Biodegradable Mineral Plastics

Mineral plastics are a promising class of bio‐inspired materials that offer exceptional properties, like self‐heal ability, stretchability in the hydrogel state, and high hardness, toughness, transparency, and non‐flammability in the dry state along with reversible transformation into the hydrogel by addition of water. This enables easy reshape‐ability and recycling like the solubility in mild acids to subsequently form mineral plastics again by base addition. However, current mineral plastics rely on petrochemistry, are hardly biodegradable, and thus persistent in nature. This work presents the next generation of mineral plastics, which are bio‐based and biodegradable, making them a promising, new class of polymers for the development of environmentally friendly materials. Physically cross‐linked (poly)glutamic‐acid (PGlu)‐based mineral plastics are synthesized using various alcohol‐water mixtures, metal ion ratios and molecular weights. The rheological properties are easily adjusted using these parameters. The general procedure involves addition of equimolar solution of CaCl2 to PGlu in equal volumes followed by addition of iPrOH (iPrOH:H2O = 1:1) under vigorous stirring conditions. The ready biodegradability of PGlu/CaFe mineral plastic is confirmed in this study where the elements N, Ca, and Fe present in it tend to act as additional nutrients, supporting the growth of microorganisms and consequently, promoting the biodegradation process.


Introduction
More than 6.3 billion metric tons of plastic waste has been generated so far by humankind, ≈79% of which is accumulated in DOI: 10.1002/smtd.202300575landfills or in some other form in the environment. [1]Most polymers are expected to persist over thousands of years and several attempts are being made to reduce the ever-increasing plastic pollution in the environment. [2]Microplastic is found in marine and freshwater environments, the atmosphere, and soil and has eventually cropped up in the food chain, which is a serious threat to life on earth. [3,4]It is therefore crucial to switch to substitutes that employ more environmentally friendly plastic materials, such as biodegradable polymers. [2]n living organisms, biominerals with a combination of biopolymers and strong minerals in hard-soft architectures result in reinforced materials with exceptional strength and toughness such as nacre, consisting of ≈95 wt% aragonitic CaCO 3 and 1-5 wt% chitin and fibroin protein in a "brick-and-mortar" architecture. [5,6]][13][14] These liquid precursors were identified to be polyacid stabilized liquids [15] resulting from the prenucleation cluster pathway. [16,17]Despite all of their outstanding features, biominerals including biogenic amorphous calcium carbonate (ACC) are formed irreversibly.
However, reversible design of mechanically strong ACC-based materials would have a number of significant advantages over the corresponding biominerals including easy recyclability.As a solution to this question, in previous work, Sun et al. presented the synthesis of polyacrylic acid-based (PAA) hydrogel called "mineral plastic" (MP), physically cross-linked via extremely small amorphous calcium carbonate nanoparticles (ACC). [18]his material is shapeable, stretchable and self-healable in the hydrated state, and reversibly becomes a hard, transparent and non-flammable material in the dry state.Mineral plastic was also reported to be formed by a low molecular weight electrolyte ibuprofen and magnesium carbonate. [19]Besides the obvious applications resulting from the above-named properties, mineral plastics were used in further interesting applications like an adhesive in dry and wet states and underwater glue, [20] artificial skin in an alginate-modified hydrogel form, [21] fireretardant, ultrastrong and tough structural organic-inorganic hybrids with high silica content and delignified wood, [22] mineral plastics with improved mechanical and rheological properties by Cu 2+ addition, [23] fluorescent mineral plastics when crosslinked with rare earth carbonates, [24] heat-insulating mineral plastic foams, [25,26] and graphene oxide/reduced graphene oxide mineral plastic 3D printable hydrogels as electronic skins, noses and laser controlled actuators. [27]uhrer et al. have shown that such typical mineral-plastic properties are achieved if solid-and fluid-like characteristics are present to the same extent in the material and the rheological storage and loss moduli are of the same order of magnitude. [28]chupp et al. have shown in addition that other alkaline earth or transition metal ions are also suitable to achieve these structures and properties with simple ions instead of metal carbonates. [29]ue to the alkaline pH used for mineral plastic preparation, CO 2 from air dissolves in the polyacid-metal salt solution and also leads to metal carbonate formation. [25]owever, all reported syntheses of mineral plastics employed PAA and are therefore fossil-fuel-based and can be expected to be only very poorly biodegradable. [30]In this work, we present a (poly)glutamic-acid (PGlu)-based mineral plastic, which forms upon mixing PGlu, CaCl 2 and alcohol (iPrOH, EtOH or MeOH) in water, and which is biodegradable.Significant work has been done with PAA-based mineral plastics as described above.So, the prime focus here