3D printing of lignin: Challenges, opportunities and roads onward

As the second most abundant biopolymer on earth, and as a resource recently becoming more available in separated and purified form on an industrial scale due to the development of new isolation technologies, lignin has a key role to play in transitioning our material industry towards sustainability. Additive manufacturing (AM), the most efficient‐material processing technology to date, has likewise made great strides to promote sustainable industrial solutions to our needs in engineered products. Bringing lignin research to AM has prompted the emergence of the nascent “lignin 3D printing” field. This review presents the recent state of art of this promising field and highlights its challenges and opportunities. Following a review of the industrial availability, molecular attributes, and associated properties of technical lignins, we review R&D efforts at implementing lignin systems in extrusion‐based and stereolithography (SLA) printing technologies. Doing so underlines the adage of lignin research that “all lignins are not created equal,” and stresses the opportunity nested in this chemical diversity created mostly by differences in isolation conditions to molecularly select and tune the attributes of technical lignin systems towards desirable properties, be it by modification or polymer blending. Considering the AM design process in its entirety, we finally propose onward routes to bring the full potential to this emerging field. We hope that this review can help promote the unique value and overdue industrial role of lignin in sustainable engineered materials and products.


| INTRODUCTION
While additive manufacturing (AM) emerged in the late 1970s to 1980s with the pioneering work of Charles Hull for stereolithography (SLA), Scott Crump for extrusion-based 3D printing and Ross Householder for powder bed fusion, the last two decades have seen exponential research and development in the technology and its implementation in various application fields such as rapid prototyping, aerospace, tooling, spare parts, customized medical tooling and some mass production. [1] Ceramics, metals, inorganic glasses, polymers and composites are all suitable AM feedstocks and several technologies are amenable to multimaterial printing. [2] With the pressing need to transition our society's consumption behaviors towards sustainability, some of the most recent efforts have turned towards combining this material-efficient manufacturing process with environmentally friendly materials such as biobased and biodegradable polymers. [3] Among the bio-based polymers considered, lignocellulosic polymers have attracted particular attention for AM over the past 5 years. [4][5][6] This stems in part from their large natural abundance-cellulose and lignin are the two most abundant biopolymers on earth-but also from their recent availability in purified and/ or nanoparticulate colloidal form from traditional pulp and paper processing and from biorefineries. [6] While most efforts in this arena have concentrated on AM of complete lignocellulose as fillers [5] or cellulosics, in particular nanocellulose and cellulose derivatives, [6] attempts at implementing lignin in AM remain scarce. Yet, AM is a promising avenue to add value to lignin in its polymeric form and could significantly contribute to "making lignin great again". [7] In this review the focus will be on using lignin in its oligomeric or polymeric form as a feedstock for AM. The aim is to present research, which uses lignin as an additive (more than 30% of total volume), rather than a filler (up to 5% of total volume). [8] After briefly presenting the commercial availability and structural features of technical lignins, this review attempts to highlight the recent advances in AM of lignin-based feedstocks.

| Overview of lignin resources
With a worldwide generation of 100 million dry tons/ year as by-product of pulp and paper-making and a price ranging from 200 to 500 USD/dry ton (Table 1), [10] well below that of polyethylene (1000 USD/dry ton), lignin represents a vast and inexpensive feedstock that could substitute synthetic polymers currently processed with AM, such as ABS, PET, PC, PEEK, PP, and Nylon. Additionally, the inherent properties of lignin, viz. antioxidant and antibacterial properties, but also its propensity to return to humus upon biodegradation, can be expected to be retained in lignin-based parts. Two principal categories of lignin are derived from wood pulping, viz. lignosulfonates (~88%) and kraft lignins (~9%). [10] A new category, organosolv or biorefinery lignin (~2%) is further gaining popularity and is expected to experience the highest growth over the coming years. [10]  Additionally, as a natural polymer not "designed" for mankind manufacturing, technical lignins lack the essential processability of engineered polymers. Although viscous flow can be achieved upon heating especially for the less branched hardwood lignins, their amorphous morphology precludes the possibility of melting and crystallization. Therefore, polymer blending and/or derivatization is needed to endow melt-processability or solubility to technical lignins. [7] In this endeavor, the specific structural features of the technical lignins influence their amenability to a particular AM technology (Table 1 and Figure 1). In addition, this inherent processing challenge necessitates that one designs a lignin system, in which lignin is either blended with another polymer or reactive monomer, or derivatized, rather than utilizing pure lignin. Alternatively, and while not yet commercially transferred, progresses in producing lignin colloids, in the form of aqueous dispersions of lignin micro-and nano-particles, open new prospects for AM of lignin systems. [12] 2 | OVERVIEW OF AM TECHNOLOGIES FOR POLYMERIC SYSTEMS 3D-printing generates three-dimensional parts by stacking up layers on top of each other. In comparison to conventional manufacturing techniques, little waste material is generated and the production of molds is not necessary, as the geometric data is retrieved from a computer aided design (CAD) file. [28] Furthermore, the scale and raw material used can be adjusted to a certain extent by modulating the computer file. [29] The freedom of modeling data enlarges the design space, enabling the manufacture of a wide range of structures and complex geometries. [1] On the other hand, postprocessing is often required to improve part quality and mechanical properties are often below those obtained by traditional polymer processing technologies such as injection molding. [30] For polymeric feedstocks, different types of 3D-printing technologies are available including material extrusion, vat-photopolymerization, powder bed fusion and material jetting [9] (Figure 2).
