1 A biological approach to metalworking based on chitinous colloids and composites Ng Shiwei1 Ng Guan Zhi Benjamin1 Robert E. Simpson2 Javier G. Fernandez1

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A biological approach to metalworking based on chitinous colloids and composites

Ng Shiwei1†, Ng Guan Zhi Benjamin1†, Robert E. Simpson2, Javier G. Fernandez1*

1 Engineering and Product Development, Singapore University of Technology and Design, 8

Somapah Road, 487372, Singapore

2 School of Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

† These authors contributed equally to this work

E-mail: javier.fernandez@sutd.edu.sg

Keywords: aggregates; colloids; composites; chitosan; metals; ultra-low binder content;

sustainability

Abstract

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, a new method of metalworking that

uses chitosan from the arthropod cuticle to aggregate colloidal suspensions of different metals

into solid ultra-low-binder-content composites is demonstrated. These composites, which can

contain more than 99.5% metal, simultaneously show bonding affinity for biological

components and metallic characteristics, such as electrical conductivity. This approach stands

in contrast with existing 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, the

composites’ ability to be printed and cast into functional shapes with metallic characteristics is

demonstrated. The affinity of chitometallic composites for other biological components also

allows them to infuse metallic characteristics into other biomaterials. The findings and robust

manufacturing examples go well beyond basic demonstrations and offer a generalizable new

approach to metalworking. The potential for a paradigm shift toward biomaterials based on

their unique characteristics and the principles of their manufacturing methods is highlighted.

1. Introduction

The field of bioinspired materials represents our attempt to learn from nature’s design of

biological composites, which result from millennia of evolution. Understanding how biological

systems integrate complex hierarchical designs on multiple length scales enables the replication

of the same principles and novel functionalities using artificial materials[1, 2]. However,

beyond the adaptation of specific strategies at the product level, we are still in the nascent phase

of applying the essential lessons taught by nature’s use of local components, favor of activities

with minimal metabolic energy, and seamless integration within Earth’s ecological cycles[3],

which could ultimately lead to a biological transformation of manufacturing systems[4]. 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 from common components while minimizing the metabolic

cost[5]. Manufacturing with metals is a paradigmatic example of the different approaches to

artificial and biological production.

The existing approach to manufacturing metals is based on melting and shaping processes

requiring substantial energy sources to reach high temperatures and pressures. These processes

are not unique to metalworking, rather they are a signature of our path of industrialization,

which is characterized by an ever-increasing dependence on highly energy-demanding

processes[6]. Our use of metals within a paradigm of significant energy sources contrasts with

the rare, but real, use of metals in biological structures. Found and studied mainly in the

chitinous cuticles of arthropods[7], these metals (mostly Zn, Mn, and Fe) are incorporated at

ambient conditions into critical areas, such as the tips of the claws and fangs or the shells of

eggs, to exploit their mechanical properties[8-10]. Metals, however, are not generally

incorporated into arthropod cuticles at the first stage of molting[11]. Initially, the organic phase

is secreted into a water-based environment, producing a soft hydrogel that transitions to a stiff

shell through tanning and controlled dehydration. The anisotropic shrinking and associated

strong intermolecular forces generated by the removal of water are essential aspects of the

cuticle’s consolidation and extraordinary mechanical properties[12, 13]. Indeed, they compel

the rearrangement of chitinous molecules and fibers to strengthen their intermolecular bonds[13,

14]. It is in this late stage of development when metals are incorporated from the

environment[15]. While this metal embedded in the cuticle is generally associated with proteins,

chitinous polymers also show a strong affinity for metals, which are 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[16].

In this paper, we explain how this affinity of chitinous biomolecules for metals goes far beyond

the removal of metallic elements from water. We show the aggregation of metal particles from

a colloidal suspension into a solid composite without using external pressure, thus creating an

agglomeration that is hundreds of times the weight of the original chitin. This process enables

conventional manufacturing techniques, such as casting, coating, and 3D printing, to be adapted

to a new context of biological conditions (i.e., metabolizable chemicals under ambient

temperature and pressure) in a unique example of a large-scale, ecologically integrated, and

generalizable approach to metalworking.