is to come up with an environmentally safe alternative, while retaining the interesting properties of the original mineral plastics.The water-soluble polymer, PGlu was a wise choice because it is well-known in literature to be non-toxic and biodegradable [31,32] and reinforces our sustainable approach.PGlu is an amino acid biopolymer that is known to offer biocompatibility and non-immunogenicity.It is readily available and can also be biosynthesized. [31]PGlu is synthesized, for instance, by microorganisms such as Bacillus subtilis as a component of their capsules and slimes or as intracellular carbon and nitrogen storage compounds. [33,34]As such, PGlu is a ubiquitous compound in various natural environments and microorganisms capable of its degradation are equally wide-spread.The degradation pathways of PGlu have been studied in the past and are found in various natural environments and in all organisms capable of synthesizing PGlu. [34]Further, PGlu and its composites including nanostructures, hydrogels, and monoliths have found extensive applications in the medical field. [35]PGlu has also found effective use as an adhesive, cryoprotectant, heavy metal absorbent, humectant [32] etc. some of which have also been targeted by PAAbased mineral plastics.
As stated before, mineral plastic has only been achieved from PAA and with ibuprofen so far none of which are sustainable or biodegradable.Also, the fact that it has only been obtained from two materials further indicates that getting a mineral plastic is very tricky.The challenge that comes along is that there are so many factors such as molecular weight, concentration, temperature, rate of addition and so on that are responsible for the formation of mineral plastic that it leaves a very narrow window for the favorable conditions, which eventually form the mineral plastic with the desired rheological characteristics.Furthermore, the implementation of the concept of biodegradability makes it even more complicated.
Here, we have diligently examined all the factors and report the conditions suitable for the formation of PGlu-based mineral plastics.Moreover, we also illustrate how the rheology of mineral plastic can be effectively tuned using the different parameters like molecular weight, ion percentage, temperature etc. Subsequently, we also successfully confirm the ready biodegradability of our material, which was our prime motivation.Hence, this piece of work amalgamates a series of concepts with utmost ingenuity by assuring the intended properties of the mineral plastic of PAA in biodegradable PGlu-based mineral plastic (MP), which can be easily tuned using an effective toolbox.

Results and Discussion
PGlu/Ca MP was synthesized using a simple and straightforward procedure described in the experimental section.It has the characteristic properties of mineral plastics.It is stretchable (Figure 1a), deformable, and also reversibly swellable in isopropanol-water (1:1) mixture.The MP retains its shape in the dry, unswollen state (Figure 1b).Drying was carried out under 1 atm at 20-30 °C.However, drying at elevated temperatures and freeze drying are also possible.When MP in the dry state was subjected to a flame test, it was observed that the material undergoes charring, which was accompanied by swelling of the material (Figure 1c, Video S1, Supporting Information).The MP also exhibited self-healing, which was demonstrated by using MP dyed with pink and blue dye, which fused together as soon as they were brought in contact with each other (Figure 1e, Video S2, Supporting Information).These properties match well with those of the first-generation mineral plastics [7,18,25,29] and show that the preparation of a PGlu-based mineral plastic (Figure 1d,f) is possible using an alcohol as a precipitating agent.However, MP in the case of PGlu is non-transparent unlike the ones in first generation (Figure 1b).SEM images of a freeze-dried MP sample showed the typical microporous surface of hydrogels (Figure 1g).
Physical crosslinking is primarily responsible for the formation of MP and is mainly achieved by dynamic ionic coulomb interactions.The carboxylic side groups of the polymer PGlu have a pKa value of 4.5. [36]Experiments were conducted at pH 8, which was adjusted using 1 m NaOH.As a result, more than 99.9% of carboxylate groups were in a deprotonated anionic state.Sodium ions and calcium ions were present in the solution as potential counter ions.Calcium ions competed with the sodium ions to bind to the carboxylate groups.According to the Schulze Hardy rule, the critical coagulation concentration decreases with the inverse sixth power of the counter ion valence. [37,38]Hence, the ability of calcium ions to crosslink with carboxylate ions is much stronger than that of sodium ions.With trivalent ions like iron(III), the cross-linking ability increases further.Iron(III) was chosen as the trivalent ion because it is non-toxic, cheap, water-soluble, and because it is an essential nutrient for microorganisms and, thus, may support the microbial degradability of the MP (see below). [39,40]Figure 1d illustrates these interactions in MP.The polymer backbone chain, represented by the black line with carboxylate side groups (pink), is electrostatically crosslinked with positively charged calcium(II) (blue) and iron(III) (orange) ions.
