A colloidal suspension and method of forming the same

Technical Field

The present invention relates to a colloidal suspension and method of forming the same.

Background

Conventional metalworking processes typically require intensive energy for melting and shaping of the precursors, which is not environmentally friendly. Safety issues also arise during various stages of the processes, for example, welding fumes, chemical exposure, and hazardous noise levels.

Thus, there is a need for an improved precursor and method of forming metallic materials.

Summary of the invention

The present invention seeks to address these problems, and/or to provide an improved precursor and method of forming metallic materials.

According to a first aspect, the present invention provides a colloidal suspension comprising a biopolymer and a metal, wherein the ratio of the biopolymer to the metal is < 1 :50 by weight.

According to a particular aspect, the biopolymer may comprise polysaccharides, polynucleotides, and polypeptides. The biopolymer may comprise chitinous molecules.

According to a particular aspect, the suspension has a viscosity of 10-10,000,000 cps.

The metal may have an average particle size of < 1 mm. The metal may comprise a transition metal, a non-transition metal, or an alloy.

According to a particular aspect, the suspension may form a composite. The composite may further comprise a biomaterial.

The composite may have an electrical conductivity of > 0.05 S/cm. The composite may have a thermal degradation temperature of > 1600 °C.

According to a second aspect, there is provided a method of forming the suspension, comprising mixing a metal and a biopolymer, wherein the ratio of the biopolymer to the metal is < 1 :50 by weight. The method may comprise adding the biopolymer to a dispersing medium prior to the mixing the metal.

According to a particular aspect, the dispersing medium may not react with the metal. According to another particular aspect, the method may comprise removing excess dispersing medium prior to adding the metal.

The metal may have an average particle size of < 1 mm.

According to a particular aspect, the method may comprise drying the suspension to form a composite. In particular, the drying may be under ambient conditions. The method may comprise casting, coating, moulding and/or 3D printing the suspension prior to the drying.

According to a particular aspect, the method may further comprise polishing the composite after the drying.

The method may comprise heat treating the composite after the drying. In particular, the heat treating may be at a temperature of > 40 °C.

Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

Figure 1 shows a schematic diagram of the formation of chitosan-metal colloidal suspensions and their transformation into ultra-low-binder-content composites by internal compaction;

Figure 2 shows scanning electron images of the metal powders and the colloidal suspensions after freeze-drying;

Figure 3 shows images of the solid composites, with the left column showing scanning electron micrographs of the composites, and the right column showing reconstructed volumes of the composites recorded by x-ray microtomography;

Figure 4 shows flexural strength of the metallic aggregates with respect to the chitosan- to-metal ratio, where all samples were cast under the same conditions; Figure 5 shows scanning electron images of the surface of a chitosan-metal composite before and after polishing;

Figure 6 shows the mechanical performance of a FLAM tip dip-coated with a 1 :250 w/w chitinous colloidal suspension of tin compared to an uncoated tip;

Figure 7 shows heat flow of pure chitosan, pure metals, and chitosan-metal composites as measured by differential scanning calorimetry (DSC); and

Figure 8 shows the electrical resistance of the different samples while heated to 350°C on a hot plate (open atmosphere) and when cooled down to room temperature. The arrows in the graph represent the direction of the cycle (i.e. , from left to right is the heating process, and from right to left the cooling process); Figure 8(a) shows the electrical resistance of pure chitosan; Figure 8(b) shows the electrical resistance of chitosan-tin composite; Figure 8(c) shows the electrical resistance of chitosan-copper composite; Figure 8(d) shows the electrical resistance of chitosan-stainless steel composite.

Detailed Description

As explained above, there is a need for an improved precursor and method of metalworking.

In general terms, the present invention provides a colloidal suspension comprising a polymer and a metal, wherein the ratio of the polymer to the metal is < 1 :50 by weight. The polymer may be a biopolymer. Thus, the colloidal suspension of the present invention may advantageously contain a biopolymer as a binder, which is sustainable and biodegradable, and thus environmentally friendly.

According to a first aspect, the present invention provides a colloidal suspension comprising a polymer and a metal, wherein the ratio of the polymer to the metal is < 1 :50 by weight. The polymer may be any suitable polymer, for example, a biopolymer.

