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A biological approach to metalworking based on chitinous colloids and composites

Biological systems evolve with minimum metabolic costs and use common components, and they represent guideposts toward a paradigm of manufacturing that is centered on minimum energy, local resources, and ecological integration. Here, we demonstrate a new biological method of working metal that exploits the unique ability of chitin to incorporate metals into the arthropod cuticle in order to aggregate colloidal suspensions of different metals into solid ultra-low-binder-content composites. These composites, which can contain more than 99.5% metal, simultaneously show both biological affinity and the electrical conductivity and durability of metals. This approach stands in contrast with existing energy-demanding metalworking methods, taking place at ambient temperature and pressure and being driven by water exchange. Furthermore, all the nonmetallic components involved are metabolized in large amounts in every ecosystem. Under these conditions, we demonstrate the ability of the composites to be printed and cast into functional shapes with metallic characteristics. The affinity of chitometallic composites for other biological components also allows them to infuse metallic characteristics into other biomaterials. Our findings and robust manufacturing examples go well beyond basic demonstrations and offer a generalizable new approach to metalworking. We highlight the potential for a paradigm shift toward structural biomaterials based on their unique characteristics and native manufacturing methods.

metabolic energy, and integrate with the Earth's ecological cycles. Unlike the modern manufacturing paradigm, which depends upon complex global supply chains and seemingly consequence-blind resource exploitation, natural systems have evolved to produce outstanding functional structures using common components and without significant consumption of energy 6 . Manufacturing with metals is a paradigmatic example of the different approaches of artificial and biological production.
The existing approach to manufacturing metals requires substantial amounts of energy for melting and shaping. These energy-craving processes are not unique to metalworking but rather are the signature of our path of industrial development, which has been characterized by an increasing dependency on highly energy-demanding industries that goes beyond mere economic expansion 7 . Using metals within a paradigm of uncapped energy demand contrasts with the rare, but real, structural use of metals in biological systems. Found and studied mainly in the chitinous cuticles of arthropods, metals are incorporated into critical areas, such as the tips of the claws and fangs or the eggs, in order to exploit their mechanical properties [8][9][10] . This affinity of chitinous molecules for metals is known to be retained after their extraction. In fact, one of the most extensive uses of chitinous waste, such as that from the fishing industry, is in flocculating heavy metals in water remediation systems 11 . In this paper, we explain how this affinity of chitinous biomolecules for metals goes beyond the removal of metallic elements from dispersive media to the aggregation of metal particles from a colloidal suspension into a solid composite; achieving an agglomeration that is hundreds of times the original chitin weight without using external pressure. This enables conventional manufacturing techniques, such as casting, coating and 3D printing, to be adapted within a new context of biological conditions (i.e., metabolizable chemicals and ambient temperature and pressure) in the first example of a large-scale, ecologically integrated, and generalizable biologically driven approach to metalworking.

From colloids to solids
The process of producing chitometallic composites is illustrated in Figure 1a. Briefly, chitosan, which was extracted from shrimp, was dissolved at 3% concentration in a weak acetic acid solution at the anaerobic fermentation limit 12 (1% v/v, 4-8% table vinegar). A key factor in this dissolution is that, despite its considerable molecular weight (150-200 kDa), chitosan can be dispersed by the presence of easily protonated amine groups, enabling the introduction of repulsive inter-and intramolecular forces. Notwithstanding the low concentration of chitosan, the resulting solution behaves as a liquid crystal, simultaneously able to flow as a liquid and to interact intermolecularly at long range like a solid 13 .
In this state, we added metallic particles of tin, copper, and stainless steel in varying ratios (from 50 to 250 times the weight of dry chitosan), forming colloidal suspensions. Copper and tin were selected as examples because they are key metals in electrical engineering and because copper is a transition metal while tin is a non-transition metal, each thus having a different chelation mechanism with chitosan 14 . Stainless steel was selected because of its known inert nature and its lack of interaction with chitosan, requiring a surface modification to show affinity 15 . As the water evaporated, the interplanar space between the chitosan chains was reduced proportionally (Figure 1b) 16 , dragging and compressing the metallic particles into a granular solid. An illustrative example of the forces exerted by chitosan during dehydration-which, in the arthropod cuticle, provides stiffness and uniaxial strength through strain hardening 17 -can be found in our previous work in which these forces were employed to move large architectural constructions in response to ambient humidity changes 18 .
For all three of the metals, the result was a compact but porous aggregate of metallic particles (Figures  1c and d). The minimum amount of chitosan required to consolidate the particles into a solid ranged from 0.4% to 1.0% of the metal's 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 lower by an order of magnitude than the binder concentrations in previously reported ultra-low-binder-content (UBC) composites 19 . However, unlike UBC composites, which are made using synthetic polymers and high external pressures, the chitometallic composites reported here achieved these results without external forces and under ambient conditions, highlighting the unparalleled efficiency of this biological approach to bind particles.