There are several parameters that play a crucial role in the formation of MPs and that can be used to tune their properties effectively in a targeted manner.In this work, the effect of molecular To analyze the structural and mechanical properties of MP, rheological measurements were carried out.The samples were measured 3 h after preparation at 20 °C using an 8 mm parallel Peltier plate geometry (see Experimental Section).Figure 2a displays the effect of MW, ion composition, and the type of alcohol used, on the storage and loss modulus, as is discussed in the following.
In the first series, we examined the role of MW on the rheology of the PGlu/Ca MP, which was synthesized in water-iPrOH ratio of 1:1.Low MW, high MW and a 1:1 ratio of both MW were used to have three different variations of the MP.MP with a MW of ≈20-22 kDa (referred to as Low MW) has a two times higher loss modulus (0.46 MPa) than storage modulus (0.23 MPa) at 10 Hz and has, therefore, more liquid-like behavior.MP with a MW of ≈27 kDa and 74 kDa (referred to as High MW) also has a 1.9 times higher loss modulus (0.64 MPa) than storage modulus (0.34 MPa), while offering an increase in both moduli of ≈40%.A 1:1 mixture of high and low MW results in a behavior almost identical to that of the high MW (storage modulus 0.34 MPa vs 0.34 MPa and loss modulus 0.64 MPa vs 0.63 MPa).This behavior can be explained by the molar mass distribution.The cross-links in the hydrogel structure are mainly formed by higher molecular weight chains due to their higher tendency to cross-link, while the shorter chains are incorporated to a lesser extent, [28] hence similar values are obtained in both cases.
In the second series, we studied the influence of the metal ions on the rheology of the PGlu/Ca MP synthesized in iPrOH-water ratio of 1:1 without any iron(III).Trivalent iron has a higher tendency to cross-link than calcium(II) and should therefore result in a harder, more solid-like material.In Figure 2a, it can be seen that an increasing ratio of iron(III) to calcium(II) ions indeed leads to an increase in storage and loss modulus.For 1% of iron(III) and 99% of calcium(II) ions, the storage modulus is already 44% higher than with calcium(II) ions alone (0.34 MPa vs 0.24 MPa).The trend continues with an increase in storage modulus for 2% (0.86 MPa) and 10% (2.65 MPa) of iron(III) ions.It is remarkable that the storage modulus even surpasses the loss modulus for 10% iron(III) (2.65 MPa storage and 1.94 MPa loss modulus) and has therefore more solid like behavior.The observations above are very well in agreement with the fact that trivalent ions have a higher tendency to cross-link than bivalent ions, as mentioned above, which can also be clearly seen in the illustration in Figure 1d.
Even though iron(III) is known for strong cross-linking, the mixture of PGlu, FeCl 3 , and CaCl 2 remains soluble and no MP is formed.Ionic interactions are strengthened by adding a less polar solvent like alcohol.This is the third parameter that was used to attune the properties of MPs, besides MW and metal ion composition.Figure 2a confirms that the storage and loss moduli increase with decreasing carbon-chain length of alcohol from iPrOH to EtOH to MeOH.In the third series, the water to alcohol mixture was 3:1 and storage and loss moduli were orders of magnitude smaller than for a water to alcohol mixture of 1:1 in all the previously described samples where the variation of MW and metal ions was analyzed.Solvent type and ratio are therefore crucial for the determination of rheological behavior of MPs.Since in second and third series, the storage and loss moduli were very similar, especially in third series, the samples showed mineral plastic behavior. [28]ext, the complex viscosity was measured using the rheometer and the results achieved are depicted in Figure 2b.Here again, three different cases are evaluated; (a) PGlu/Ca MP; (b) PGlu/Ca MP with water-iPrOH ratio of 3:1 and (c) PGlu/Ca MP with 1% Fe(III).So, the first case (orange line) displays a higher viscosity over the whole frequency range as compared to the second case (blue line).Adding 1% Fe(III) leads to a further increase in viscosity (pink line).Therefore, even with different solvent ratios, the increasing trend of viscosity is consistent with the previously described increase in storage and loss modulus upon the addition of iron(III).The observed drastic reduction of the viscosity with increasing shear rate indicates shear thinning as already described for PAA-based mineral plastics. [18]other interesting rheological observation of MP was the reversible time-dependent shear thinning, also known as thixotropy which was also apparent from the graph in Figure 2b showing a decrease in viscosity with frequency.Figure 2c shows shear thinning as a hysteresis in a cycle of shear-rate sweep between 0 and 100 Hz.Thixotropy is typically observed for pseudoplastic fluids. [18,41]From this behavior it can be assumed that ionic bonds are broken by shear forces, thereby causing thinning.However, structural changes regress after some time if no shear forces are applied.
