffolds	39
	Compressive strength: 43.91 MPa, enhanced cell attachment and proliferation	Scaffolds	22
	Strength 120–140 MPa, compression 100 MPa, porosity ∼ 52%	Scaffolds	41
	Fracture strength = 21.01 MPa (sintering 1200 °C, 3 h, porosity = 51.49%) and 76.82 MPa (sintering 1300 °C, 3 h, porosity = 51.49%) and biological properties	3D printed synthetic bone implants	108
	Porosity = 55%, compressive strength = 0.12–0.88 MPa, improved biological properties such as osteoconduction and osteointegration	Bone tissue engineering	42
	Biocompatible scaffolds, compressive modulus = 44.19–99.81 MPa	Bone tissue engineering	18
	Bending strength = 70 MPa, Young's modulus = 80 GPa, reduced and isotropic shrinkage from the sintering process	Bone and tissue replacement	10.
	Ink viscosity = viscosity is ∼200 Pa s, Young's modulus = 8.06–24.54 GPa, strength = 43.9–66.2 MPa, light weight, exceptional mechanical properties	Bio inspired composite structures	53
	Porosity 38%, compression strength = 0.59–0.70 MPa, improved biocompatibility	Patient specific bone implants	35
	Mechanical strength 12.01 MPa, enhanced osteoid like new bone formation, and mineralization	Bone tissue engineering	20
	Compressive strength: calcium phosphate/poly(hydroxybutyrate-co-hydroxyvalerate) = 0.55 MPa, carbonated hydroxyapatite/poly(L-lactic acid) = 0.64 MPa, compressive Young's: calcium phosphate/poly(hydroxybutyrate-co-hydroxyvalerate) = 6.6 MPa carbonated hydroxyapatite/poly(lactic acid) = 6.2 MPa, enhanced cell proliferation	Bone scaffolds	21
	Compression strength of ∼15 MPa	Bone scaffold fabrication	43
	Compressive strength 0.26–0.47 MPa, enhanced proliferation	Bone tissue engineering	45
	Porosity = 55–57%, improved hydrophilicity, improved mechanical properties	Tissue engineering	40
	Improved bone tissue growth, Young's modulus 3.42 MPa, max. tensile strength 0.11 MPa	Tissue engineering	109
	Target drug delivery, improved mechanical and biological properties	Cell scaffolds	50
	Porosity 72–88%, equilibrium modulus 130 kPa, stiffness 214 kPa	Scaffolds	49
3D printed composites with enhanced mechanical properties	Strength = 43.9–66.2 MPa, Young's modulus = 8.06–24.54 GPa	Highly stiffed sandwich panels, energy absorbers, catalyst supports, and insulation	53
	Stress 15 MPa, fracture resistant composites electrical conductivity increased from 10^−12 to 10^−5 S cm^−1	Bone implants	2
	Enhanced mechanical properties	Fully functioning end use parts	55
	Porosity 57.9%, ultra-high melting metal composites (>2500 °C), rapidly prototyping of near net shapes and dimensions	Aerospace vehicle, refractory rocket nozzle	60
	Tensile stress 24 MPa, strain ∼ 2%	High mechanical materials and components	64
	Porosity 32%, increased osteoblast growth, improved tensile modulus (1–2 MPa) and stress (0.1–0.25)	Bone tissue regeneration and drug delivery	34
	10% increment in Young's modulus, improved mechanical strength	Engineering and technology education	110
	Flexural strength > 500 MPa	Frameworks for all-ceramic dental restorations, complex structures	63
	Flexural stress ∼ 60 MPa, flexural modulus ∼ 50 GPa	Aerospace, automotive, and medical	52
	For carbon–ABS composites = tensile modulus 19.5 MPa, strength 185.2	Load-bearing components	56
	For plant fibre–ABS composites tensile modulus 5.11 MPa, strength 57.1 MPa	Load-bearing components	56
	Improved stiffness, strength and hardness	Applications in engineering disciplines	62
	Enhanced mechanical strength, multifunctional material	Fabrication of multifunctional materials	58
	Bending strength ∼ 233 MPa, fracture toughness 4.56 MPa m^1/2	Near net shape composites	61
3D printable composite materials with enhanced bending/twisting strength	Large deformations, coupling of in-plane/bending/twisting deformations	Mobile phone, robotics	68
	Lower deformation tendency, improved elastic properties and enhanced shape preserving	Flexible electronics, memristor etc.	65
	Lower Young's modulus than in the compressed samples, porosity (around 20%)	Bio composites	23
	Large deformations, complex morphed shapes	Morphing for overcoming the geometric constraints of structures or actuators	111
3D printed electrically active composite materials	Sheet resistance 8.1 and 4.9 Ω sq^−1, optical transmittance of 79% and 65% at 500 nm for PdCu and AgCu respectively, electrically conducting	Smart phones, touch screens, light-emitting diodes and thin-film solar cells	88
	High bulk conductivity, preserved at large deformations (σ ≈ 2200 S cm^−1 at 100% strain for a 150 μm-thick mat), stretchable & electrically conducing	Stretchable antenna, sensor and light emitting diodes	86
	Optical transparency 85%, sheet resistance of 100 kΩ^−1 electrically conducting	Electronics	78
	Tensile strength 69.4 MPa, electrically conducting	Electrical and mechanical	13
	Resistivity in-plane with the layers = 0.0960.01 ohm m^−1 and perpendicular to the layers = 0.1260.01 ohm m^−1	Electronic sensors, capacitive buttons	79
	Transmittance 99%, resistance of composites (Ramb > 12 MV) lower compared to the resistance of gellan (Ramb > 350 MV), electrically conducting	Actuators and sensors	80
	Electrical conductivity 1 × 10^−4 S cm^−1, flexural modulus 1450 MPa	Electrically conducting material	85
	Electrical conductivity 611 S m^−1, strength 10–50 MPa	Scaffolds	71
	Electrical conductivity was about 23 S m^−1	Helical sensors	73
	Electrical conductivity = 10–100 S m^−1	Conductive micro structures	82
	Resistance 1 ± 0.2 Ω	3D connector of an electrical circuit	90
Dielectric material	Electrical conductivity 18 Ω sq^−1	Flexible and lightweight paper electrode	89
	Piezoelectric coefficients (d33) of ∼40 pC N^−1	Fabrication of bio diagnostics, nano and microelectromechanical systems, imaging, sensors, and electronics	87
	Improved electromagnetic properties, X-ray radiation shielding	Space-based applications	74
	Electrical conductivities greater than 800 S m^−1, enhanced cell adhesion, viability, and proliferation	Electronics and scaffolds	76
	Permittivity 2.6–8.7, loss tangents 0.005–0.027, dielectric material	Photonic crystals, electronics, communications	95
	Improved permittivity, low dielectric loss tangent value	Insulating spacers/components	94
	Dielectric permittivity = 5–7, dielectric loss tangent = 3 × 10^−2 to 5 × 10^−3	Artificial insulating material	93
3D printed composite materials with controlled thermal properties	Linear thermal coefficient > 75 ppm °C^−1, electrical conductivity = 1.05 × 10^−3 S m^−1. Thermally conducting	Thermally conducting materials	97
	Tensile strength 47.82 MPa, tensile modulus 2362 MPa, the flexural strength 73.12 MPa	Thermally conducting materials	98
	Improved heat transfer ability, co-efficient of linear thermal expansion 48 × 10^−6 ± 3.6 × 10^−6 K^−1, increase in contact angle 98.7 θ	Heat sink and cooling system	99
	Thermal conductivity of 0.2 W m^−1 K^−1	Thermoelectric material	101
	Porosity 27.4%, improved thermal stability	Rocket motors	66
	Coefficient of thermal expansion = 50 ppm °C^−1, tensile stress 26–45 MPa, tensile modulus 906–978 MPa	Thermo-mechanical materials	100
3D printed composites with enhanced optical properties	Luminous efficiency 0.1 lm W^−1 for the blue phosphor driven with 420 V at 31 kHz, enhanced optical properties	Luminescent light-converting thin films for AC electroluminescent displays	106
	Optically active	Full colour AC-driven displays	105
	Optically active	Electrochromic devices