For the purposes of the present invention, the use of the singular includes the plural unless specifically stated otherwise. It should be noted that, as used in the specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Further, the use of the term “including”, “comprising”, and “having” as well as other forms, such as “include”, “comprise”, “have” are not considered limiting. For the purposes of the present invention, references to colloidal suspension refers to a mixture of insoluble solid particles in a fluid.

For the purposes of the present invention, references to a biopolymer refers to a natural polymer produced from natural sources, either chemically synthesized from a biological material, or entirely biosynthesized by living organisms.

The ratio of the biopolymer to the metal may be < 1 :50 by weight. In particular, the ratio of the biopolymer to the metal may be 1 :50 to 1 :100, 1 :50 to 1 :150, 1 :50 to 1 :200, 1 :50 to 1 :250, 1 :50 to 1 :300, 1 :50 to 1 :350, 1 :50 to 1 :400, 1 :50 to 1 :450, 1 :50 to 1 :500 by weight. Even more in particular, the ratio of the biopolymer to the metal may be 1 :50 to 1 :250 by weight.

According to a particular aspect, the biopolymer may comprise polysaccharides, polynucleotides, and polypeptides. In particular, the biopolymer may comprise chitinous molecules. For example, the biopolymer may comprise chitin, chitosan, and/or modifications thereof.

The suspension may have any suitable viscosity so as to enable any suitable metalworking steps to be performed. For example, the suspension may have any suitable viscosity so as to enable casting, coating, moulding, and/or 3D printing of the suspension. In particular, the suspension may have a viscosity of 10-10,000,000 cps. In particular, the suspension may have a viscosity of 100-1 ,000,000 cps, 100-1 ,000 cps, 500-1 ,500 cps, 1 ,000-900,000 cps, 1 ,000-5,000 cps, 1 ,000-10,000 cps, 1 ,000-20,000 cps, 10,000-25,000 cps, 10,000-50,000 cps, 10,000-75,000 cps, 10,000-100,000 cps, 100,000-800,000 cps, 200,000-700,000 cps, 300,000-600,000 cps, 400,000-500,000 cps. The viscosity may be at standard conditions (1 atm and 20 °C).

According to a particular aspect, the metal may be in powder form. According to a particular aspect, the metal may have an average particle size of < 1 mm. For the purposes of the present invention, references to an average particle size refers to the typical value in a set of particles used, in particular the mode of the sizes. In particular, the metal may have an average particle size of 1-1000 pm, 50-950 pm, 100-900 pm, 150-850 pm, 200-800 pm, 250-750 pm, 300-700 pm, 350-650 pm, 400-600 pm, 450-550 pm. The metal may be a transition metal, a non-transition metal, or an alloy. A non-transition metal refers to a metal in Groups 1-2, and 13-16 of the periodic table. For the purposes of the present invention, references to an alloy refers to a mixture of metals, which may be a mixture of transition metals, non-transition metals, or a combination thereof.

According to a particular aspect, the suspension may form a composite. For example, the suspension may be dried using any suitable method to form a composite. The draying may comprise removing water and/or other solvents from the suspension. In particular, the suspension may be dried in ambient conditions or freeze-dried to form a composite. Advantageously, the suspension may be dried in ambient conditions, which eliminates the need for energy-intensive heating and further processing steps. The composite may require a very low amount of binder content based on total weight, thus advantageously imparting a large degree of metallic characteristics into the composites. The composite may have any suitable porosity. For example, the composite may have a porosity of 0.5 to 99.5%.

The composite may further comprise a biomaterial. For example, the composite may be applied on a biomaterial. In particular, since the composite can advantageously be formed at ambient conditions, the composite may be applied on biomaterials that are sensitive to temperature. This advantageously allows the application of the composite to impart metallic characteristics to biomaterials which cannot withstand high temperatures in traditional metalworking processes. The biomaterial may be any suitable material derived from biological sources, and which has affinity with the biopolymer. For example, the biomaterial may comprise cellulose. In particular, the biomaterial may be fungal-like adhesive materials (FLAMs). The biomaterial may be coated by or encapsulated within the composite.

