Conductive inks are generally composed of a functional material (or its precursor), which gives the ink its electrical conductivity properties. If the ink is composed of the precursor, the metal particles are prepared with bottom-up methods, decomposing the precursor’s molecules thermally or through the reduction of metal salts reacting with a reduction agent. Depending on the functional material used, conductive inks could be classified into three separate groups: (1) conductive inks formed of metallic particles, (2) carbon-based conductive inks, or (3) particle-free conductive inks.

Most conductive inks use metallic materials, such as silver or copper, to achieve good electric conductivity. However, some of the used materials are considered critical raw materials or are harmful to some species (such as silver for submarine life), entailing a risk to the environment [18].

Carbon-based materials are the second family of materials used to give inks electrical conductivity. These materials have demonstrated good conductivity and can come from an inexhaustible source. Their biodegradability is secured in certain conditions; for example, carbon nanotubes (CNTs) degradation is produced by macrophages [19]. These allotropic forms of carbon (graphite, graphene, CNTs, or carbon black) can be used separately or by combining their properties to ensure good conductivity. Carbon is an element present in nature that can be found in fossil form, in the air, or in the ocean, and can be processed and afterward recycled and returned to nature.

The third family of inks contains those that do not have particles, such as Poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) inks (PEDOT:PSS inks). This material is not biodegradable at all, but, used in a small proportion, the final ink could be considered biodegradable.

Depending on the selected material, the conductivity of the final ink will vary.

A strategy to synthesize biogenic silver particles has been developed in recent years. This sintering method is low cost, causes less toxic waste, and consumes less energy while a higher yield is obtained. The process is based on the ability of certain organisms, such as bacteria, yeasts, or fungi, to alter the chemical nature of metals and reduce them into nanoparticles [24]. Depending on the specific organism used and the environmental conditions, the nanoparticles obtained may have different physicochemical properties [24].

Nevertheless, the attainment of silver nanoparticles depends on numerous factors, such as the genetic properties of the organisms or the environmental conditions [11].

Carbon can be provided from biomass produced from agricultural waste. This source is abundant, sustainable, renewable, and rich in this material (up to 55% of biomass is carbon). It is necessary to apply thermal treatments to biomass to obtain graphitic structures [26].

Other carbon sources are vegetable oils, chicken oil, or camphor (C₁₀H₁₆O) after being correctly processed [27,28,29,30]. Carbon nanostructures could also be formed through the thermal treatment of cellulose using a nickel salt during the process [31].

It has been demonstrated that graphene could be obtained from daily materials such as food, waste, plastics, or plants, treating them at hot temperatures in an H₂/Ag atmosphere to obtain high-quality graphene layers [30].

CNTs are generally obtained through the Chemical Vapor Deposition (CVD) process. The sustainability of this process can be improved by using more sustainable catalysts or renewable carbon sources. Metals or metallic oxides that can be found in nature, such as lava or sand, could be used as catalysts [32,33]. These sources are not commonly considered renewable but are abundant in nature and low cost. However, the use of these catalysts does not lead to uniformity in the morphology or the nanostructure of the CNTs [30]. CNTs could also be produced using iron extracted from plants such as sesame seeds as organic precursors [34]. The obtained CNTs present a uniform size [35]. Pol et al. described a process to obtain CNTs from polymer waste without solvents [36]. This process is based on a thermal dissociation in a closed system within autogenic pressure and catalysts. The procedure described could, in addition, solve another current environmental problem: plastic degradation [30].

Focusing on the biodegradability of carbon-based materials, there are studies that demonstrate the degradation of CNTs. This degradation occurs not only chemically with strong oxidants or thermal treatments in an oxygen atmosphere but also through enzymatic oxidation with horseradish peroxidase. These studies demonstrate the total degradation of the CNTs in in vitro systems (using different animal tissues, cells, or molecules) without cytotoxicity. However, for in vivo systems (evaluating the degradation of CNTs in living organisms), the degradation that occurs is partial, and there is still long-term concern about its toxicity [19].

Other metals that are considered to be biodegradable, such as magnesium (Mg), zinc (Zn), or iron (Fe), can be used to develop conductive inks [7]. These are corrodible metals, and they degrade relatively quickly. Mg and Zn are more often used due to their lower cost and ease of processing. However, they degrade more quickly than Fe, making the last one more suitable for applications with a longer lifetime [7]. Lee et al. developed bioresorbable systems using Zn microparticles sintered electrochemically. The substrate used to deposit them was a bioresorbable polymer: a sheet of polylactic-co-glycolic acid (PLGA) [37]. Hwang et al. manufactured biodegradable electronics using Mg, a biodegradable polymer, among other materials [38].