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3D printable composite materials for biomedical applications

The key issues for consideration in the 3D printing of composite objects for biomedical applications include biocompatibility,^18–23 mechanical strength,^24,25 and improved biomimicking of tissues structures.²⁶ Bone scaffolds, porous structures with highly interconnected networks of channels, are used for bone tissue engineering and to provide a template for initial attachment of patient-derived cells, to facilitate subsequent formation of tissues.²⁷ The precise control of a scaffold's pore size, shape, interconnections and other architectural parameters is essential to ensure efficient nutrient diffusion, blood flow, manipulation of tissue differentiation, to achieve optimal mechanical strength,^24,25 and where necessary biodegradability. The ability of 3D printing technology to fabricate such features both internally and externally upon a structure, more or less independently of shape and design²⁸ makes this technique highly suitable for the fabrication of complex-shaped porous ceramic scaffolds, with such well-defined internal 3D architecture and pores structures. Thus 3D printing has recently been applied to the generation of highly specific bone implants,^{12,17–19,25,27,29–34} with size and shape precisely matching the bone defects in patients, often utilising the patients' scanned anatomical information to optimise and produce a custom-made implant.³⁰ Fig. 1 shows one such example of a custom made 3D printed implant for oral and maxillofacial surgery.¹⁷

Fig. 1 An example of 3D printed implant fabricated for oral and maxillofacial surgery. Reproduced with permission.¹⁷

Inkjet-based 3D printing has been frequently employed to fabricate calcium phosphate scaffolds (CPS).^{19,33,35–38} In inkjet printing, the calcium phosphate powder is temporarily bound by an adhesive polymer, typically followed by sintering of the printed body at high temperature for permanent binding.^27,29 Composite materials for scaffold fabrication by inkjet printing have been developed to create objects with varying geometric complexity. For example, Khalyfa et al. (2007) developed a 3D printable composite powder consisting of tetracalcium phosphate (Ca₄(PO₄)₂O) as a reactive component, and β-tricalcium phosphate (β-TCP) or calcium sulfate as biodegradable fillers. An aqueous solution of citric was used as a binder. Using the developed powder–binder system, various light weight objects with complicated internal and external architecture and designs were fabricated (see Fig. 2(a)). Composite objects with complex geometries and channels with various diameter (1–2 mm) and inter-channel distances were fabricated to study the printing parameters, such as accuracy and feature resolution (see Fig. 2(b)). However, the authors reported the fabrication of channels smaller than 1 mm as ‘challenging’ due to difficulties in the removal of unbound materials from within the printed channels. The objects produced using the developed powder binding system also showed relatively low compression strengths due to the high microporosity of the material, and required a post-processing procedure to improve the mechanical strength.³⁵