The extrusion-based techniques viz. fused deposition modeling (FDM) and direct ink writing (DIW) are layer additive manufacturing processes, in which the printer head rises. In FDM a continuous, thermoplastic filament, is heated in the nozzle to a semi-liquid state and extruded to the build platform on the previously printed layer. The thermoplastics are typically heated slightly above their melting temperature, so that solidification is reached directly after extrusion. [28] FDM enables large scale fabrication; windmill blades have for example been printed in the Oak Ridge National Lab Big Area Additive Manufacturing (BAAM). In contrast, it is limited in details with the smallest layer thickness in the order of~0.1 mm and invariantly delivers anisotropic parts. At the smaller scale, DIW or robocasting is a microscale extrusion AM, which can process thermoplastic materials as well as solution-based hydrogels and pastes. For both FDM and DIW, the rheology of feedstock plays a crucial role and its engineering to concomitantly display strong shear-thinning for nozzle extrusion and form dimensionally stable self-supporting features upon printing is a major challenge. [31,32] As an extrusion-based method, DIW also generates anisotropic parts and alike FDM is amenable to multimaterial printing. As processing can occur at room temperature, DIW is ideal for tissue-engineering, whereby living cells can be incorporated in the feedstock a priori. Also, a layer-wise method, vat-photopolymerization, including SLA and DLP, relies on the light-induced polymerization of a photoactive and low viscosity resin, typically comprising acrylic-or epoxy-based monomers. A rapid free radical or cationic photopolymerization is initiated by application of light (UV light, electron beam, light emitting diodes in combination with projectors, or two photon) onto a layer of resin or monomer solution.
Once the 3D model is generated, the unreacted resin is removed. [1] Vat-photopolymerization delivers much finer features, better surface quality and enables a broader range of length scales (from the micrometer to the meter scale) than extrusion-based AM, while producing isotropic parts. However, it is restricted to date to a small range of photosensitive liquids. [33] In powder bed fusion of polymers, or selective laser sintering (SLS), a focused energy source such as a laser is scanned over a bed of powder feedstock, causing local sintering or melting of the polymer powder. [34] In SLS, the build area is preheated to just below the polymer melting point, enabling a much higher throughput than extrusion and vat-photopolymerization at the same resolution. SLS is ideally suited for semicrystalline thermoplastics although amorphous polymers can also be utilized. It can produce beautiful complex shapes and is ideally suited for fine lattice structures. Material jetting follows the principle of inkjet printing, in which droplets (as small as 10 μm in diameter) of feedstocks are deposited from an inkjet printhead with spatial control and subsequently lightcured. Because feedstock placement and composition can be controlled one droplet at a time, multimaterial complex shapes can be F I G U R E 1 Overview of the chemical structure of isolated technical lignins (A: kraft [25] , B: soda [26] , C: lignosulfonate [25] , D: organosolv lignin [27] ) that are currently commercially available for AM printed at high speed, with fine surface finish and spanning small length scales from~0.1 mm to meter length scales, while producing zero waste.