Casting and extending metallic properties to other biocomposites
Casting is an essential metal manufacturing process that, until now, requires high temperatures. However, the unique ability of the chitometallic colloids presented here to "self-aggregate" into solids from a flowing state allows casting of metal composites at ambient conditions. To illustrate this, we printed 3D negative molds of different geometrical shapes and filled each with a colloidal suspension. We then allowed the crystallization of the chitosan and thus allowed consolidation of the composite in the mold, resulting in a metallic replica (Figure 2a). Casting a colloidal suspension into a solid requires a balance of the 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. Likewise, in its solid form, low amounts of chitosan result in loose aggregates (Figure 2b), while an excess of chitosan results in the metallic electrical, thermal, and abrasion resistance properties of the filler being overshadowed by the properties of the chitosan.
All three tested metals produced complex shapes (Figure 2c), confirming the general ability of chitinous polymers to bind metals 20 . 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 partially overcome by neutralizing the excess acetic acid before introducing the metal. Therefore, further applications of the technology described here should consider the targeted metals' interactions, not only with the biopolymer, but also with the dispersing medium.
Compared to continuous (i.e., melted) pieces of metal, the granular and porous chitometallic composites do not excel in terms of their standalone mechanical strength, which is similar to the discrete use of metals in a cuticle. However, they acquired enhanced resistance to thermal degradation (Supplementary Figure  1) and abrasion from its metallic elements. The latter enables the pieces to be polished and thus acquire a characteristic metallic shine and, more importantly, surface continuity and metallic electrical conductivity (Supplementary Video 1). Furthermore, despite the low amount of biopolymer, the chitometallic composites retain chitosan's compatibility 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. We used this property to provide metallic characteristics to inexpensive, sustainable, large-scale constructions of cellulose-based fungal-like adhesive materials (FLAM) 21 .

Figure 2d
shows the process of incorporating the properties of tin into the surface of two solid cellulosic objects: a replica of a honeybee's (Apis mellifera) head and the Merlion mascot of Singapore (Figure 2e). Because of the natural affinity of chitosan's amine groups to biological materials, FLAM objects can be dipped or "painted" in a 1:250 chitinous colloidal suspension of tin. The colloid self-consolidates (i.e., crystallizes) in a conforming metallic layer which can then be polished or reworked (Figure 2f,  Supplementary Video 2). 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 simple water to drive the assembly.
The chitinous metallic coating of cellulosic constructs goes beyond merely aesthetic or electrical modifications. In the arthropod cuticle, metallic deposits are incorporated at sharp tips to increase their durability 10,22 . Inspired by this biological use of metal deposits, we produced a sharpened FLAM tip and dip-coated it 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) (Figure 2g). It is worth noting that the affinity of chitosan for biological construction is not limited to cellulose. Most notably, the straightforward use of the principles described here to provide electrical properties to other biomaterials that have a demonstrated affinity with chitosan-such as internal organs and skin 23 , or cotton 24 -suggests their potential inclusion in smart textiles and medical devices.