Fig. 2 (a) 3D printed cranial segment, (b) 3D printed cuboid object consisting of channels with various diameter and channel distance fabricated from composite powder binder mixture of tetra-calcium phosphate and β-TCP. Reproduced with permission.³⁵

The issue of low compressive strength of ceramic scaffolds comprised of highly porous calcium phosphate³¹ has been addressed by blending TCP powder with hydroxyapatite (HA),³⁹ calcium sulphate,³⁸ and polycaprolactone (PCL)⁴⁰ to develop composite materials exhibiting significantly enhanced mechanical properties. Following this approach, Shuai et al. (2013) prepared calcium phosphate scaffolds consisting of TCP/HA with composition ratios varying from (0/100, 10/90, 30/70, 50/50, 70/30 and 100/0) and observed an improvement in fracture toughness and compressive strength with increasing TCP from 0 to 30 wt%, and a decrease in mechanical strength above 30 wt%.³⁹ Similarly, calcium phosphate powders were blended with the calcium sulfate (CaSO₄)-based powder to develop composite materials. Using this printable material, highly resolved objects with improved compressive strength could be produced with inkjet printing.³⁸

In addition to mechanical strength, biodegradability⁴¹ and controlled bioactivity are other important concerns when developing materials for the fabrication of scaffolds with potential to stimulate bone regeneration, cell proliferation or combat infection.^18–22 Such biological properties of scaffolds can be enhanced by the addition of appropriate fillers into the matrix powder. For example, Fielding et al. (2012) incorporated ZnO and SiO₂ into calcium phosphate powder for the development of composite material with enhanced mechanical and biological properties. Using these materials, various composites and pure tricalcium phosphate scaffolds were fabricated by inkjet printing, followed by sintering at high temperature (1250 °C)¹⁹ which is usually carried out to facilitate the permanent binding of the calcium phosphate powder.³⁸ The scaffolds developed using the new composite materials showed up to 250% improvement in compressive strength and enhanced cellular attachment characteristics in comparison to pure TCP scaffolds. Similarly, the addition of SrO and MgO, and Ce to calcium phosphate powder have been investigated for the development of composite scaffolds with improved osteogenesis capability and compressive strength.^20,107

The formation of glass/ceramic composites using both inkjet printing³⁷ and indirect 3D printing⁴³ and has been described for applications in bioresorbable, largescale bone implants. In indirect printing, a negative mould is constructed based on the scaffold design, which is later used for casting of the desired biomaterial and then removed to acquire the final scaffold.⁴⁴ However, each of these printing methods require sintering at high temperature, which has been reported to cause shrinking of the final material and thus requires pre-calculations of dimensional changes to achieve perfect fit. This is particularly the case for glass materials, where the sintering process depends on viscous flow and is usually associated with greater anisotropic shrinkage. Large and complex structures may significantly deform because of a number of factors, such as gravity, intrinsic strain or temperature, surface tension and density gradients.¹⁰ Such effects seen during the sintering of glass materials can be minimised by reducing the glass fraction of the composite materials.

Winkel et al. (2012) developed bioactive glass composites with HA powder to fabricate 3D scaffolds. Glass ceramic scaffolds with 40% HA content were fabricated by inkjet printing. The incorporation of HA to the bioactive glass material provided effective stabilisation of complex 3D-printed structure and a significant reduction in shrinking was observed during viscous sintering.¹⁰ Fig. 3 shows changes in the dimensions of the 3D printed composite scaffold, following the sintering process. As can be seen, sintering caused relatively small overall axial shrinkage (20%) in the pore size of the object fabricated from this composite material (as compared to the non-composite material).¹⁰

Fig. 3 (a) Computer model, (b) a photograph of 3D-printed green body, (c) sintered glass/HA composite structure (after heating to 750 °C at 2 K min⁻¹) for testing the viability of the 3D printing process and the sinter model for optimized HAp content. Reproduced with permission.¹⁰

In addition to shrinkage and deformation, sintering at high temperature also precludes the options for integration of bioactive molecules and drugs that are useful for bone regeneration or preventing infection.³¹ However, low temperature inkjet printing has been employed for incorporation of bioactive materials. For example, formation of calcium phosphate composites with collagen, a bioactive polymer, has been applied to produce scaffolds with improved mechanical strength, cellular compatibility and bone ation into a non-load bearing bone defect. Furthermore the regenerative properties. Incorporation of collagen in TCP produced sufficiently strong scaffolds for handling and implant-degradability and osteoconductive properties of collagen enabled the final replacement of these scaffolds with the newly formed bone.³³ Additionally, HA and poly(vinyl)alcohol (PVOH) were mixed (ratio 50 : 50) to produced porous scaffolds to facilitate osteoconduction and osteointegration. The excessive powder from the pores of the 3D printed scaffold was effectively removed with the help of compressed air through a syringe needle (see Fig. 4). The pore size, surface roughness and non-designed porosity within the scaffold were considered promising factors for the promotion of osteoconduction and osteointegration.⁴²