The last type of conductive inks found in the market are those composed of a conductive polymer, such as PEDOT:PSS. Their conductivity is lower than particle-based ones, but it could be interesting to study the possibility of developing a conductive ink based on a biopolymer. PEDOT:PSS is a conductive polymer exhibiting biocompatibility, electrochemical properties, good electric conductivity, and versatile processing, and it is commercialized in water dispersion. There are studies demonstrating the biodegradability of montmorillonite/PEDOT:PSS composites (MMT/PEDOT:PSS) after being processed by specific super worms [39]. PEDOT:PSS does not fulfill the biodegradability ISO 14852 rule [9], which measures its degradation in aqueous media. Nevertheless, Pietsch et al. developed PEDOT:PSS biodegradable electrodes according to the ISO 14855 rule, which measures de degradation of the complete system in compost media [9]. This happens because the quantity of the PEDOT:PSS in the device is less than the non-degradable quantity accepted by the ISO 14855 rule to consider it biodegradable. Therefore, to consider a PEDOT:PSS ink biodegradable, it is necessary to combine a low quantity of PEDOT:PSS with an appropriate biodegradable binder [40]. The main strategy for using the PEDOT:PSS as functional material is to mix it with a biodegradable polymer, keeping conductivity. Conductivities up to 4 7 × 10⁻¹ S/m have been reached in particle PEDOT systems dispersed in poly(L-lactic acid) (PLLA) [41]. Mantione et al. developed different PEDOT:biopolymer dispersions, achieving conductivities up to 7 × 10² S/m.

Hereinafter, different biobased and/or biodegradable materials that exhibit dielectric properties and are good candidates for formulating dielectric inks are collected.

There are biodegradable inorganic dielectric materials, such as silicon dioxide (SiO₂), magnesium oxide (MnO), or silicon nitride (Si₃N₄), that have been used as dielectric materials due to their dielectric properties [7]. SiO₂ has a dielectric constant of 3.9, while the dielectric constant of Si₃N₄ is 7.5 [48]. This type of material could be dispersed in a binder to develop a dielectric ink for printed electronics.

Cellulose is a natural biopolymer found abundantly on Earth and is obtained from a vegetal source (annually, between 10¹¹ and 10¹² tons of nanocellulose are produced) [49]. The nanocelluloses are cellulose-based materials characterized by having nanoscale dimensions taken from plants, such as wood, coconut husk, sisal, algae, etc., or obtained from animals or bacteria [50,51]. They can be classified into three groups: (1) cellulose nanocrystals (CNCs), obtained chemically from plants or animals, (2) cellulose nanofibrils (CNFs), obtained mechanically, also from plants or animals, or (3) bacterial nanocellulose (BNCs), obtained from bacteria.

Cellulose is typically an insulating material whose dielectric properties are affected by its morphology. As an example, algae-containing nanocellulose, in comparison to wood-containing CNFs, has more crystallinity and less water sorption capability. Nevertheless, having more porosity, it has less electric resistance and more dielectric loss [66]. Thus, it is necessary to correctly select the cellulose type used in the formulation of a dielectric ink. Cellulose is resistant to hydrolysis due to its inter- and intra-molecular hydrogen bonds. For this reason, to ensure the biodegradability of cellulose, it is necessary to process it with microbes and fungal enzymes [67]. Williams et al. printed crystalline nanocellulose as a dielectric isolator [68].

Keratin protein is a protein that exhibits dielectric properties that can be used to produce green electronics. One source of keratin is chicken feathers, which are composed of 90% keratin. Singh et al. developed an aqueous keratin dispersion for dielectric coatings. King et al. developed various keratin aqueous dispersions, achieving dielectric constants up to 7.4 with frequencies of 10⁶ Hz [69].

Chitosan is a linear polysaccharide biopolymer, which also exhibits dielectric properties. Bonnard et al. developed biocomposites of chitosan and nitrile-modified cellulose nanocrystals, increasing its dielectric constant from 5.5 (pure chitosan) to 8.5 (composite with 50%wt.), measured at 10³ Hz [53,70].

Natural starches (such as tapioca, corn, wheat, or rice) also present dielectric properties. Ndife et al. prepared different starch dissolutions and measured their dielectric properties at 2.45 × 10⁹ Hz, obtaining a dielectric constant between 40 and 65 [54].

Allgén et al. studied the dielectric properties of sodium alginate in aqueous solution, obtaining dielectric constant values between 80 and 90, depending on the concentration of the sample [71].

Piezoelectricity is the property of certain materials to produce an electric current when suffering a deformation or to deform slightly when an electrical current is applied.

Piezoelectric materials could be classified into two distinct types: (1) materials of primordial piezoelectric nature (such as quartz) and (2) those known as ferroelectrics, which need a process of poling to present piezoelectric properties.