| FDM WITH TECHNICAL LIGNINS
Not every polymeric material can undergo FDM and deliver a 3D printed part. For example, low T g rubbery materials do not form good printable pellets or extruded strands that can undergo FDM. Some semicrystalline polymers such as polylactic acid (PLA) and polyether ether ketone (PEEK) are commonly printed using FDM, yet it is difficult to print polypropylene without using an additive. One reason for this is that in FDM the solid strand material must meet sufficient rigidity (high enough Young's modulus) to avoid buckling at the entrance of the temperature-controlled melting chamber (see Figure 2). Pellets of polymer feedstock can be used for FDM to overcome this buckling obstacle; however, it requires a special screw-type feeding device for melting, extrusion, and deposition on a build plate. In addition to the filament's high Young's modulus, the melt must have low enough viscosity to facilitate extrusion, which depends on the solid filament to seal the melt from escaping through the top and act as a piston exerting force on the melt. [35] Furthermore, the deposited layer must have high enough zero shear viscosity to avoid any flow or dimensional change. To meet these requirements for FDM printability, lignin systems must form a rigid fiber as well as a stable melt. [36] To further complicate this task, not all lignins are created equal as highlighted in section 1 and however, a similar strategy cannot be adopted for lignin because of its susceptibility to undergo thermal degradation. Nonetheless, various lignin fractions with <1000 Pa.s viscosities are available and those can be consistently extruded to a fiber or large diameter filament form. [36,37] Yet, filament extrudability does not necessarily assure 3D printability of the lignin. Oligomeric filaments or deposited layers of lignin are very brittle and difficult to handle. Therefore, in most cases, lignin is either blended with a printable plastic matrix or copolymerized with a soft segment that induces toughness to the composition, but the interaction between the different lignins and the matrix must be taken into consideration. For example, thermally stable lignin blended with ABS renders the matrix more brittle, primarily due to inadequate compatibility between the phases. However, use of nitrile rubber [38] or polyoxyethylene [39] in lignin loaded blends of ABS makes the composition adhesion. [38] The rheological data of a few of the mentioned lignin samples and polymer-modified lignin compositions are displayed in Figure 3.
An equal mass alloy of lignin with nitrile rubber shows excellent toughness and yield stress and a high tensile strength. [40] This soft composition is, however, very difficult to 3D print. The factors that hinder printability include high viscosity of the composition caused by the high molecular weight rubber component and its crosslinking with lignin. To mitigate this issue, the composition was further modified by blending with a rigid plastic such as high impact polystyrene, and the composition showed excellent 3D printability. Traditional use of polymer extenders or impact modifiers to enhance processability can also be applied to induce printability of various bio-based plastics including lignin. [41,42] Ongoing research suggests modification of these ligninbased compositions with natural fibers produces printed composites that can find application as wood substituents. [43] A few lignin modified thermoplastic matrices, on the other hand, exhibit excellent 3D printability without needing additives. For example, Overview of 3D printing techniques available for polymeric feedstocks. DLP, digital light processing; SLA, stereolithography organosolv hardwood lignin shows excellent compatibility with PLA matrix, and its 15 wt% lignin loaded composition shows very good printability by FDM. Contrastingly, an equivalent composition with a softwood kraft lignin shows worse printability, primarily due to the degradation of PLA caused by residual alkali in the lignin. [44] Because of the inherent antioxidant nature of the lignin, its slight loading in PLA along with a wound healing compound delivered a printed mesh with significant use as wound dressing material. [45] Other work suggests significantly loaded lignin (40 wt%) in PLA exhibiting very good printability and high free radical scavenging capacity. [46] 4 | DIW OF LIGNIN SYSTEMS With DIW, a multitude of inks can be printed, as long as these are in the form of pellets or paste, strongly shear thinning, and able to form self-supporting parts at rest. [31] Ink prescreening protocols have been recently proposed, enabling a fast optimization of ink formulations. [47] DIW is well-suited for lignin systems as it allows processing at room temperature, well below the degradation temperature of lignin around 200 C. [48] Yet, implementation of DIW on technical lignins remains scarce ( and demonstrated to be biocompatible with HepG2 cells. [49] In this work, lignin nanoparticles were found to increase shape fidelity and bestow antioxidant properties to the printed parts, even in concentrations as low as 0.5 wt%. [49] While such use of colloidal lignin is promising, its marginal fraction in the ink and the niche application considered restrict its potential to low volume handling of lignin. Soda lignin has also been combined for DIW with an acrylatecontaining soft triblock copolymer, Pluronic F127, which acts as crosslinking agent. [50] After printing, parts were freeze-dried and subsequently oven-cured at 120 C to yield crosslinked, water-insoluble objects. The parts, comprising up to 83 wt% lignin content, exhibited high Young's modulus (5 GPa) and UV-blocking properties, [50] paving the way to promising applications and high-volume usage of soda lig- nin. Yet, its combination with a petroleum-derived photopolymer limits the environmental-friendliness of this lignin-based ink.