Fig. 4 Efficiency of compressed air for powder removal from 50 wt% HA–PVOH composites. (a) Printed scaffold before powder removal, (b and c) scaffolds after powder removal. Reproduced with permission.⁴²

Composite materials consisting of polymers and ceramics has also gained increased popularity for scaffold fabrication because of the synergistic advantages of the mixed materials, such as high strength via the ceramic phase and toughness and plasticity induced by the polymeric phase.²⁵ Liu et al. (2011), developed polymer scaffolds consisting of well-dispersed nano-sized ceramics. This nano-ceramic dispersion in the polymer matrix provided enhanced mechanical and biological properties and a close resemblance to the natural bones. These scaffolds facilitated greater cell adhesion, synthesis of collagen and deposition of calcium containing minerals.³⁴ Aerosol based 3D printing, in which a dense aerosol consisting of tiny droplets is sprayed layer by layer on the deposition platform, was employed to allow the fabrication of highly ordered bone like structures, while preserving the dispersion of the nanoparticles in the polymeric matrix.³⁴

Fused deposition modelling (FDM) based printing methods, in which a material in molten form is extruded layer by layer on the printing bed, has also been applied for scaffold fabrication. 3D printed polymeric scaffolds consisting of HA,⁴⁵ bioactive glass, magnetic particles incorporated into mesoporous bioactive glass materials,¹² TCP,⁴⁶ and zirconium oxide,⁴⁷ have all been manufactured using FDM printing. The composite scaffolds produced present a number of promising platforms for bone regeneration, with enhanced properties reported, including superior bioactivity, osteogenic ability, and mineralization of human bone marrow-derived mesenchymal stem cells. Fig. 5 shows mineralization of human bone marrow-derived mesenchymal stem cells on one such composite scaffold. As it can be seen, a larger number of mineralized nodules were observed on the composite scaffolds consisting of 5, 10, 15 wt% of Fe₃O₄. These scaffolds have application in the reconstruction of bone defects generated by bone tumour resection or any other means.¹² Polymeric composite material consisting of HA and bioactive glass support local angiogenesis and bone healing, and improve mechanical strength, cell adhesion and proliferation properties of the composite scaffolds.^45,48

Fig. 5 Mineralisation of human bone marrow-derived mesenchymal stem cells on the bioactive glass/PCL, composite with Fe₃O₄ (a) 5% wt, (b) 10% wt, (c) 15% wt, (d) cultured scaffolds after 14 days. Reproduced with permission.¹²

A major limitation of melt extrusion based printing processes is that the production of smaller diameter filaments is challenging, as the minimum filament diameter is typically 50–100 μm. However, an alternative 3D printing technique, capable of printing well below the limits of traditional FDM technology is known as melt electrospinning writing, and has also been employed for development of composite polymer scaffolds. In this printing process, fibrous structures are produced from polymer melts with filament diameters as small as 5 μm. Using this technology composite scaffolds consisting of soft hydrogels reinforced with structured and highly porous micro-fibre networks have been manufactured. The stiffness and elasticity of these composites scaffolds is reported as being comparable to articular cartilage tissues.⁴⁹

An exciting development in 3D printing is the capability to print biofunctional composite structural materials. The development of polymeric material blended with both drugs⁵⁰ and antibiotics,⁵¹ have recently been reported. For example, the development of a 3D printable material loaded with antibiotics for application in treating chronic diseases was recently described by Shim et al. (2015). For preparation of this biofunctional composite material, PCL and poly(lactic-co-glycolic acid) (PLGA) pellets were melted at 110 °C in a glass dish (Fig. 6).

Fig. 6 Schematic illustration of preparation and printing of antibiotic loaded composite material. Reproduced with permission.⁵¹

To this molten polymeric mixture, tobramycin (antibiotic) powders were added and stirred to achieve homogeneous mixing of all components. The composite mixture was extruded from a 3D printer for the fabrication of scaffolds for chronic osteomyelitis treatment.⁵¹ A schematic of the material preparation and printing process is given in Fig. 6.

3D printed composites with enhanced structural and mechanical properties

3D printing materials with intrinsically limited mechanical strength and durability have been combined with fillers to develop composites with enhanced performance in these areas.^52–59 Notably these enhanced materials have been developed for applications in aerospace, the automotive industry, wind energy and similarly high material performance demanding industries. A very recent and typical example is that reported by Ning et al. (2015), who added carbon fibres to a commercially thermoplastic resin matrix for FDM printing, to develop a new printable composite with enhanced mechanical properties.⁵² Using this technique, composite materials consisting of fibre content ranging from 3–15 wt% were developed and various mechanical properties, including flexural and tensile strength, were examined. Composite objects with 5 wt% carbon fibre (150 μm) content showed enhanced flexural stress, flexural modulus, and flexural toughness with an increase of 11.82%, 16.82%, and 21.86%, respectively, in comparison to the pure polymeric material. However, in this particular study the composites with 10 wt% carbon fibres actually showed worst tensile properties, due to the large porosity values in comparison to the other composites, as can be seen in SEM images of the fracture interface of the composites with various fibre contents, following tensile testing (see Fig. 7).

