Engineering advanced cellulosics for enhanced triboelectric performance using biomanufactured proteins

The study addresses how to enhance the triboelectric performance of sustainable, biodegradable cellulosic materials by integrating engineered, biomanufactured proteins inspired by squid ring teeth (SRT). Cellulose is abundant, renewable, and versatile but its triboelectric output can be limited compared to certain synthetic polymers. With growing demand for renewable materials and the need to replace less sustainable synthetic fibers, the authors hypothesize that recombinant SRT proteins—tunable via synthetic biology—can be blended with or coated onto cellulose to increase charge generation through amino-acid-derived charges, induced crystallinity, and polar functional groups, without compromising mechanical integrity. The work aims to create scalable, high-performance, biodegradable triboelectric fibers suitable for smart textiles, energy harvesting, and filtration.

Background highlights include: (1) Cellulose’s prominence as the most abundant biopolymer, with expanding applications from packaging to medical uses and advanced forms like nanocrystals and nanofibrils. (2) Protein–cellulose interactions in nature (e.g., cellulose-binding domains) motivate engineering proteins to tailor cellulose’s properties. (3) Triboelectric effect fundamentals and construction of the triboelectric series; polysaccharides and polypeptides can generate triboelectric response via polar hydroxyls and charged amino acids. (4) Prior triboelectric fibers span synthetic polymers (nylon, PTFE), natural polysaccharides (cellulose), and proteins (wool, silk), but synthetics pose sustainability issues and natural resources are limited. (5) Prior blending of proteins (e.g., keratin, silk fibroin) with cellulose in ionic liquids showed dispersion and sometimes phase separation at higher loadings; studies link crystallinity/chain packing to improved triboelectric output. These works motivate using biomanufactured, sequence-controlled SRT proteins to sustainably enhance triboelectric cellulose fibers.

• Bioengineered SRT protein design and production: A segmented copolymer was designed with crystal-forming (AAASVSTVHHP) and amorphous (YGYGGLYGGLYGGLGYGP) segments. SRT tandem repeat variant with n = 11 (MW ≈ 39.9 kDa) was selected for high toughness/extensibility. Proteins were produced via heterologous expression in E. coli BL21 (pet14b plasmid) in a 100 L fermenter; DNA verified; biomass processed to purified proteins following prior protocols.
• Dope preparation and wet spinning: Cellulose triacetate (CTA) and microcrystalline cellulose were dissolved with SRT protein in 1-ethyl-3-methylimidazolium acetate (EmimAc)/dimethyl sulfoxide (DMSO) 1:1 (v/v). Dope solids: 13% w/v for CTA, 16% w/v for cellulose. Protein fractions: 0%, 1%, 5%, 10% (w/w relative to cellulose/CTA). Solutions stirred 3 h at 65 °C (avoiding >90 °C). Spinning on a lab-scale continuous line (Alex James & Associates Inc.): stainless steel spinneret with 100 orifices (100 µm); pump at 65–80 °C, flow 4.2 cc/min, jet velocity 5–6 m/min. Coagulation in DI water (RT), two washing baths at 75 °C and 60 °C, drying/annealing on heated roller at 85 °C; continuous filaments at ~35 g/h.
• Draw and fiber dimensions: As-spun cellulose diameter 56.3 ± 1.6 µm; drawn cellulose 28.6 ± 0.6 µm (draw ratio ≈ 2.0). Drawn CTA diameter 21.5 ± 1.2 µm (draw ratio ≈ 2.6). Visual inspection showed uniform, well-blended dopes.
• Protein-coated fibers (bi-composites): ~120 mg of cellulose fibers immersed in SRT solution in DMSO (20 mg/mL) for 2 h at RT, solvent dried, washed in ultrapure water at 60 °C, and desiccated. SEM confirmed surface-localized protein.
• Structural/chemical characterization: SEM on cross-sections and surfaces; WAXS to evaluate anisotropy and crystallinity (cellulose-II identified). Crystallinity estimates: cellulose ~56.4%, CTA ~44.2%. FTIR confirmed composition (β-glycosidic link 895 cm⁻¹; carbonyl 1730 cm⁻¹) and protein retention (Amide I/II growth with protein fraction). FTIR Amide I deconvolution on 10% protein fibers quantified secondary structures; moisture-corrected deconvolution provided similar results.
• Mechanical testing: Monofilament tensile tests per ASTM; cellulose blends showed strength ~256–266 MPa and higher stiffness than CTA (~2.4×). Protein blending up to 10 wt% did not significantly change mechanical properties.
• Triboelectric device fabrication and testing: Multifilaments cut to staple fibers and deposited on Kapton tapes (2 × 1.2 cm²) as top layers; Kapton films on Cu sheets as bottom electrodes with 0.5 cm polyethylene foam insulation. Protein thin films (10% w/v in HFIP) cast on 3 × 3 cm² Cu, rinsed and dried. Measurements: voltage via Siglent SDS 1104X-E oscilloscope; current via MetroOhm Autolab PGSTAT128N potentiostat. Contact-mode finger tapping at 1, 2, and 3 Hz; durability up to 2000 cycles at 1.25 Hz; two configurations tested: single-electrode and two-electrode. Power density vs load resistance; capacitor charging: 0.1 µF through full-bridge rectifier at 3 Hz; LED circuit demonstration.
• Figure-of-merit normalization: Outputs standardized by planar area of tribo layers/electrodes; comparisons limited to fibrous morphologies to mitigate geometric effects.
• Biodegradation and protein recovery: Domestic waste composter (with food waste) used for biodegradation of cellulose fibers (0–10% protein); post-test mechanical characterization where possible. Protein recovery by DMSO leaching: protein dissolves in DMSO while cellulose swells; protein precipitated with water; FTIR verified removal (loss of Amide I/II in leached fibers; Amide I/II in recovered residue). SEM/optical imaging assessed morphological changes (diameter increase ~2 µm; porosity increase ~10%).