Blending the lyotropic gel-forming hydroxypropyl cellulose (HPC) with up to 25 wt % beech organosolv lignin (OSL) in an acetic acid aqueous solution delivered liquid crystalline shear thinning inks, which could be successfully direct ink written. [51] Thermal postcuring of ink formulations with polycarboxylic acid crosslinkers such as citric acid and a dimerized fatty acid, endowed parts with water insolubility and anisotropic swelling. While both HPC and lignin alone were found not to be amenable to DIW, blending these two bio-based polymers bestowed the ink with both the required shear thinning for extrusion by virtue of the HPC liquid crystalline formation, and the needed shape retention at rest by virtue of lignin stabilizing effect. Lignin stabilizing effect in this blend was further found to stem from its phenolic-OH groups and low molecular weight fraction, which enhanced lignin miscibility with HPC. [52,53] The anisotropic swelling imparted by molecular orientation and the resulting band texture formation endowed parts with morphing properties upon exposure to humidity, paving the way to the manufacture of sensing and soft robotic devices from fully bio-based lignin systems. However, dragging of the outer layers to the center during printing limited dimensional stability and restricted the design space to continuously-printed structures such as rings. [51] In the aim to improve solidification, shape freedom and shape retention, ethanol was further used as cosolvent of the aqueous acetic acid solution for this lignin/ HPC system. [54] Prescreening of ink formulations according to Paxton

| SLA WITH LIGNIN-CONTAINING RESINS
At its core, SLA is a photopolymerization process that requires a photoinitiator and a reactive monomer suitable for chain-growth  [49] Alkali lignin 38-56 wt% Pluronic F127 Chemical crosslinking using Pluronic F127 [50] Organosolv lignin 25 wt% Hydroxy-propyl cellulose Esterification with dimerized fatty acid [51] Organosolv lignin 50 wt% Hydroxy-propyl cellulose Chemical crosslinking with citric acid and borate ions [54] polymerization reactions. However, it is a complex process that entails simultaneous control of resin photoproperties, rheology, and chemistry to provide crosslinked thermoset objects. This control is imparted by multicomponent resins, which are precisely engineered mixtures that provide photo-, rheological, and mechanical properties required to ensure a successful print. For commercially available SLA printers, there are two general configurations as shown in Figure 2. [55] In a free-surface configuration, the light source is above the resin bath and the layers are printed on top of each other as the build platform moves down into the bath. The size of the object is limited by the vertical height of the resin bath, and more resin must be used to ensure the object is always coated. [56] Many commercial printers employ the Sajab and coworkers also studied the effects of adding small amounts of lignin to SLA resins. [58] After isolating organosolv lignin from oil palm empty fruit bunches using a formic acid-catalyzed solvolytic extraction, they incorporated it without modification into a commercially available resin whose primary component was a ure- In addition, the group constructed working curves for each of the resins to compare critical energy dosage to cure each resin ( Figure 4C). Since the myriad aromatic groups in lignin impart a high photon cross section, the light dose (405 nm) required to cure lignincontaining samples was lower than the unadulterated resin using diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) as the F I G U R E 4 Images of lignin-containing samples printed by SLA. Unmodified lignin dispersed in resin components affords a heterogenous mixture (A), whereas acetylation with methacrylic anhydride affords a homogenous solution (B). A working curve "windowpane" consisting of 32 squares of varying thicknesses of 5 to 20 layers (C) can be used to generate resin working curves that interrogate photoproperties of lignin-containing resins. ASTM D638 Type V tensile testing specimens (D) can be 3D printed to test performance properties of lignin-containing resins. Adapted with permission from. [59] Copyright 2018 American Chemical Society photoinitiator. In fact, lignin has been recently reported to be useful as a photoinitiator itself. [60,61] This is an exciting development regarding SLA with lignin, whereby potentially the oligomers and the photopackage could consist entirely of properly modified lignin.