Fig. 7 SEM images of fracture interface of objects after tensile testing showing different porosity in different objects consisting of various carbon fibre content. Reproduced with permission.⁵²

The organised alignment of micro-particulate and filimentaceous fillers within printed composites is also an advantage of 3D printing currently being explored. 3D printing potentially allows manipulation of the filler's alignment in the polymeric matrix during the printing process, in order to digitally integrate physical parameters such as composition, stiffness and toughness within the design components. For example, the 3D printing of cellular composites, consisting of a highly interconnected network of solid supports, has been enabled using composite material consisting of silicon carbide (SIC) whiskers and carbon fibres included within an epoxy resin. The alignment of fibres inside the composite matrix was controlled during printing, to manufacture hierarchical honeycomb structures with a close resemblance to natural balsa wood (see Fig. 8). This approach shows great potential for fabrication of a variety of bio-inspired composite objects with precise architecture and mechanical properties.⁵³ Furthermore, reinforcement of 3D printable material with natural fibres has also been described, in one example for the fabrication of hygromorphic biocomposites that are capable of bending in response to a moisture gradient.²³

Fig. 8 (a) 3D printing of the triangular honeycomb object fabricated from composite material, (b) schematic representation of the controlled alignment of fillers in the polymer matrix within the printer's nozzle during deposition of the composite ink on the printing bed. Reproduced with permission.⁵³

In addition to introducing various fillers to the polymer matrix, the development of tough composites based upon two polymers of contrasting properties has also been described. For example, Dimas et al. (2013) fabricated tough biological composites with exceptional fracture resistance by integrating two acrylic photopolymers with significantly different mechanical properties. The resulting composite comprised of specific topological arrangements of soft and stiff phases. In comparison to traditional materials that are delicate and prone to catastrophic failure, these 3D printed synthetic composites exhibited superior mechanical strength with fracture and deformation mechanisms comparable to mineralised biological composites. Furthermore, in this instance 3D printing provided the added advantages of large scale, low cost fabrication of complex topologies, which could not be achieved using traditional fabrication methods.²

The use of mould free rapid 3D printing technology has also been described for fabrication of high strength,^60,61 light weight,⁶² near net shape composites with complex geometries. ‘Near net shape’ is an industrial manufacturing term where the first fabrication of the object is very similar to the final (net) shape, preventing/minimising the requirement for surface finishing and therefore resulting in reduction of the production costs. These composites are traditionally prepared by hot pressing or cold pressing and are capable of only producing components with simple structures. However, using 3D printing complex structures with enhanced mechanical properties can be realised. 3D printing technology has been integrated with other manufacturing process to generate composites with extraordinary properties such as light weight, dense, and ultra-high melting capabilities.⁶⁰ Metal printing techniques have also been used to enhance the mechanical properties of composite ceramics, such as the production of mixed zirconia–alumina based ceramics using selective laser melting (SLM) based printing. The printed crack free materials displayed a flexural strength of more than 500 MPa.⁶³

Research have also been reported investigating the effect of fabrication parameters on the mechanical properties of such composite materials. For example, Christiyan et al. (2016) studied the influence of printing parameters, such as layer thickness and printing speed on the mechanical properties of an acrylonitrile-butadiene-styrene (ABS)–hydrous magnesium silicate composite. In this instance better results showing improved mechanical properties were obtained at a low printing speed and minimum layer thickness due to a better bonding between the layers.⁶⁴

Nanoparticles have also been added to various printable resins for the production of composite material exhibiting greater mechanical flexibility, with enhanced shape memory when exposed to repeated stretching and/bending.^23,65,66 For example Cai et al. (2011) added TiO₂ into molten vaseline to produce organic composite inks for fabrication of structures, with improved rheological properties and stretching and bending strength.⁶⁵

3D printed composites fabricated from multiple materials are becoming increasingly popular to develop objects with enhanced morphing and viscoelastic properties to overcome the geometric limitations of certain structures.^67,68 For example, Ahn et al. (2012) developed so-called smart soft composites by combining a shape memory alloy (SMA), that served as an actuator, and ABS, which was used as an anisotropic material, with PDMS providing the matrix.⁶⁸ A multi-nozzle 3D printer was employed to deposit the matrix, electrical circuits and anisotropic material. Other components such as the batteries, smart actuators and controllers were embedded in the morphing structure between the layers (see Fig. 9). Using this method, morphing composites structures with high magnitude of deformation were manufactured.