3D printing at ambient temperature and heat treatment
So far, we have demonstrated that chitometallic composites produced under ambient conditions allow electrical conductivity along the surfaces of solid chitometallic or cellulosic objects, and these can easily be used to create conductive tracks and patterns (Supplementary Video 3). However, these metallic properties were only apparent after polishing, suggesting that the porous interior lacks continuity. We now show how chitometallic structures can become internally electrically conducting through their bulk.
We achieved internal continuity, which would be needed for applications that require bulk electrical properties, such as batteries and electrodes, by exposing the chitometallic composites to increased temperatures for a short time. After heat treatment at temperatures typically lower than 300°C, we observed a drastic reduction in electrical resistance in both tin and copper composites, while stainless steel composites were unaffected (Supplementary Figure 2). This change in conductivity, which was ten orders of magnitude for tin and six orders of magnitude for copper, was initially hypothesized to result from the carbonization of chitosan, reduction of a metal oxide, or sintering. However, a comparison between thermal analyses of all the involved components and the temperature at which the composites changes conductivity suggested otherwise (Figures 3a and b). The existence of a critical threshold for inducing a permanent change in the bulk material at a temperature that has no apparent relevance to any of the involved components seems to suggest further internal compaction and the achievement of continuity by percolation. 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, suggests that the affinity of the metal for the chitinous binder might be a fundamental aspect of this process. The percolation hypothesis agrees with the SEM-EDS analysis, which revealed low traces of carbon and distinct metallic particles with closer packing than before heat treatment (Figure 3c). While the cumulative evidence seems 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) suggests that there could be a combination of effects. Further experiments, which are beyond the scope of the current study, will be needed to confirm these observations and to fully understand the phenomenological aspects of the process through which chitometallic composites acquire bulk electrical properties.
The ability to safely manipulate and dry the material in ambient conditions and to then apply a 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. Bulk 3D printing with the tin-chitosan colloid was found to be particularly straightforward because of its shear-thinning characteristics, which facilitate extrusion and help it to retain its shape thereafter. We pneumatically extruded the colloid using a standard 150 ml plastic syringe, actuated by computercontrolled stepper motors assembled on a cartesian system to control the position (Supplementary Figure  3). 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, shown in Figure 3d. The process consisted of two well-defined stages: bioprinting and heat treatment, with the latter performed by raising the temperature of the printing surface to 320°C for 10 minutes. The success of the process was demonstrated visually by lighting a regular bulb. A more detailed, uncut version of the process is shown in Supplementary Video 4. Although we conducted the whole process in an open atmosphere, the heating of some metals might lead to the formation and reduction of oxidated states, which would require, in those cases, the use of controlled (e.g., neutral) atmospheres.

Discussion and outlook
In this paper, we have described the concepts and technological applications of an approach to metalworking that is based on biological principles and components, rather than high temperatures. The bridge between the biological and metal domains-chitosan-is a common biopolymer that can be derived from multiple sources-including urban waste 25 -within a circular, sustainable economy and can be safely handled without toxic chemicals. We showed that chitosan's extraordinary ability to integrate metals, which was originally evolved to enhance the performance of the arthropod cuticle, enables the production of functional organic-inorganic composites under ambient conditions from colloidal suspensions of metal powders.
Traditionally, working with metals has been characterized by high-temperature processes that have been defined by the melting temperatures of the metals in use. In contrast, the approach presented here is characterized by the frugality and sustainability of biological systems 6 , and it has been demonstrated on a large scale, using industrial-quality metallic particles (i.e., low cost and with inconsistent shapes and sizes), at ambient temperature and pressure, and-for the optional thermal treatments-under atmospheric conditions. The decoupling of thermal treatment from manufacturing enables a new and more accessible approach to working with metals, reserving the use of high heat for post-processing, if needed. We have demonstrated how this enables models of metal manufacturing that are safe for users and the environment without specialized laser sintering equipment or industrial facilities. This approach also has the potential to integrate easily with other biological components and systems operating under ambient conditions.
The differences between the proposed approach and traditional metalworking are significant at a fundamental level, defining a technology that does not replace the old, but rather enables a new complementary production paradigm based on the biological transformation of industrial manufacturing 26 . The proposed technology is separate from typical applications of traditional metalworking, like the fabrication of standalone load-bearing structures, but overlaps with other uses, such as producing durable electrical components. It also enables new fields, such as integration with biological systems and sustainable modes of production. As society transitions to a more sustainable development model, being able to selectively use materials and identify appropriate processes is critical for curbing energy demands. While modern society is built on high performance technical materials, alloys, and composites, an enduring society would need to be built on materials and technology that also seek to preserve. As this study demonstrates, nature has a library of strategies and materials that, if wisely employed, could bring us to a new technological age 27 . Fig.1| Chitinous metallic colloids and ultra-low-binder-content composites. a, Diagram of the formation of chitometallic colloidal suspensions and their transformation into ultra-low-binder-content composites by internal compaction. A liquid crystal solution of chitosan is used as the continuous phase, to which metallic particles are added as the dispersed phase. The resulting colloidal suspension, which can flow into and retain shapes, transitions to a solid as the water evaporates under ambient conditions. During this process, the crystallization of the relatively low amount of chitin (<0.4% w/w) brings all the metallic particles together into a porous aggregate that exhibits metallic characteristics. b, X-ray diffraction spectra of chitosan. The top row shows the spectrum of the raw chitosan extracted from shrimp shells, with the typical L-2 chitosan polymorph. The next three rows present the spectra of the liquid crystal chitosan and the evaporated film. Note the change in crystallite size with the water content in the bottom two rows. c, The two left columns are images of the three types of metallic particles employed here, showing the diversity of sizes and aggregation states. The two right columns are images of the colloidal suspensions and their colored scanning electron images after freeze-drying. The right-most column shows the initial intermingled state between particles and biopolymer and the existing connections between the chitinous molecules in suspension. d, Images of the solid chitometallic composites. The left column shows photographs of 2×2cm square composite samples of different metals. The central column shows scanning electron micrographs in which the chitosan has been colored. The right column shows reconstructed volumes of the composites recorded by x-ray microtomography, illustrating the high porosity of these chitometallic aggregates.