If ionic cross-linking and bond breaking is a dynamic process, the temperature should also influence the rheological behavior.Figure 2d shows a temperature ramp with increasing and decreasing temperature at a constant frequency of 10 Hz for MP.An increase in temperature from 5 to 55 °C leads to a decrease in storage and loss modulus.Above this temperature and up to 75 °C, the moduli showed a slight increase.Cooling the system from 75 to 5 °C showed a reversible behavior.However, below 15 °C, storage and loss moduli were lower in ramp-down than in rampup experiments.This behavior is explained by the superposition of the two concepts of thixotropy and temperature dependence.During the temperature measurement, some stress must be applied for oscillation, so shear thinning will be observed.The thinning at rising temperatures can be explained by a more dynamic ionic bond breaking and forming, leading to a lower storage and loss modulus.This behavior is in contrast to the temperature thickening observed for the PAA-based mineral plastic hydrogels, [18] which show an increase in the storage and loss moduli by three orders of magnitude when increasing the temperature from 30 to 70 °C. [18]otentiometric titrations were performed to assess the Ca 2+ affinity of PGlu.Experiments were conducted by the stepwise addition of CaCl 2 (0.02 mm) to a solution of PGlu (10 mg L −1 ) in water at pH (9.0).Ca 2+ potential was measured via an ion selective electrode and the activity of free Ca 2+ in solution was inferred from a calibration curve realized by adding free calcium ions to Milli-Q water also at pH (9.0). Figure S1, Supporting Information, demonstrates the complexation behavior where 0.44 Ca 2+ ions were observed to interact per COOH group.This is in good agreement with the binding values reported in the case of a block copolymer containing PGlu by Keckeis et al. [42] Here, the atomic absorption spectroscopy (AAS) results demonstrated a range of 0.39-0.53Ca 2+ /COOH.However, the values obtained from potentiometric titrations for the block copolymer were between 0.06 and 0.29, which is lower but was addition rate dependent.This is also in accordance with other carboxylic acid group containing polymers in literature like PAA where 0.2-0.3Ca 2+ is bound per COOH group for low molecular weight of PAA (≈15 kDa) and increased up to 0.5 for higher molecular weights (450 kDa). [28]easurements of Fe 3+ binding were not successful because the ions readily hydrolyze and condense at the applied pH leading to oxo-bridged negatively charged clusters on the way to the formation of FeOOH.These could interact with Ca 2+ and thus lead to the obtained irreproducible titration results (not shown).
The mechanical performance of dry PGlu/Ca MP was found to be quite promising.The dry PGlu/Ca MP has a Vickers Hardness of 44.8 ± 2.6 HV at room temperature and hardens to 81.4 ± 4.0 HV upon heating in a heating oven for 5 h at 150 °C, measured at room temperature (Figure S2, Table S1, Supporting Information).These high temperatures over a long period of time lead to better cross-linking and harder material.At this point, one can say that the rheological behavior depends on the temperature to a large extent.From the literature, it is known that the different everyday plastics like PMMA, polypropylene and unplasticized PVC [43] have a Vickers Hardness number of 16.75, 7.04, and 14.05 HV, respectively.Dentine [44] in teeth, on the other hand, has a Vickers Hardness number of 63.01 HV.These values clearly lead us to the conclusion that the MP is exceptionally hard.The Young's modulus of a film of dry PGlu/Ca MP was determined using atomic force microscopy (AFM) (Figure S3, Supporting Information).It was found to be 2.58 ± 0.89 GPa, which is higher than the biomineral, shrimp shell (1.87 ± 0.41 GPa) [45] and the conventional plastic, PMMA (0.187 MPa). [46]The dry mineral plastics can be transferred into the shapeable hydrogel by using a 1:1 (by volume, V) water/isopropanol mixture enabling easy recycling of the material.The thermal behavior of the prepared MPs (PGlu/Ca and PGlu/CaFe) was studied using thermogravimetric analyses (TGA) (Figure S4, Supporting Information).Interestingly, both materials undergo similar thermal characteristics before 300 °C indicating loss of water at 100 °C.The major degradation begins only after 300 °C where the materials behave differently.