The composite may have a suitable electrical conductivity to produce functional electrical components. For example, the composite may have an electrical conductivity of > 0.05 S/cm. In particular, the composite may have an electrical conductivity of > 0.1 S/cm, > 1 S/cm, > 10 S/cm, > 50 S/cm, > 100 S/cm, > 500 S/cm, > 1000 S/cm, > 1000 S/m, > 5000 S/m, > 10,000 S/m, > 100,000 S/m, > 1 ,000,000 S/m.

The composite may have a suitable thermal degradation temperature to enable use in high temperature environments. For the purposes of the present invention, references to thermal degradation temperature refers to the temperature at which the composite is chemically or physically damaged, thereby affecting its properties. For example, the composite may have a thermal degradation temperature of > 1600°C. In particular, the composite may advantageously remain thermally stable when exposed to high temperatures, and retain structural integrity even at temperatures higher than the carbonisation and vaporisation temperatures of most organic compounds.

According to a second aspect, there is provided a method of forming a suspension of the first aspect, the method comprising mixing a metal and a biopolymer, wherein the ratio of the biopolymer to the metal is < 1 :50 by weight. The biopolymer may be as defined above. The ratio of the biopolymer to the metal may be 1 :50 to 1 : 100, 1 :50 to 1 : 150, 1 :50 to 1 :200, 1 :50 to 1 :250, 1 :50 to 1 :300, 1 :50 to 1 :350, 1 :50 to 1 :400, 1 :50 to 1 :450, 1 :50 to 1 :500 by weight. Even more in particular, the ratio of the biopolymer to the metal may be 1 :50 to 1 :250 by weight.

The mixing may be at any suitable temperature for any suitable time. For example, the mixing may be at a temperature of 30-70 °C. In particular, the mixing may be at a temperature of 40-60 °C. Even more in particular, the mixing may be at a temperature of 50 °C. The mixing may be for 24-120 hours. In particular, the mixing may be for 48-96 hours. Even more in particular, the mixing may be for 72 hours.

According to a particular aspect, the method may further comprise adding the biopolymer to a dispersing medium prior to the mixing the metal. The dispersing medium may be any suitable medium for dispersing the biopolymer. According to a particular aspect, the dispersing medium may be selected as one that does not react with the metal. For example, the dispersing medium may comprise an alcohol, an acid, a base, and/or water. According to another particular aspect, if the dispersing medium selected will react with the metal, excess dispersing medium may be removed prior to the adding the metal. For example, the dispersing medium may comprise a carboxylic acid. In particular, the dispersing medium may comprise acetic acid.

The metal may be in powder form. The metal may have an average particle size of < 1 mm. The average particle may be as defined above. In particular, the metal may have an average particle size of 1-1000 pm, 50-950 pm, 100-900 pm, 150-850 pm, 200-800 pm, 250-750 pm, 300-700 pm, 350-650 pm, 400-600 pm, 450-550 pm..

The method may further comprise drying the suspension to form a composite. The draying may comprise removing water and/or other solvents from the suspension. The drying may be at any suitable temperature for any suitable time. According to a particular aspect, drying may be at a temperature of 20-30 °C. In particular, the drying may be at a temperature of 25 °C. The drying may be for < 48 hours. In particular, the drying may be for < 36 hours, < 24 hours, < 12 hours, < 6 hours, < 3 hours, < 1 hour. In this way, the method can advantageously be performed at ambient conditions, such as at standard temperature and pressure, without the need for externally applied pressure or temperature. Accordingly, new methodologies from which metals were previously excluded due to temperature requirements (for example, spraying, spin coating, use in combination with natural materials) can now be performed, and may be used in semiconductors and ceramics, metal mixtures whose components possess complementary characteristics, or exploration of optical properties using particles with well-controlled geometries. According to another particular aspect, the drying may be freeze drying.