Fig. 9 (a) Fabrication process for the development of smart soft composites, (b) 3D printed morphing structure for phone/robot. Reproduced with permission.⁶⁸

Similarly, Hardin et al. (2015) utilised multi-material 3D printing, employing a microfluidic print-head, capable of effortlessly switching between two viscoelastic materials during the printing process, thus allowing the 3D printer to control the composition and property gradients within the printed objects. Using this approach, various compositions of PDMS based viscoelastic inks were used for the fabrication of various structures.⁶⁷ Additionally, Duigou et al. (2016) developed hygromorphic material via the incorporation of natural fibres into a printable biopolymer, developing materials with the ability to bend in response to moist environments.²³

3D printed electroactive composite materials

Electrically active composites are usually produced by adding fillers, such as carbon nanotubes (CNTs), with unique electrical and mechanical properties to the polymeric or structural matrix^13,69–77 (see Tables 1 and 2 for detailed information on matrix, fillers, composites, their properties and applications). However, the processing/incorporation of nanotubes presents considerable challenges in the development of such composites when using traditional techniques such as controlled growth on a substrates, or wet processes involving nanotube dispersions, such as drop deposition, filtration, and fibre spinning.⁷⁸

3D printing has been used as an alternative fabrication method for the development of versatile, inexpensive and environmentally benign electronics derived from composite materials. Some of the poor processability issues as presented by the traditional techniques, have been circumvented by employing a 3D printers (inkjet) capable of the precise spatially controlled deposition of such filler fibres onto substrate surfaces. In the case of inkjet printers, this approach requires the preparation of fillers as a dispersion with in a solvent to allow their use as an ink.⁷⁸

To improve the dispersion of CNTs in common solvents, natural gums and biopolymers are often added to the solvent,⁸⁰ or chemical modification of the nanotubes is carried out. For example, carboxylation of CNTs to allow efficient dispersion in water has been described, and said to provide a number of advantageous applications, including environmentally friendly and low cost ink for the printers, and ease of handling and storage.⁸¹ Similarly, to overcome the poor dispersion of CNTs, Panhuis et al. (2007) employed a biopolymer dispersant, including xanthan and gellan. Excellent dispersion and stabilisation of CNTs in aqueous solutions was achieved using this biopolymer. Using ultra small quantities (0.0001% w/v) of these dispersants they were able to prepare an electrically conducting composite ink compatible with inkjet printing.⁸⁰ Development of similar electrically conductive composite films consisting of CNTs and a water soluble electrically conducting polymer, namely poly(2-methoxyaniline-5-sulfonic acid), have also been described. The conductive polymer enabled efficient dispersion and stabilisation of the CNTs, and facilitated the use of the aqueous dispersion as an ink for the fabrication of composite prints with high electrical conductivity.⁷⁸ Taking this approach further, the use of FDM printing has also been explored for the production of conductive composite materials. In one such study, thermoplastic PLA was first dissolved in dichloromethane (used as dispersion medium) followed by the addition of CNTs. This method allowed the fabrication of conducting composite features as small as 100 μm, exhibiting high electrical conductivities (10–100 S m⁻¹).⁸²

However, use of such CNT based composites may not be completely suitable for conductive electrodes and electric circuits that are required to be functional and electrically stable under high mechanical deformations, e.g. for applications in flexible displays, field-effect transistors, energy-related devices, smart clothing and actuators. Although there has been efforts to improve conductivity and elasticity using composite elastomers, consisting of one dimensional nanostructures such as CNTs coated on a flexible fabric metal stripes,^83,84 generally conductivity under high mechanical stress is too low to fulfil requirements for practical applications.

The development of such electrically conducting composite elastomers from the use of carbon black incorporated into 3D FDM printable resins has also been described.^79,85 Using this material, a variety of functional electronic sensors were fabricated, including piezoresistive sensors capable of sensing mechanical flexing and capacitive sensors with ability to sense the presence and amount of liquid inside. The applicability of this material was demonstrated by fabrication of flex sensors and gloves capable of capable of sensing mechanical stress.⁷⁹ A variation in the resistivity of the printed material on the application of stress confirmed the piezoresistive nature of the composite objects (see Fig. 10).

Fig. 10 Flex sensor (ai) CAD design, (aii) 3D printed sensor, (aiii) sensor undergoing mechanical stress, (aiv) the resistance response of the sensor during stress. 3D printed glove (bi) CAD design, (bii) 3D printed ‘glove’, (biii) 3D printed ‘glove’ before flexing, (biv) during flexing and (bv) the resistance response of different fingers during flexing. Reproduced with permission.⁷⁹

Similarly, Park et al. (2012), developed a conductive composite of elastomers with silver nanoparticles (AgNPs) for creating highly stretchable circuits. To develop the highly flexible circuits the precursor (AgCF₃COO and alcohol) for the AgNPs was absorbed in the rubber fibres, followed by formation of AgNPs directly on the fibre mat (see Fig. 11). High bulk conductivity was achieved on percolation of the AgNPs inside the fibre mat and was retained during large deformations. For 3D printing the precursor solution was directly sprayed/nozzle printed on the fibre mat to fabricate both an antenna and light emitting diode with high elasticity, and additionally a strain sensor to demonstrate the potential applications of the developed material.⁸⁶ The authors claimed the compatibility of the fabrication process with any substrate for the development of arbitrary patterns over a large surface area.