PGlu is known to be non-toxic and completely biodegradable, for example, to humans as it has successfully been employed for medical applications. [31,32,35]Hence, also the PGlu-based MP prepared in this study can be considered to be easily biodegradable, for example in natural environments by microbial communities, which would be a significant benefit compared to the previously generated, hardly biodegradable (see below) PAA-based mineral plastics.In water, the MP dissolves into ions and gammapolyglutamic acid.Furthermore, PGlu with a molecular weight in the used range of 100 to 1000 kDa can be biologically produced by microbial fermentation, [47] and thus, PGlu may serve as wellaccessible and an environment-friendly alternative for developing MPs.
As a part of this study, the microbial degradation of PGlu/CaFe MP was examined, in order to ensure that it exhibits similar degradation behavior as known for PGlu (see Introduction) and that its degradation is not, for example, hampered by the cross-linking, by the high molecular weight of PGlu, and/or by the Ca and Fe ions released during its degradation.Therefore, we monitored CO 2 production as an indication of complete degradation (mineralization) of the substrate-carbon provided by MP, that is, as a source of carbon (and electrons) for energy generation via aerobic respiration, concomitant with CO 2 production and microbial biomass formation.Forest soil samples were used in the first experiment (Figure 3), and in the second .CO 2 production in a biodegradation assay using mineral-salts culture medium spiked with MP.Based on the amount of organic carbon that was added with the different substrates to the liquid cultures, the percentual (%) conversion of substrate-carbon to CO 2 was calculated.The CO 2 production in cultures without added substrate (blanks, n = 3) was used as baseline and subtracted.The liquid cultures were inoculated with microbes of a forest-soil slurry (see Experimental Section).a) The mineral salt medium contained all essential elements in excess for microbial growth other than carbon (e.g., N, P, S, Ca, Fe, and other trace metals); see Table S2, Supporting Information.For a positive control (blue), easily degradable carbon substrates were used (a mixture of succinate, glycerol and glucose).The other substrates added were the MPs PAA/Ca (orange) and PGlu/CaFe (red), or free PGlu (green).b) The carbon-limited mineral salts medium contained no N, Ca, and Fe source.Easily degradable carbon substrates (succinate, glycerol and glucose) were used (blue), and PAA/Ca (orange), PGlu/CaFe (red) or PGlu (green).The error bars indicate the standard deviations of three replicates (n = 3).experiment a carbon-limited mineral salts medium (Figure 4a,b), for which a slurry of forest soil was used as microbial inoculum.Furthermore, in order to assess whether the additional N, Fe, and Ca present in the PGlu/CaFe MP may indeed serve as additional essential elements for microbial growth and, thus, improved MP biodegradation, mineral salts medium lacking appropriate sources of N, Ca and Fe for growth was also tested.The experiments were conducted in comparison to PAA/Ca MP, as well as in comparison to natural cellulose in the soil setting or a mixture of easily degradable carbon sources in the soil-slurry setting (glucose, glycerin, and succinate), as references.