The most used piezoelectric materials are piezoelectric monocrystals, such as lithium niobate (LiNbO₃) or lithium tantalate (LiTaO₃), piezoelectric ceramics, such as barium titanate (BaTiO₃) or lead titanate-zirconate (PZT), and piezoelectric composites, such as ceramic piezoelectric particles embedded in a polymer matrix.

Piezoelectric polymers have advantages over piezoelectric ceramics; they are weaker and can be cut into different shapes at low temperatures. Four classes of piezoelectric polymer materials are known: (1) polyvinylidene fluoride (PVDF) and its copolymers trifluoroethylene (TrFE) and tetrafluoroethylene (TFE); (2) distinct types of vinylidene cyanide copolymers (VDCN); (3) aromatic copolymers; and (4) aliphatic polyurea. These polymers are ferroelectric materials; it is necessary to apply a poling process to obtain their piezoelectric properties.

PVDF is the most well-known piezoelectric polymer and the one with the best possibilities due to its piezoelectric properties [72]. It is a semicrystalline polymer that exhibits distinct phases (α, β, γ y δ). At room temperature, α and β phases coexist; the β phase is responsible for the piezoelectric properties of the polymer [73]. The polymer needs a poling process to achieve piezoelectric properties. Ismail et al. demonstrated PVDF manufacturing from biobased and biodegradable carbonated solvents (ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC)) [74]. However, the degradation of PVDF could produce harmful gases that damage the atmosphere and are harmful to human beings [67].

The quantity of piezoelectric inks available on the market is limited. While several companies sell conductive or dielectric inks, there are only a few companies that sell piezoelectric inks for printed electronics. These inks use P(VDF-TrFE) as a piezoelectric material.

Although wood’s piezoelectricity has been known for decades, the piezoelectric properties of CNFs and CNCs have only been studied during the last few years [78]. The piezoelectric responses measured in nanocellulose-based films are 2–8 pC/N.

Films based on plain chitosan exhibit piezoelectric properties with a sensibility of 4 pC/N. As with nanocellulose, it is necessary to apply a poling treatment to chitosan to develop its piezoelectric properties [70]. Chitosan is a biobased and biodegradable polymer and is becoming a good option for the development of piezoelectric inks.

Animal-based polymers are another material family showing piezoelectric properties. Two examples of these types of materials are silk, with a coefficient of up to 1 pC/N, and collagen, showing 2.64 pC/N. Both materials degrade enzymatically in physiologic conditions and in the presence of catalysts.

PVA is a water-soluble polymer known for its physicochemical properties, such as film formation, flexibility, and thermal stability [79]. There are studies showing a method for manufacturing high-quality crystalline thin layers of piezoelectric γ-glycine crystals between PVA layers. These thin films show a macroscopic piezoelectric response. The films are water-soluble, and, correctly packed, they could be used as part of an energy-harvesting biodegradable device [80]. PVA degrades under the presence and action of the phytopathogenic fungi Fusarium lini, producing carbon dioxide and water [81].

Poly (lactic acid) (PLA) is a biopolymer whose degradation occurs through hydrolysis in moist environments. This degradation could be accelerated with the addition of an enzyme such as bromelain or proteinase K. Using PLA, it is possible to manufacture films with piezoelectric behavior showing a sensibility between 3 and 5 pC/N. The measured sensibility, when flexed, is between 30 and 80 pC/N. This becomes a good option in applications requiring piezoelectric properties, especially when these properties are used in a flexion sensor [82].

Other biological materials with piezoelectric properties can be found in nature. As an example, 16 amino acids and their compounds hold piezoelectric properties at room temperature. Among them, the one with the most promising piezoelectric characteristics is γ-glycine, showing a coefficient of up to 10.4 pC/N. These responses could be compared with zinc oxide’s response, which could present a piezoelectric coefficient between 14.3 and 26.7 pC/N in specific conditions [67,83].

Another piezoelectric amino acid is DL-anime, whose coefficient is 10.34 pC/N, although it is decreased to 4 pC/N, on average, due to the random growth of the crystals [67,78]. Peptide also has piezoelectric properties that can be compared with perovskite [67]. These materials not only can be processed in a water solution, but they also decompose gradually, becoming basic molecules in water environments.

To obtain the piezoelectric properties of the mentioned materials, a poling process must be applied to them. This process consists, commonly, of the application of an electric field. This electric field rotates the dipoles present along the material in the direction of the applied electric field to obtain the piezoelectric properties [84]. Therefore, these types of materials are good candidates for developing piezoelectric sustainable inks, but it is necessary to process them correctly.

The binder is one of the essential parts of ink. Thus, it is necessary to select a biobased or biodegradable binder to formulate an ink. Depending on the selected dispersion media, the composition of the ink will vary. 