Fig. 11 (a) Schematic illustration of the electrically conductive fibre mat fabrication process. Fibre mat with adsorbed silver precursor (AgCF₃COO) and alcohol, and reduction of precursor into AgNP, (b) highly stretch composite AgNP-fibre mat. Reproduced with permission.⁸⁶

Recently, Kim et al. (2014) developed a piezoelectric nanoparticle–polymer composite material by introducing barium titanate (BaTiO₃, BTO) nanoparticles into polyethylene glycol diacrylate solutions for development of printable sensors, and electronics.⁸⁷ The 3D printing of graphene composites using the biocompatible elastomer, polylactide-co-glycolide, has also been described for applications in printed electronics and conducting scaffolds. Utilising this liquid composite ink, robust and flexible electrically conducting graphene structures of variable size (as small as 100 μm) and thickness (<300 μm) were realised using extrusion-based 3D printing.⁷⁶

The fabrication of optically transparent composite conductive films comprised of palladium–copper and silver–copper, printed on a stretchable poly(ethylene terephthalate) (PET) substrates, has also been achieved using inkjet printing. These composite films provided a low cost alternative to indium tin oxide (ITO), an expensive and scarce material, used frequently in the optoelectronic devices. These 3D printed conductive composite films provided minimum resistance and exceptional optical transmittance.⁸⁸

The coating of printed porous substrates with solutions consisting of conducting filler particles^89,90 and injection of electrically conducting solutions into a 3D printed matrix⁹¹ is another approach which has been investigated for the production of electrically active composites.^89,90 For example, Cooperstein et al. (2015), developed electrically conducting composite objects, again using AgNPs. For development of these conductive composites, porous structures, fabricated using a printable oil-in-water emulsion, were dipped in a dispersion of AgNPs and then sintered at room temperature, to facilitate the development of conductive percolation paths inside the 3D structure.⁹⁰ Abueidda et al. (2015) similarly prepared interpenetrating phase composites by injecting an electrically conductive solution into the 3D printed matrix with the help of a syringe. The three dimensional ‘reinforcing’ phase was prepared by dispersing carbon nanostructures (0.8 wt%) in to the epoxy resin. The electrical conductivity of the reinforcing phase was reported to be 3.02 × 10⁻² S m⁻¹.⁹¹

In addition to introducing conducting fillers into 3D printed structures/scaffolds, efforts have also been made to perform chemical modification of the printed material to render it electrically conductive. For example, Carrico et al. (2015) chemically activated Nafion, a commonly used material for formation of ionic polymer metal composites, to produce electrically active composites. Nafion was printed in its precursor form which is melt processable but electrically inactive. Thus after the precursor polymer has constructed the desired geometry, it is hydrolysed to Nafion to make it electrically conductive. The developed composite material was used for applications in soft robotics.⁹²

There has also been efforts to develop 3D printable dielectric composite materials.^93–95 Kurimoto et al. (2015) prepared insulating material by adding alumina to a photo-resin for a stereolithographic (SLA) 3D printer,⁹⁴ in which the layers of the liquid composite (consisting of filler) are cured via a ultraviolet laser, until the entire structure is fabricated.⁹⁶ The developed material exhibited high permittivity and low dielectric loss tangent values.⁹⁴ Similarly, fabrication of dielectric composite materials with relative real permittivities have been achieved by incorporation of BaTiO₃ into ABS material for use with FDM printers.⁹⁵ Isakov et al. (2015) employed a multi extruder FDM based printer to create anisotropic dielectric composites. Two extruders of the printer were loaded with ABS material as the low dielectric permittivity phase and a mixed inorganic ceramic–polymer powder composite material as the high dielectric phase within the final extruder. This approach demonstrated great potential for employing FDM based printers for fabricating novel electromagnetic devices.⁹³

3D printable composite materials with thermal properties

3D printing provides the opportunity to incorporate fillers with high thermal conductivity^66,97–100 or insulating properties¹⁰¹ for the fabrication of composites for temperature dependent applications (see Tables 1 and 2 for detailed information on matrix, fillers, composites, their properties and applications). For example, recently a 3D printed graphene–polymer composite has been fabricated by introducing graphene flakes into a commercially available polymer matrix for FDM printing. In this work, graphene was first oxidised to graphene oxide (GO), which was then dispersed in N-methylpyrrolidone (NMP). ABS was then also dissolved in NMP. The two solutions were homogeneously mixed to form GO–ABS dispersion, followed by chemical reduction of GO to develop G–ABS dispersion which was fractionally precipitated via the addition of water (see Fig. 12). The developed G–ABS composite was melted (210 °C) in the FDM 3D printer nozzle, for fabrication of the thermally conducting composites. However, using the FDM process it was not possible to introduce high filler concentrations as formation of graphene aggregates was reported at concentrations at and above 7.4 wt%, which resulted in blocking of the printer nozzle.⁹⁷

Fig. 12 Composite preparation and 3D printing. (a) Graphite flakes, (b and c) dispersions of GO and ABS in NMP solvent, (d and e) a homogeneous mixture of GO–ABS in NMP before and after chemical reduction by hydrazine hydrate at 95 °C for 1 h, (f) G–ABS coagulations obtained after isolation, (e) with water, (g) G–ABS composite powder after washing and drying, (h) schematic illustration of FDM 3D printing process (i) a typical 3D printed model using 3.8 wt% G–ABS composite filament, scale bar: 1 cm. Reproduced with permission.⁹⁷