For the soil experiment, the CO 2 production was tracked for 32 days (Figure 3).The PAA/Ca MP showed no significant, additional CO 2 production compared to soil samples that were incubated without any added materials (blanks as a baseline, subtracted), hence, there was no indication that PAA was degraded over the whole incubation period.For the cellulose added to the soil, a significant increase in CO 2 production compared to the controls was detectable after ≈10-12 days, and at the end of the growth experiment, the amount of additional CO 2 produced corresponded to about 22% of the provided, additional substrate-carbon in cellulose; hence, cellulose was degraded, but yet incompletely.For PGlu/CaFe added to the soil, increased CO 2 production was visible after ≈5 days, and at the end of the incubation, the amount of additional CO 2 corresponded to about 60% of the substrate-carbon of MP.The free PGlu led to increased CO 2 production after 1-2 days and the amount of additional CO 2 had reached about 80% of the PGlu-carbon at the time when the incubations were stopped.Overall, the results indicated that, at least in a soil-microbial community setting, the mineralization of free PGlu and PGlu/CaFe is faster than mineralization of cellulose while no significant mineralization of PAA-based MP was detectable, under the conditions we used.The sigmoid CO 2 production curves exhibited by PGlu/CaFe and by PGlu, that is, being slowest at the beginning, followed by exponential increase and then formation of a plateau, are clear indications that their mineralization was accompanied by microbial growth [48] until all substrate carbon was depleted, respectively, in both cases.The increased maximal CO 2 production observed for PGlu compared to PGlu/CaFe can, however, not necessarily be interpreted as an increased degradation efficiency.The carbon use efficiency, that is, the proportion of carbon incorporated into biomass as opposed to being released as CO 2 during substrate degradation, is dependent on various factors including, but not limited to, the degrading organisms, the substrate and other environmental factors. [49,50]or the experiments with defined culture medium and a soil slurry as inoculum, the respiration was tracked for 21 days (Figure 4a,b).Two types of culture media were used.First, our standard, sodium/potassium-phosphate buffered (50 mM phosphate, pH 7.2) carbon-limited mineral salts medium (Figure 4a), and second, a carbon-limited mineral salts medium with reduced phosphate concentration (2 mm, pH 7.2), in which also N, Fe, Ca sources were omitted (Figure 4b).Succinate/glycerol/glucose (3C) were rapidly mineralized to CO 2 , within 3 days to >50% of the carbon provided by 3C (Figure 4a).In the N-, Ca-and Felimited medium (Figure 4b), 3C resulted in only 12% final CO 2 production, confirming that in this experimental setup, the microbial growth and thus biodegradation was severely restricted.The PGlu as substrate was compensating for at least N-limitation in the cultures, as indicated by its mineralization to about 50% CO 2 in the nutrient-limited (Figure 4b) and to 60% CO 2 in the nutrient-rich setup (Figure 4a), each within 21 d.However, the linear, rather than sigmoidal, degradation curve in the nutrientlimited condition indicates a less efficient degradation and restricted growth of the degrading organisms.This is likely linked to the lack of Ca and Fe as essential components for cellular maintenance and growth; complete inhibition of growth was likely prevented by trace amounts of these nutrients present in the soil slurry inoculum.The PGlu/CaFe, on the other hand, exhibited sigmoid degradation curves in both conditions, reaching 70% and 50% of CO 2 production after 21 days in the nutrient-rich and nutrient-limited condition, respectively (Figure 4a,b).The lag phase, that is, the time until the maximum mineralization rate was reached, as well as the time until a plateau was reached, was in fact shorter in the nutrient-limited condition for PGlu/CaFe, indicating an increased degradation efficiency under these conditions (Figure 4b).It is tempting to speculate whether the nutrientlimited conditions resulted in a less diverse and more specialized degrader community capable of accessing the Fe and Ca in the MP, while the excess of these nutrients in the nutrient-rich condition likely sustained an initially more complex degrader population.This may be supported by the significantly higher standard deviation in the nutrient-rich condition, reflecting differences in degrader populations between the replicates.Likewise, the difference in total CO 2 production between the nutrient conditions is likely explained by differences in the degrader population.Notably, after the degradation of PGlu/CaFe proceeded, a precipitate had formed in the culture medium; this may resemble insoluble Ca and/or Fe salts formed from the excess Ca and Fe released through degradation of the PGlu matrix.
Finally, in both nutrient-rich and nutrient-limited medium, the PAA-based mineral plastic showed no significant mineralization (Figure 4a,b), as with the soil setting (Figure 3), confirming that PAA-based MP is difficult to degrade.
The results of the biodegradation experiments demonstrate the ready degradability of the newly developed MP by natural microbial communities.The results furthermore exhibit an added value of the additional N, Ca, and Fe incorporated into this new polymer, which can act as essential nutrients to facilitate microbial growth and rapid biodegradation.