Fig.2|
Manufacturing of chitinous and chitinocellulosic metallic objects at standard temperature and pressure. a, Diagram of a casting process using a chitometallic colloid. In the example, the model was printed in nylon using a commercial selective laser sintering (SLS) 3D printer for rapid prototypes of high quality finish. A flexible silicone mold in the shape of the object's negative was then produced, which was used to cast the colloidal suspension. b, Flexural strength of the metallic aggregates with respect to the chitosan-to-metal ratio. All samples were cast under the same conditions. In the case of copper, the poor mechanical strength resulted from premature precipitation of the metal due to the slightly acidic chitinous continuous phase. c, Results of casting models of Marie Curie in tin, stainless steel, and copper. d, Process for using the chitometallic colloids to incorporate metallic properties into other biocomposites, specifically the coating of cellulosic objects. e, Example of two large objects (a Merlion and a bee head) produced in fungal-like adhesive material (FLAM) and coated with tin under ambient conditions. Insets show the cellulosic objects before coating. The fabrication process is depicted in detail in Supplementary Video 2. f, Scanning electron images of the surface of a chitometallic object before and after polishing. g, Mechanical performance of a dip-coated FLAM tip. In the chitinous arthropod cuticle, metallic composites are used to enhance durability rather than to form structures. The coated tip in the image doubles its elastic region (linear slope) when crushed against a solid surface, requiring more than five times the energy (area under the curve in the elastic region) to undergo permanent deformation.

Fig.3| Thermal and electrical properties. a,
Heat flow of the chitosan film, tin, and chitosan-tin composite, as measured by differential scanning calorimetry. Due to only small amounts of chitin in the composite, there is no observable difference in the intrinsic thermal characteristics of the composite and the constituent metal. b, Effect of thermally treating tin-and copper-based chitometallic objects. The composites transition to being electrical conductors at moderate temperatures, showing 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 appears 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. For further information and a discussion of the process in tin and other metals, refer to Supplementary Figure  2. c, Physical appearance and composition of a chitometallic composite based on tin, before and after thermal treatment, as measured by energy dispersive spectroscopy. d, Bioprinting of a bulb socket at room temperature using a chitometallic colloid, followed by its thermal treatment in an open atmosphere to achieve electrical functionality. For further details on this fabrication, refer to Supplementary Figure 3 and Supplementary Video 4. (STP refers to standard temperature and pressure)

Supplementary Information
The supplementary information comprises materials and methods, three additional figures and four videos.

Supplementary Information
A biological approach to metalworking based on chitinous colloids and composites Ng Shiwei, Ng Guan Zhi Benjamin, Robert E. Simpson, Javier G. Fernandez* * To whom correspondence should be addressed. E-mail: javier.fernandez@sutd.edu.sg This material includes:

Preparation of metallic chitosan composite
To produce a material that can be economically scaled, accessible and relevant outside of laboratory environment, the materials sourced were in their unideal but most common form; And experiments were performed with a focus on its practical application. As the second most ubiquitous organic material in the world and a part of every ecosystem, potential sources of chitin are numerous; especially after the demonstration of its extraction from Black Soldier Fly which facilitates bioconversion of organic waste in urban environments. In our experiments, we used deacetylated chitin from the abundant processing waste of shrimp in South East Asia. 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 initially viscid solution was submerged in a 50℃-water bath for 72 hours to fully disperse chitosan.

X-Ray Diffraction (XRD)
XRD measurement was conducted using Bruker Eco D8 Advance diffractometer at CuK radiation (wavelength = 1.542Å) at a voltage of 40 kV and 25mA. Scans between a 2 range of 5 and 55 were performed with at step size of 0.02 and 5.min -1 . Presented data were results of post processing on DIFFRAC.EVA software that includes stripping K2 contributions, smoothening and removing background.