Another important consideration regarding lignin modification is related to the ability of lignin to react with radical species to form stable quinone methide structures. Kim and coworkers demonstrated that methylation of the phenolic hydroxyl group in lignin prevents the formation of quinone methides. [62] The blocking action imparted by chemical modification of these hydroxyl groups by acetylation with methacrylate moieties perhaps prevents competitive radical quenching reactions during photopolymerization, leading to cured objects with superior mechanical properties. Additional research regarding the differences in curing kinetics imparted by lignin modification could elucidate these structure-function relationships.
In addition to biorefinery lignin, researchers have demonstrated the utility of chemical modifications of lignin-based phenolics to create new photoactive resins for 3D printing. For example, Reineke and coworkers created a slate of mono-and di-functional photoactive monomers from guaiacol and vanillyl alcohol, respectively. [63] The group was able to show that these monomers, when suitably modified with methacrylate moieties, could be used to generate photoactive SLA resins, and that changing the concentration of these multifunctional monomers allowed for precise control of resin photoproperties and rheology, and subsequent performance properties of printed objects. Likewise, Stanzione and coworkers showed that photocurable resins based on vanillin could be used for SLA. [64] These

| CHALLENGES AND OPPORTUNITIES OF AM FOR LIGNIN
Coined as the fourth industrial revolution, AM is now also fueling innovations in the biomass utilization field, starting with the development of lignin-based feedstocks for these processing technologies. such as the optimization of support structures for parts and overhangs, the optimum use of build volume for manufacturing productivity, the removal of support structures and the part finishing. In this endeavor, using relevant artifacts such as the NIST artifact, [66] ligninbased feedstocks need to be benchmarked against established feedstocks to assess feature resolution in pores, steps, columns etc. and shape-forming flexibility for an AM process. Thus, further development of lignin-based feedstock remains necessary to alleviate some of the listed challenges and expand precision, shape forming flexibility and parts performance. In this endeavor, it appears worth to the authors to continue exploring lignin in its polymeric form while capitalizing on the unexpected compatibility between a compressionmodulus building aromatic lignin and Young s modulus building carbohydrates. [7] Certainly, current industrial advances in wood-based nanotechnology, with the industrial availability of cellulosic nanoparticles and the laboratory scale preparation of lignin but also tannin colloids, also invite opportunities for the design of fully biobased feedstock from such colloidal assemblies. Here, collaborations along the product life cycle, from its conception with chemists, physicists, engineers and designers, to its manufacturing and finally its commercialization with entrepreneurs, industrial partners are economists will be critical. This review has thus the ambition to spark interest in this field from a wide range of actors and stakeholders along the chain supply and product life cycle of the lignin AM products with a view to finally "making lignin great again". [7] 7 | CONCLUSIONS R&D to implement lignin-based systems in AM is in its infancy but has already shown outstanding promise to add-value to a vastly available and underutilized bio-based polymer while fostering sustainability.
Extrusion-based and VAT-photopolymerization technologies have already been successfully demonstrated with lignin-based systems, enabling the production of 3D parts with moderate to high lignin content (from a few % to 50%), with partial to total biobased carbon content, and with at times drastically improved mechanical performance. Yet, much R&D remains to be done to unleash the full potential and synergies of lignin and AM, as a sustainable bioresource and as a material-efficient processing technology and bring new bio-based innovations into the real world with applications in construction materials, lightweight structures, sensing and biomedical materials among others.
With the recent scientific and industrial advances in biorefining, the forest product industry is ripe to empower the chemical and material industry to produce lignin-based products on the industrial scale.
Leveraging 5 decades of fundamental research and development on lignin-based polymers and lessons from the past [7] with this disruptive technology, opens vast opportunities. Wood scientists, chemists, engineers, designers and entrepreneurs need to come together and seize these opportunities towards a more efficient use of mother earth resources and a more sustainable society. This entails a thorough knowledge and molecular manipulation of the highly tunable resource that lignin truly is, as a well as close collaborations along the chain supply and life cycle of the 3D printed lignin parts, from suppliers of the raw resource to parts end users. With this review, we hope we can contribute some excitement for this nascent field, and motivate the various stakeholders to further advance the science, technology and industry of AM for lignin-based systems in the near future so that, together, we can finally find commercial value of lignin and bring it to reality in industry and in our consumption habits.