Conclusion
Biodegradable PGlu-based mineral plastics were synthesized and rheological properties were adjusted by varying molecular weight, using mixtures of bivalent calcium(II) and trivalent iron(III) ions as well as different water-to-alcohol ratios.Higher molecular weight results in higher storage and loss modulus.Ions with higher valencies, like iron(III) compared to calcium(II) lead to an increase in storage modulus and a less pronounced increase in loss modulus.Using a higher ratio of alcohol to water mixture and preferring methanol over ethanol and isopropanol also contributes to an increase in storage and loss modulus and a harder material.The biodegradability of PGlu-based mineral plastics as confirmed in this study, makes it a very attractive polymer to be used for the development of environmentally safe materials.The elements N, Ca, and Fe present in the PGlu-based mineral plastics act as additional nutrients, supporting the growth of microorganisms and consequently, promoting the degradation process.Easy tunability makes PGlu-based mineral plastics a highly interesting class of materials with numerous potential applications.They can effectively be used as adhesives, cell phone screen covers, non-flammable, insulating foams as opposed to styrofoams, parts in hot environments like in ovens, mo-tors etc. since they are non-flammable, and many more.Hence, the PGlu mineral plastics can be efficiently tailored for a number of applications with the valuable advantage of being biodegradable.Further, it also helps us in ensuring a very rational, sustainable, and eco-friendly approach to coming up with plastic alternatives.

Experimental Section
Chemicals: The low and high MW (poly)glutamic-acid used were obtained from a commercial supplier (company; Alibaba) and characterized using NMR (Figure S5, Supporting Information) and analytical ultracentrifugation (AUC) (Figures S6-S9, Supporting Information).The chemicals CaCl 2. 2H 2 O (>99%) and FeCl 3 •6H 2 O (>98%) were purchased from Carl Roth.The analytical grade solvents isopropanol (iPrOH), ethanol, and methanol were purchased from VWR.The dyes Rhodamine B and methylene blue for staining the MPs for self-healing experiments were obtained from Sigma Aldrich.All the glassware was cleaned with water of MilliQ quality (18.2 MΩ cm).The pH values were adjusted with hydrochloric acid (Merck, 0.1 and 1 m) and sodium hydroxide (Merck, 0.1 and 1 m).For the biodegradation tests, all mineral salts used for preparation of culture media were of at least p. A. purity grade or higher.Polyacrylic acid (PAA) 100 000 g mol −1 (Sokalan PA 80 S, BASF)-based mineral plastic and cellulose powder (Sigmacell 100, Sigma Aldrich) were used as control substrates for the biodegradation tests.
Preparation of Cross-Linked (poly)Glutamic-Acid-Based Mineral Plastics: First, the pH value of a 5.0 mL solution of low MW PGlu (0.2 m, ≈20 kDa) dissolved in water was adjusted to pH 2.8 using HCl (1 m), in order to protonate the carboxyl groups of the polymer.Then, an equivolume, equimolar solution of CaCl 2 (0.2 m, 5.0 mL) was added to it using a syringe under vigorous stirring conditions.Next, the pH was adjusted to 8.0 using NaOH (1 m), in order to deprotonate carboxyl groups and enable ionic interactions with the calcium(II) ions.Last, iPrOH (10.0 mL) was added using a syringe pump under vigorous stirring conditions, resulting in the formation of a sticky white solid, which on kneading resulted in MP denoted as PGlu/Ca; the kneading is necessary to homogenize the MP.
PGlu/Ca was the standard MP which was modified in order to study the influence of iron (III), solvent type and the molecular weight.The abovementioned general procedure was modified as described in the following to obtain the variants of PGlu/Ca MPs.
Variation with Iron(III): After adjusting the pH of low MW PGlu (0.2 m) to 2.8 (see above), 5.0 mL of the mixture of CaCl 2 (0.2 m) and FeCl 3 (0.2 m) was added to it under vigorous stirring using a syringe.Three different ratios of iron(III) and calcium(II) ions were used.The ion ratio of iron(III): calcium(II) was 1:9, 1:49, 1:99, which was 10, 2, and 1 atom%, respectively.The pH was adjusted to 8 using NaOH (1 m).A sticky yellow-brown solid was formed upon addition of iPrOH (10.0 mL) for the ratio of 1:99.The materials obtained were kneaded for about 5 min.As the ratio of iron (III) was increased to 1:49 and 1:9, the sticky solid became a brittle solid, which was eventually not easily mouldable.Hence, the sample with the ratio of 1:99, denoted as PGlu/CaFe exhibited stretchability (Video S3, Supporting Information) and was used for biodegradation studies.The EDX images illustrated that iron (III) and calcium (II) are distributed evenly in the sample (Figure S10, Supporting Information).