Scanning Electron Microscopy (SEM) & 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 100uA.

Coating samples and loading measurement
Dried 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 a off-shelf pencil sharpener to obtain a sharp tip. For coated samples, the sharpen 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 sharpen 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 mins and subsequently ramped down at the same rate.

Electrical Resistance as a function of temperature measurement
The electrical resistance of 1 cm by 1 cm by 3 mm chitometallic 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, , 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, Δ , 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 = (2) Δ . The hotplate was programmed to increase in temperature at a rate of 5 ⁰C/min to a maximum temperature of 350 ⁰C. 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 is measured to decrease at this transition temperature.
Additive manufacturing of 3D objects 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 gcode 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 midprint 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.

Figures
Sup. Fig. 1| Cold casting and flame test. a, Examples of chitometallic slabs produced at room conditions by casting colloids in silicone molds. b, The sequence presents the effect of a butane flame (∼1500 °C) on a chitosan-copper slab produced at ambient conditions. Even at those temperatures, well above those for the carbonization/vaporization of most organic components, the chitometallic aggregates remain solid and preserve their geometry.
Sup. Fig. 2| Thermal and electrical tests. a, Differential scanning calorimetry (DSC). Of all the metals used, only tin has an endotherm peak at its melting temperature. Due to the extremely low amounts of chitosan in the composites, there are no observable differences between the heat flow of the metal alone and the chitometallic composites. b, Thermogravimetric analysis (TGA) of chitosan films in air shows three stages of weight loss. The first stage below 100°C relates to the initial dehydration of film, followed by drastic weight loss in the range of 280°C to 300°C (degradation of acetylated/ deacetylated units, depolymerization and dehydration of saccharide rings) and finally the complete thermal oxidative degradation before 600°C c, Electrical Resistance and thermal treatments. The data represents the electrical properties of the different samples while heated to 350°C on a hot plate (open atmosphere) and their cooling 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 gives rise to a more resistant material, suggesting that carbonization of the organic binder does not play a relevant role in the process. Tin and copper have a permanent drop in resistance of about 10 and 6 orders of magnitude, respectively. In contrast, Stainless Steel shows an order of magnitude change. It is worth noting that the size of stainless steel particles is several times larger than those of the other metals (see Fig. 1c). The transition occurs sharply at temperatures unrelated to the metal used (see panel a). For the same metal, changing the ratio of chitosan to metal (See graph for copper) or the size of the metallic particles (See graph for tin) results in shifts in the electrical transition temperature. All the data suggests that the process in which the chitometallic composites acquire 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 could be described, for the lack of a better definition, as a "self-percolation" or "self-compaction" process.
Sup. Fig. 3| Printing set-up. a, The printing set-up consists of a pneumatically actuated syringe dispensing the chitometallic colloid and assembled on a cartesian positioner, printing on a hot plate. The hot plate is included to perform the printing and thermal treatment in the same set-up. However, the hot plate could be removed since printing and treatment are two disjunct processes (the latter optional). In a simplified set-up, the printing could be done on a flat surface, and the resulting sample thermally treated somewhere else. The lack of temperature during printing also enables low requirement parts, such as a regular polypropylene syringe for dispensing. The cartesian positioner is actuated by three stepper motors, one at the base controlling a horizontal axis, one at the syringe carrier controlling the second horizontal axis, and one controlling the vertical position of the syringe. b, Example of the process of printing a piece at room temperature. c, The printed piece of the previous panel after polishing. No thermal treatment has been performed in this case. For an example of thermal treatment, refer to Supplementary Video 4.

Videos
Video 1 -Electrical continuity test. Testing, using a multimeter, the electrical conductivity of several chitometallic objects, produced by different methods and with and without different surface and bulk treatments.
Video 2 -Production of electrically conductive biological objects at ambient conditions. Metallic functionalization of a replica of a bee head and a Merlion made out of cellulose (Fungal-like adhesive materials) using a tin-based chitometallic colloidal suspension. The process, happening at ambient temperature and pressure, covers the path from the digital model to the functional biometallic object.
Video 3 -Easy production of conductive tracks in chitometallic composites. Demonstration of localized surface continuity on a chitosan-tin composite produced with a blunt tool.
Video 4 -Bioprinting of a functional E27 bulb socket. The printing process of a bulb socket at ambient conditions and its thermal treatment to achieve bulk electrical properties. The video covers, without cuts, the whole process from printing the first layer to using the socket to screw and light a commercial bulb.