The proposed approach, which is heavily influenced by the processes that allow the use of

metals in the cuticle, prioritizes the adaptation of biological tools and strategies to meet the

requirements of an industrial production system that is integrated with biological systems and

cycles. It does not aim to precisely replicate the exact biological processes that enable the use

of metals in chitinous animals. Therefore, there are several significant differences between the

described results and the incorporation of metals in the cuticle: (i) Metals in the chitinous cuticle

are commonly incorporated through binding to the histidine residues of proteins, possibly via

chelation at the imidazole functional group[15, 17]. Here we use the metal affinity of chitosan

as the main bridge of the organic-inorganic links[18], favoring the use of a ubiquitous molecule

to achieve industrially relevant production scales[19]. (ii) In the arthropod cuticle, large

amounts of metals are incorporated at the molecular level at concentrations that can reach 25%

of the cuticle weight by the end of the animal’s life[20]. Unbounded by the limits of metal

bioavailability, we explore concentrations that are orders of magnitude larger than those in the

cuticle and the use of metallic particles, thus enabling the formation of organometallic

composites that have the fundamental functional properties of metals rather than those of a

modified organic material[21-23]. (iii) We use some of the principles behind chitinous cuticle

stiffening, which is achieved at ambient temperature and pressure, to consolidate the composite

from a colloidal form to a compacted porous metal through water exchange at biological

conditions, embodying an approach that can be incorporated into other bioinspired

manufacturing technologies[24]. However, in the arthropod cuticle, metals are incorporated

after or during the consolidation of the organic matrix, not before.

2. Results

2.1. 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 a 3% concentration in a weak

acetic acid solution at the anaerobic fermentation limit[25] (1% v/v, table vinegar ranges

between 4 and 8%). 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[26], simultaneously able to flow as a liquid and to interact intermolecularly at long

range like a solid.

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[27]. Stainless steel was selected because of its

known inert nature and its lack of interaction with chitosan, requiring a surface modification

to show affinity[28]. As the water evaporated, the rearrangement of the chitosan chains, the

new intermolecular direct bonds, and overall shrinking due to the lost volume resulted in

strong forces (Figure 1b)[12, 29], internally compacting the metallic particles into a granular

solid. For all three metals, the result was a compact but porous aggregate of metallic particles

(Figures 1c, d). An illustrative example of the forces exerted by chitosan during

dehydration—which, in the arthropod cuticle, provides stiffness and uniaxial strength through

strain hardening[30]—can be found in our previous work in which these forces were

employed to move large architectural constructions in response to ambient humidity

changes[31].

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 was largely independent of the metal chosen,

with particle size being the determining factor. This suggests that the formation of the solid is

likely to be a predominantly 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[32]. 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.

2.2. 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 the 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 vitrification of

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, where 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, the chitosan:metal ratio influences the characteristics of the solid form

(Figure 2b), as low amounts of chitosan result in loose aggregates, 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 chitosan to bind metals[33]. 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 and noticeably significant 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 porous chitometallic composites

have a very poor standalone mechanical strength. However, they acquired enhanced resistance

to thermal degradation (Figure S1) and abrasion from their 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 (Video S1). Interestingly,

despite the natural hydrophilicity of chitosan, the chitometallic composites show

unanticipated moisture stability, with no observable mass loss or shape change after several

days immersed in water (Figure S2). 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

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1AbiologicalapproachtometalworkingbasedonchitinouscolloidsandcompositesNgShiwei1†,NgGuanZhiBenjamin1†,RobertE.Simpson2,JavierG.Fernandez1*1EngineeringandProductDevelopment,SingaporeUniversityofTechnologyandDesign,8SomapahRoad,487372,Singapore2SchoolofEngineering,UniversityofBirmingham,Edgbaston,Birm...

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1 A biological approach to metalworking based on chitinous colloids and composites Ng Shiwei1 Ng Guan Zhi Benjamin1 Robert E. Simpson2 Javier G. Fernandez1

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