Variation of Solvents Used: In the general procedure followed for making the low PGlu/Ca MP, the system had a total solvent ratio (by volume, V) of 1:1 (V[H 2 O]:V[iPrOH]).In order to analyze the effect of alcohol types used and their ratio, the general procedure was followed until the pH was adjusted to 8 and then iPrOH, EtOH, or MeOH was added under vigorous stirring using a syringe pump.The solvent ratio was kept at 3:1 ).A comparative study of MP generated at a ratio 1:1 could not be done, because the addition of more EtOH and MeOH led to the formation of hard coacervates, which were solid and therefore not kneadable.

Figure 1 .
Figure 1.Properties of PGlu/Ca mineral plastic.a) Stretchability of PGlu/Ca mineral plastic (MP).b) Dry PGlu/Ca.c) PGlu/Ca burned using a lighter as a source of heat (see also Video S1, Supporting Information).d) Different interactions in the MP, where the bold black line corresponds to the PGlu backbone, the pink spheres symbolize negatively charged carboxyl groups, the blue spheres symbolize Ca(II) ions, and the orange spheres Fe(III) ions.The dashed lines indicate dynamic coulomb interactions.e) Self-healing experiment with MP dyed in pink and blue (see also Video S2, Supporting Information).f) Structural representation of polyglutamic acid (PGlu).g) SEM images of a freeze-dried PGlu/Ca, for which the inset shows a magnified section; scale bars are indicated.

Figure 2 .
Figure 2. Rheological data of PGlu/Ca (low MW, 0% Fe(III), water:iPrOH = 1:1) MP and its variations.a) Storage and loss modulus at 10 Hz and 100 Pa for three different series of PGlu/Ca MP analogs.Each series has one variable factor whereas the rest is constant.The three series depict the variation of MW (low, mixed, high), Fe content (1, 2, 10%) and the alcohol (iPrOH, EtOH, MeOH) used with the third series having a water:alcohol ratio of 3:1.b) Viscosity at different frequencies for three selected PGlu/Ca MPs; the standard PGlu/Ca, with water:iPrOH = 3:1 and with 1% Fe. c) Thixotropic loop for PGlu/Ca.d) Storage and loss moduli of PGlu/Ca at 10 Hz in a temperature loop.

Figure 3 .
Figure 3. CO 2 production in a biodegradation assay using forest soil spiked with MP.Based on the amount of organic carbon contained in the test materials that were mixed into the soil samples, the percentual (%) conversion of substrate-carbon to CO 2 was calculated.CO 2 production in soil samples without any added materials (blanks, n = 3) was used as the baseline, which was subtracted.Cellulose powder (blue) and traditional PAA/Ca MP (orange) were used as reference materials, in comparison to the PGlu/CaFe MP produced in this study (red).Free gamma-polyglutamic acid was also tested (green).The error bars indicate the standard deviations of three replicates (n = 3).

Figure 4
Figure 4. CO 2 production in a biodegradation assay using mineral-salts culture medium spiked with MP.Based on the amount of organic carbon that was added with the different substrates to the liquid cultures, the percentual (%) conversion of substrate-carbon to CO 2 was calculated.The CO 2 production in cultures without added substrate (blanks, n = 3) was used as baseline and subtracted.The liquid cultures were inoculated with microbes of a forest-soil slurry (see Experimental Section).a) The mineral salt medium contained all essential elements in excess for microbial growth other than carbon (e.g., N, P, S, Ca, Fe, and other trace metals); see TableS2, Supporting Information.For a positive control (blue), easily degradable carbon substrates were used (a mixture of succinate, glycerol and glucose).The other substrates added were the MPs PAA/Ca (orange) and PGlu/CaFe (red), or free PGlu (green).b) The carbon-limited mineral salts medium contained no N, Ca, and Fe source.Easily degradable carbon substrates (succinate, glycerol and glucose) were used (blue), and PAA/Ca (orange), PGlu/CaFe (red) or PGlu (green).The error bars indicate the standard deviations of three replicates (n = 3).