According to a particular aspect, the method may further comprise casting, coating, moulding and/or 3D printing the suspension prior to the drying. The casting may comprise pouring the suspension into a mould cavity. The casting requires a balance of the properties of both states. In the colloidal form, the initial concentration of biopolymer in the dispersing medium determines rheology and manufacturability, while an excess of biopolymer in the dispersing medium makes it easier for the colloid to conform to a shape but harder to retain that shape after shrinking. In the composite form, low amounts of biopolymer in the dispersing medium result in loose aggregates, while an excess results in the electrical, thermal, and abrasion resistance properties of the metal being overshadowed by the properties of the biopolymer.

The coating may comprise any suitable coating methods. For example, the coating may comprise dip coating, painting, and/or spraying.

The 3D printing may be at any suitable temperature. For example, the 3D printing may be at a temperature of < 100 °C. According to a particular aspect, the 3D printing may be at a temperature of 25 °C. According to another particular aspect, the 3D printing may comprise printing on a hot plate, to provide simultaneous printing and thermal treating. The printing on a hot plate may be at any suitable temperature. For example, the printing on a hot plate may be at a suitable temperature to effect drying to form the composite, and/or thermal treating of the composite. In particular, the printing on a hot plate may be at a temperature of 200-400 °C. The 3D printing may be in any suitable atmosphere. According to a particular aspect, the 3D printing may be in open atmosphere. According to another particular aspect, when the 3D printing comprises printing on a hot plate to provide simultaneous printing and thermal treating, the heating of some metals may lead to the formation and reduction of oxidated states, and the 3D printing may be in controlled atmospheres. For example, the 3D printing may be in a nitrogen, argon, or helium atmosphere.

Thus, the suspension may have any suitable viscosity so as to enable any suitable metalworking steps to be performed. For example, the suspension may have any suitable viscosity so as to enable casting, coating, moulding, and/or 3D printing of the suspension. In particular, the suspension may have a viscosity of 100-1 ,000,000 cps, 100-1 ,000 cps, 500-1 ,500 cps, 1 ,000-900,000 cps, 1 ,000-5,000 cps, 1 ,000-10,000 cps, 1 ,000-20,000 cps, 10,000-25,000 cps, 10,000-50,000 cps, 10,000-75,000 cps, 10,000- 100,000 cps, 100,000-800,000 cps, 200,000-700,000 cps, 300,000-600,000 cps, 400,000-500,000 cps. The viscosity may be at standard conditions (1 atm and 20 °C).

According to a particular aspect, the method may further comprise polishing the composite after the drying. The polishing may be by applying pressure, friction, or both. The composite advantageously possesses enhanced resistance to abrasion from its metallic elements, which allows it to be polished and to acquire a characteristic metallic shine and, more importantly, surface continuity and metallic electrical conductivity.

The method may further comprise heat treating the composite after the drying. The heat treating may be at any suitable temperature for any suitable time. The heat treating may be at a temperature of > 40 °C. In particular, the heat treating may be at a temperature of > 50 °C, > 60 °C, > 70 °C, > 80 °C, > 90 °C, > 100 °C. In particular, the heat treating may be at a temperature of 50-400 °C, 100-350 °C, 150-300 °C, 200-250 °C. Advantageously, the composite may attain continuity even without getting close to the melting temperature of the metal, thereby achieving energy efficiency. The heat treating may be for 10-180 minutes, 20-170 minutes, 30-160 minutes, 40-150 minutes, 50-140 minutes, 60-130 minutes, 70-120 minutes, 80-110 minutes, 90-100 minutes. According to a particular aspect, the heating may be in open atmosphere. According to another particular aspect, the heating may be in controlled atmospheres. For example, the heating may be in a nitrogen, argon, or helium atmosphere. Having now generally described the invention, the same will be more readily understood through reference to the following example which is provided by way of illustration, and is not intended to be limiting.

Example

Materials and methods

Commercially available chitosan (medium molecular weight; 75-85% deacetylated) was sourced from i-CHESS chemicals Pvt Ltd., India and dispersed in 1 % (v/v) acetic acid to form 3% (w/w) chitosan solution. The solution was submerged in a 50°C water bath for 72 hours to fully disperse the chitosan.

Metallic powders were sourced from Chengdu Best New Materials Co., Ltd and used as received. Tin powder (-200 mesh, 99.6% Sn), copper powder (-250 mesh, 99.5% Cu) and stainless steel 304 powder (-300 mesh, C < 0.07, Si < 1.0, Mn < 2.0, Cr 18.0-19.0, Ni 8.0-11.0, S < 0.03, P < 0.035) were mixed in varying (w/w) ratios from 1 :50 to 1 :250, forming colloidal suspensions.