This limitation can be overcome by employing the SLA based printers. In SLA 3D printers, the layers of liquid polymer (including added filler) are photo-cured via the application of an ultraviolet laser,⁹⁶ therefore avoiding the need for nozzle or spray devices, which are likely to experience recurrent blockages from the added filler. Recently, the thermal and mechanical characteristics of photo-curable resins available for SLA 3D printers has been improved by the addition of TiO₂ nanoparticles.⁹⁸ The composite material showed a significant improvement in tensile strength, 89% in comparison to original polymer. Similar increases in other mechanical properties, including tensile modulus (18%) and flexural (6%) were observed. Differential scanning calorimetry experiments showed improvement in thermal properties by inclusion of the TiO₂ nanoparticles in the resin. Very recently, within the authors' own laboratory, Kalsoom et al. (2016), employed a SLA 3D printer for the fabrication of similar composites.⁹⁹ Using this printer, composite structures consisting of microdiamond particles, at concentrations as high as 30% (w/v) were produced. The developed polymer–diamond composite material was used for the fabrication of both a demonstration heat sink and a cooling system for heat management applications.⁹⁹ Fig. 13 shows a 3D printed composite heat sink and associated IR images for the composite heat sink and a blank polymer version, when heated at 100 °C for 10 min. As shown the composite material provided increased heat transfer rates in comparison to the blank commercially available epoxy based polymeric material.

Fig. 13 (a) 3D printed heat sink using commercially available polymer, (b) 3D printed heat sink using 30% (w/v) composite material, (c) IR images of polymer heat sink heated for 10 min at 100 °C (d) IR image of composite heat sink heated for 10 min at 100 °C. Reproduced with permission.⁹⁹

Besides the development of thermally conducting materials, production of thermoelectric materials with ultra-low thermal conductivities has also been demonstrated for a variety of applications, such as the bionic ear fabrication, reactionware for the chemical sciences, and lithium-ion micro-batteries. For the development of such composite materials, Bi_0.5Sb_1.5Te₃ (BST) powder, a traditional thermoelectric material, was mixed with the photo-curable resin for fabrication of devices using an SLA printer.¹⁰¹

3D printed composite material with enhanced optical properties

Inkjet printing has recently gained much attention as an alternative fabrication method for the manufacturing of electrochromic devices, based upon its ability to rapidly generate high resolution patterns consuming very small quantity of material.¹⁰² However, high viscosity and surface tension of the ink causes difficulties in ejecting and preserving the spherical shape of the droplet and therefore presents a major limitation to inkjet printing of electrochromic devices.^103,104 However, there has been recent efforts to overcome these limitations via the use of colloidal composites of intrinsically conductive polymers (ICPs), which can decrease the viscosity of high molecular weight polymers in commonly used solvents. For example, the inkjet printing of quantum-dot polymer composites, based upon a colloidal dispersion of quantum dots in polyisobutylene has been employed for the fabrication of patterns of robust luminescent light-converting thin films for AC electroluminescent displays. These quantum dot polymer composites facilitated the inkjet printing of full colour AC driven electroluminescent displays with maximum brightness and minimum power consumption.¹⁰⁵ Similarly, Shim et al. (2008) prepared an ICP ink using colloidal composite particles of polyaniline–silica and poly(3,4-ethylenedioxythiophene)–silica for the fabrication of electrochromic devices capable of changing colour depending upon an applied voltage.¹⁰² The print deposition of coloured thin films consisting of luminescent colloidal CdSe/ZnS quantum dot (QD)–polymer composites have also been developed. These printed films were also used for fabrication of robust, bright, full-colour AC-driven displays.¹⁰⁶

Conclusions, challenges and future direction

3D printing technology is significantly contributing to the ongoing revolution in the field of materials science research. The potential of 3D printing for the fabrication of composite material based devices has flung open the doors for a rush of new developments and possibilities in applied material science and is sparking new research into the rapid prototyping of complex objects with unique material properties.

However, there are major challenges that still need to be addressed to achieve the full potential of 3D printable composite materials. For example, current commercial 3D printers are designed for relatively simple and often printer specific resins, and not compatible with modified resins, where the inclusion of fillers can cause printer failure, through for example, agglomerate formation, heterogeneous composite formation,^5,97 blocking of printer heads, or non-adhesion and increased curing times (e.g. with SLA printing).^98,99 There is also the question of printer resolution and the impact of the composite material on such important performance parameters. All of these issues require careful optimisation, however each can be addressed and the achievements detailed within this review are testament to this. It should also be noted that 3D printer companies are beginning to supply commercial composite resins for specific applications and this trend will no doubt now continue at pace, as the diversity of potential materials and composites remains huge.

The ability to precisely control the physical, electrochemical, thermal, and optical properties of 3D complex structures is an exciting thought, and has already gained the attention of all those in the fields of biomedical devices, analytical sciences, fluidics, robotics, the automotive and aerospace industries, printed electronics, to name but a few.

Acknowledgements

This study is financially supported by the ARC Centre of Excellence for Electromaterials Science (ACES) (Grant CE140100012).

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