As presented above, nanocellulose is a biobased material. The biodegradability of functionalized nanocellulose has been demonstrated in certain conditions, and it is effective in various applications, including electronic applications [85]. It can be used both in the process of ink formulation for the synthesis of the nanoparticles (such as silver nanoparticles), acting as a template or capping agent, or as a stabilizer and binder. Brooke et al. developed a carbon nanocellulose-based ink, obtaining conductivities up to 4 × 10² S/m [86]. Hoeng et al. manufactured a silver conductive ink in a CNCs suspension binder, obtaining conductivity when silver content is higher than 3% [87].

Other cellulose materials, such as cellulose acetate, are used as binders to fabricate carbon inks. Zappi et al. developed an ink with carbon particles obtained from lignin in a cellulose acetate binder [88]. This ink had an appropriate conductivity for manufacturing printed electrodes. Considering this, cellulose could be used to formulate biobased and biodegradable inks.

Dihydrolevoclucosenone (commercial name: Cyrene) is a biodegradable, non-mutagenic, and non-toxic solvent derived from biomass [89]. Pan et al. developed a Cyrene biobased ink with graphene particles, reaching conductivities of 7.13 × 10⁴ S/m [90]. The use of Cyrene as a binder avoids the use of other toxic solvents in the ink formulation because it is biobased and biodegradable.

Shellac is an organic resin secreted by a red insect living in Thailand [91]. It is a biopolymer from a natural and renewable source. Poulin et al. used shellac as a binder in the development of a screen printing conductive ink composed of carbon black particles, obtaining conductivities of 10³ S/m [2].

Water is an abundant, renewable, and biodegradable material on Earth, composing 70% of the planet. Hence, water-based inks are considered biodegradable whether ot not the rest of the materials fulfill the established rules. The patent US 2020/0339832 A1, published in 2020, describes the formulation of a conductive, sustainable ink with carbon particles extracted from hemp [92]. Rocha et al. developed a water-based ink with chitosan as the polymeric resin and graphite as the functional material [93]. Koga et al. prepared a carbon inkjet printer ink using CNTs and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized CNFs dispersed in water [94]. Martinez-Crespiera et al. formulated a water-based conductive ink with silver particles dispersed in nanocellulose [50].

Due to the capacity of PLA to degrade, it can be considered an appropriate binder for developing biodegradable ink for printed electronics. Different inks in PLA binders have been formulated [6]. For example, Atreya et al. developed an ink consisting of wolframium particles dispersed in PLA, obtaining a conductivity of up to 4.55 × 10² S/m. The ink is water-resistant and biodegradable through hydrolysis or oxidation [6]. Najaf et al. manufactured printable conductive PLA-based inks using graphene in emulsion as the functional material. These inks presented a conductivity of 3.45 × 10¹ S/m [5].

Polyethylene oxide (PEO) is a water-soluble polymer, used by Huang et al. to develop inks for electronic devices that solubilize when they are in contact with water [1]. The inks are composed of wolframium or zinc particles embedded in a PEO binder, and the conductivities measured were up to 4 × 10⁴ S/m.

Lee et al. elaborated on an ink using molybdenum particles as the functional material, dispersed in Poly butanedithiol 1,3,5-triallyl-1,3,5-triazine-2,4,6 (1H,3H,5H)-trione pentenoic anhydride. The obtained conductivities were near 1.4 × 10³ S/m for devices that degrade after 9 days in deionized water at 37 °C [95].

The solvent is the part of the ink where the functional material is dispersed. As it is a fundamental part of the composition, it is important for it to be a biobased or biodegradable solvent (called eco-solvents), as well as compatible with the functional material and the selected binder.

Green solvents are environmentally friendly solvents, also called bio-solvents, derived from agricultural processes. As an example, ethyl lactate is a bio-solvent obtained from corn that is biodegradable and easily recycled. This solvent has been used in painting, substituting compounds such as toluene, acetone, or xylene, which are commonly used in inks. Furthermore, due to its solvency capability, it could be considered a good solvent for the development of sustainable inks [96].

Water can also be used as a solvent, obtaining a nanoparticle dispersion suitable for inkjet printing technology. For example, Mavuri et al. prepared a silver nanoparticle dispersion in water, propanol, and ethylene glycol used for inkjet printing [97]. Lin et al. dispersed CNTs in water, adding glycerol to achieve the viscosity needed [98]. Lee et al. sintered water-dispersed CNTs with methanol, which could be used for inkjet printing [99]. Other authors developed CNTs inkjet printing inks by dispersing the nanotubes in water [100,101,102].

Another bio-solvent could be natural polysaccharides obtained from bacteria, such as gellan or xantana, produced by the bacteria Spingomomas eloda and Xanthomonas campestris, respectively. Panhuis et al. developed water-based CNTs dispersed in these types of solvents [103].