Casting of colloidal suspension

The resulting colloidal suspension, which can flow into and retain shapes, transitioned to a solid as the water evaporated under ambient conditions. Figure 1 shows a schematic diagram of the formation of chitosan-metal colloidal suspensions and their transformation into ultra-low-binder-content composites by internal compaction. A liquid crystal solution of chitosan was used as the continuous phase, to which metallic particles were added as the dispersed phase. The resulting colloidal suspension, which can flow into and retain shapes, transitioned to a solid as the water evaporated under ambient conditions. During this process, the crystallization of the relatively low amount of chitin (<0.4% w/w) brought all the metallic particles together into a porous aggregate that exhibits metallic characteristics.

To illustrate this, 3D printed negative moulds of different geometrical shapes were produced, and each was filled with a colloidal suspension. Crystallization of chitosan and consolidation of the composite in the mould were then performed, resulting in a casted replica.

3D printing with colloidal suspension Bulk 3D printing was performed with the tin-chitosan colloid. The colloid was pneumatically extruded using a standard 150 mL plastic syringe, and actuated by computer-controlled stepper motors assembled on a cartesian system to control the position.

Among the several objects that were printed with this system in order to demonstrate its ability to produce functional electrical components was a bulb socket. The process consisted of two well-defined stages: (1) bioprinting and (2) heat treatment, with the latter performed by raising the temperature of the printing surface to 320°C for 10 minutes. The details are as follows.

A solid 3D model of a light bulb socket was designed in Autodesk Fusion 360, then hollowed out and sliced with Ultimaker Cura using the Spiralize Outer Contour Mode to produce geode for the printer. Printing was achieved using a modified CNC router with a cartesian motion system and a syringe-loaded pneumatic paste extruder. A chitosan-tin mixture of 1 :250 ratio was loaded into a 150 mL syringe and extruded at a pneumatic pressure of 8 to 10 psi. The material was extruded on an aluminum foil coated hot plate which was heated at 60°C and progressively increased to 100°C during printing. This facilitated drying and improved the buildability of material layer by layer.

End connector wires were placed mid-print to make the eventual socket directly functional. Once the printing was complete, the socket was dried for 15 minutes at 100°C to expel residual moisture. The hot plate was then heated to 320°C to thermally treat the socket, which reduced its internal resistance. The socket was then cooled to ambient temperature and connected to 12V and ground. A 12VDC E27 bulb was then mounted. The success of the process was demonstrated visually by lighting a regular bulb.

While the whole process was conducted in an open atmosphere, the heating of some metals may lead to the formation and reduction of oxidated states, which would require the use of controlled (e.g., neutral) atmospheres.

Characterization

X-Ray Diffraction (XRD)

XRD measurement was conducted using Bruker Eco D8 Advance diffractometer at CuKa radiation (wavelength = 1 .542 A) at a voltage of 40 kV and 25mA. Scans between a 20 range of 5° and 55° were performed with at step size of 0.02° and 5° per minute. Presented data were results of post processing on DIFFRAC.EVA software that includes stripping Ka2 contributions, smoothening and removing background.

Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS)

SEM observations of surface morphology were made using a Field Emission Scanning Electron Microscope (FESEM, JEOL, JSM-7600F). Samples were affixed on a carbon tape onto an aluminum stub and sputter coated with gold. The SEM images were cropped, levels adjusted and colorized manually using MountainsSEM® colorization tool.

SEM-EDS observations of pre and post heat treated samples were made, to detect elemental changes, and layered onto SEM images.

X-ray microtomography

To understand better the internal structure of composite, 3D imaging was performed using Bruker SkyScan 1272. Scans were conducted with a source voltage of 100kV and source current of 100pA.

Flexural strength measurement

Metallic chitosan composite of 1 :50, 1 :100, 1 :150, 1 :200 and 1 :250 chitosan to metal powder (w/w) ratio were prepared, casted into silicone molds (96 mm (length) by 13 mm (width) by 4 mm (thickness)) and dried at room ambient conditions. Specimen span length (1 :20) were determined in accordance with ASTM D7264M. Samples were subjected to a loading rate of 2.67mm/min in accordance to ASTM D790 procedure A.

Coating samples and loading measurement

Dried fungal-like adhesive materials (FLAMs) samples (1 :8 (w/w) chitosan to cellulose ratio) were cut into short bars of 7 mm by 7 mm by 5.5 mm as penetrating samples while flat sheets of 110 mm by 41 mm by 2mm were prepared as penetrating target. Penetrating samples were sharpened with an off-shelf pencil sharpener to obtain a sharp tip. For coated samples, the sharpened tip was then dipped into a chitosan-tin mixture of 1 :250 (w/w) ratio and allowed to drip dry. Similarly, casted “Merlion” and honey bee was coated. After drying, the coated surfaces were polished. The penetration target was placed on a flexural jig with supports at 3 mm length span. The coated and uncoated sharpened tips were secured between pneumatic actuated grips and pushed into the target FLAM at a rate of 4 mm/min. Load and displacement data were recorded.

Differential Scanning Calorimetry

Heat flow measurements between 30°C to 500°C were conducted using DSC Q20 (TA Instruments) at a ramp rate of 10°C/min. Upon reaching 500°C, an isothermal state was maintained for 2 minutes and subsequently ramped down at the same rate.

Measurement of electrical Resistance as a function of temperature

The electrical resistance of 1 cm by 1 cm by 3 mm samples was measured by heating the samples in air using a Torrey Pines computer-controlled hotplate, a Keithley 2410 source-measure unit (SMU), and a four-point probe (4PP) with spring-loaded tungsten tips. The tungsten probe tips were spaced 5 mm apart along a line. Since the hotplate had an aluminum surface, a glass slide was used to electrically isolate the sample from the hotplate.

The current, I, was delivered to the sample through the outer two probes of the 4PP, whilst the inner two probes were used to measure the potential difference, AV, due to the current flowing through the sample. This measurement arrangement avoids contact resistance issues. The Keithley 2410 was programmed to maintain a constant voltage of 2.1 V potential difference across the inner two probes by adjusting the current delivered through the outer two probes.

The sheet resistance was calculated as R _s = ■ The hotplate was programmed to increase in temperature at a rate of 5 °C/min to a maximum temperature of 350 °C. The current through the outer probes and the voltage drop across the inner probes was recorded every second whilst the sample was heated and whilst it cooled naturally. When the samples underwent the chitometallic to metal phase transition, there was a radical reduction in the electrical sheet resistance, and therefore the Keithley 2410 SMU compensated by substantially increasing the current delivered to the outer probes such that the voltage drop across the inner probes was maintained at 2.1V. Thus, the resistance was measured to decrease at this transition temperature. Results and

Figure 2 shows the diversity of sizes and aggregation states of the metals, with scanning electron images of the metals in powder form (left) and the colloidal suspensions after freeze-drying (right). In scanning electron images of the freeze-dried colloidal suspensions, the initial intermingled state between particles and biopolymer and the existing connections between the chitinous molecules in suspension can be seen.

Binder content

For all three of the metals, the resulting composites were a compact but porous aggregate of metallic particles (Figure 3). The right column of Figure 3 shows reconstructed volumes of the composites recorded by X-ray microtomography, which illustrates that the formed composites are highly porous. The minimum amount of chitosan required to consolidate the particles into a solid ranged from 0.4% to 1.0% of the metal weight and seemed largely independent of the metal used, with particle size being the determining factor. This suggests that the formation of the solid is a physical process (i.e. , rather than chemical) governed by the ability of chitosan to bridge and reduce the interstitial space between particles.

The concentrations of chitosan necessary to bind metallic particles are advantageously and unexpectedly lower by an order of magnitude than the binder concentrations in previously reported ultra-low-binder-content (UBC) composites. However, unlike UBC composites, which are made using synthetic polymers and high external pressures, the composites obtained here achieved these results without external forces and under ambient conditions, highlighting the unparalleled efficiency of this approach of binding particles.

Casting of colloidal suspension

The unique ability of the colloids presented here to “self-aggregate” into solids from a flowing state allows casting of metal composites at ambient conditions. Casting a colloidal suspension into a solid requires a balance of properties of both states. In the colloidal form, the initial concentration of chitosan in the solution determines rheology and manufacturability, while an excess of dissolved chitin makes it easier for the colloid to conform to a shape but harder to retain that shape after shrinking. Properties of formed composites

Likewise, in its solid form, low amounts of chitosan result in loose aggregates, while an excess of chitosan results in the electrical, thermal, and abrasion resistance properties of the filler being overshadowed by the properties of chitosan. In the case of copper, the poor mechanical strength resulted from premature precipitation of the metal due to the slightly acidic chitinous continuous phase (Figure 4).

All three tested metals produced complex shapes, confirming the general ability of chitinous polymers to bind metals. However, although the three metals were cast under the same conditions, copper reacted with the diluted acetic acid in the colloid to form copper (II) acetate. Phase separation was therefore induced during the drying process, resulting in lower detail and low mechanical strength. This effect was overcome by neutralizing the excess acetic acid before introducing the metal.

The targeted metals interactions, not only with the biopolymer, but also with the dispersing medium, need to be taken into consideration.

Compared to continuous (i.e., melted) pieces of metal, the granular and porous composites do not excel in terms of their standalone mechanical strength. However, the composites acquired enhanced resistance to thermal degradation and abrasion from its metallic elements. Composite slabs using tin, copper, and a tin-copper blend, were produced at room conditions by casting colloids in silicone molds. The effect of a butane flame (at about 1500°C) on a chitosan-copper slab was tested. At such high temperatures, which are well above those for the carbonization/vaporization of most organic components, the chitosan-copper composite remained solid and preserved its geometry.

The enhanced resistance to abrasion from its metallic elements enables the pieces to be polished and thus acquire a characteristic metallic shine and, more importantly, surface continuity and metallic electrical conductivity. Figure 5 shows scanning electron images of the surface of a chitometallic composite before and after polishing.

Compatibility with other biomaterials

Furthermore, despite the low amount of biopolymer, the composites retained the compatibility of chitosan with biological composites, which, in addition to the ability to be formed at ambient temperatures, enables the localized incorporation of chitometallic properties into other biomaterials. This property was utilised to provide metallic characteristics to inexpensive, sustainable, large-scale constructions of cellulose-based fungal-like adhesive materials (FLAMs).

The properties of tin were incorporated into the surface of two solid cellulosic objects: a replica of a honeybee (Apis mellifera) head and the Merlion mascot of Singapore. Because of the natural affinity of amine groups in chitosan to biological materials, FLAM objects can be dipped or “painted” in a 1 :250 w/w chitinous colloidal suspension of tin. The colloid crystallized in a conforming metallic layer which can then be polished or reworked. Notably, since FLAMs are based on the same bioinspired manufacturing principles and common biomolecules as the metallic colloid, the entire manufacturing process — including the formation of the cellulosic insect and Merlion — was done at room temperature, using just water to drive the assembly.

The chitinous metallic coating of cellulosic constructs goes beyond merely aesthetic or electrical modifications, and includes improving mechanical properties. A sharpened FLAM tip was produced and dip-coated with tin. This simple process resulted in a modified sharp tip that had twice the yield stress of an uncoated tip and that required more than five times the energy to suffer irreversible damage (i.e. , plastic deformation), as seen in Figure 6. The coated tip has double the elastic region (linear slope) when crushed against a solid surface, and required more than five times the energy (area under the curve in the elastic region) to undergo permanent deformation, compared to the uncoated tip.

The affinity of chitosan for biological construction is not limited to cellulose. Most notably, the use of the principles described here can be applied to provide electrical properties to other biomaterials that have a demonstrated affinity with chitosan, such as internal organs and skin, or cotton, which suggests their potential inclusion in smart textiles and medical devices.

Thermal and electrical conductivity

Figure 7 shows the heat flow of chitosan, metal, and composites as measured by differential scanning calorimetry (DSC). Of all the metals used, only tin has an endotherm peak at its melting temperature. Due to the low amounts of chitosan in the composites, there are no observable differences between the heat flow of the metal alone and the heat flow of the composites.

The chitosan-metal composites produced under ambient conditions allow electrical conductivity along the surfaces, which can easily be used to create conductive tracks and patterns. However, these metallic properties were only apparent after polishing, suggesting that the porous interior lacked continuity. Through heat treatment, the composites were able to become internally electrically conducting through their bulk.

Internal continuity of the composites was achieved, which would be required for applications that require bulk electrical properties, such as batteries and electrodes, by exposing the composites to increased temperatures for a short time. For example, after heat treatment at temperatures typically lower than 300°C, a drastic reduction in electrical resistance in both tin and copper composites were observed, while stainless steel composites were unaffected.

Figure 8 shows the electrical properties of the different samples while heated to 350°C on a hot plate (open atmosphere) and while cooled down to room temperature. The arrows in the graph represent the direction of the cycle (i.e. , from left to right is the heating process, and from right to left the cooling back to ambient temperature).

Thermally treating chitosan films alone gave rise to a more resistant material, suggesting that carbonization of the organic binder did not play a relevant role in the process. Tin and copper (Figure 8(b) and 8(c)) had a permanent drop in resistance of about 10 and 6 orders of magnitude, respectively. In contrast, stainless steel showed only 1 order of magnitude change (Figure 8(d)). It is worth noting that the size of stainless steel particles was several times larger than those of the other metals, as seen in Figure 2 (left column).

The transition occurred sharply at temperatures unrelated to the specific metal used. For the same metal, when the ratio of chitosan to metal (Figure 8(c)) or the size of the metallic particles (Figure 8(b)) was changed, there were shifts in the electrical transition temperature. The results suggested that the process in which the composites acquired the electrical properties of a metallic foam is independent of the metal used and is driven by a physical process. Because of the absence of external pressure and reliance on internal forces, the process may be described as a “self-percolation” or “self-compaction” process. The initial hypothesis that the change in conductivity was due to the carbonization of chitosan, reduction of a metal oxide, or sintering was rejected, as a comparison between thermal analyses of all the involved components and the temperature at which the composites changes conductivity suggested otherwise.

Post heat-treatment, the composites transitioned to being electrical conductors at moderate temperatures, and showed a permanent decrease in their electrical resistance of several orders of magnitude. This permanent increase in electrical conductivity occurred in all the tested metals at temperatures that have no relevance to the metal in question. Instead, the transition temperature appeared to be dependent on aspects of the packing of the composite, suggesting the existence of an internal consolidation mechanism that is driven by the chitinous phase and goes beyond the percolating threshold required to achieve volumetric electrical continuity.

The existence of a critical threshold for inducing a permanent change in the bulk material at a temperature that had no apparent relevance to any of the involved components seemed to suggest further internal compaction and the achievement of continuity by percolation. With the exception of stainless steel, which can be aggregated during the crystallization of chitosan but cannot be further packed by the same treatment as copper and tin, the results suggested that the affinity of the metal for the chitinous binder might be a fundamental aspect of this process. The percolation hypothesis agreed with the SEM-EDS analysis, which revealed low traces of carbon and distinct metallic particles with closer packing than before heat treatment (not shown). While the cumulative evidence seemed to point to a percolation process, the fact that the transition was more obvious in the metal with the lowest melting point (i.e. , tin) suggested that there may be a combination of effects.

The ability to safely manipulate and dry the material in ambient conditions and to then apply heat treatment as an optional post-process allows metal elements to be shaped without high heat. This opens up a more accessible and flexible approach to metalworking, making it easier to produce shapes and making it possible, for example, to bioprint functional electrical components under ambient conditions.

3D printing with colloidal suspension

Bulk 3D printing with the tin-chitosan colloid was found to be particularly straightforward because of its shear-thinning characteristics, which facilitated extrusion and allowed it to retain its shape thereafter. The success of the process was demonstrated visually by lighting a regular bulb (not shown). While the whole process was conducted in an open atmosphere, the heating of some metals may lead to the formation and reduction of oxidated states, which would require the use of controlled (e.g., neutral) atmospheres. Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.