• Structural/mechanical: Drawn cellulose fibers exhibited higher strength (256.1 ± 2.1 to 265.8 ± 3.9 MPa) and stiffness (~2.4× CTA; modulus ≈ 9 GPa) than CTA (136.5 ± 12.1 to 152.8 ± 6.8 MPa). WAXS: cellulose-II; crystallinity ~56.4% (cellulose) vs ~44.2% (CTA). Protein blending up to 10 wt% did not significantly alter mechanical performance or anisotropy; SEM showed no aggregation.
• Protein conformation: FTIR deconvolution (10% protein fibers) indicated substantial random coil and α-helix with reduced β-sheet content vs native hydrated protein (≤ ~34% β-sheet vs up to 55% native). Reported fractions: CTA-protein (10%): β-sheet 33.06%, random coil 24.47%, α-helix 11.99%, turns 29.24%, side chains 1.24% (note higher fit error near ~1701 cm⁻¹ due to carbonyl overlap). Cellulose-protein (10%): β-sheet 33.93%, random coil 30.9%, α-helix 11.57%, turns 21.26%, side chains 2.35%.
• Triboelectric performance (protein film): Max outputs: single-electrode 174 V, 4.82 µA; two-electrode 104 V, 1.12 µA.
• Triboelectric performance (blend fibers): Cellulose consistently outperformed CTA. Adding 10 wt% protein raised peak voltages by 72–108% (CTA) and 49–57% (cellulose) in both configurations. Enhanced performance attributed to charged amino acids and potential effects of crystallinity/chain packing.
• Protein-coated cellulose fibers: Outperformed all other tested fibers with voltage densities 60.48 V cm⁻² (single-electrode) and 25.38 V cm⁻³ (two-electrode), attributed to protein concentration at the surface and strong adhesion/self-healing behavior.
• Benchmarking: Single-electrode mode generally exceeded two-electrode outputs. Cellulose–SRT blend fibers achieved 40.19 V cm⁻² (single-electrode), surpassing prior sustainable fibrous triboelectric materials.
• Device metrics: For cellulose + 10% protein (two-electrode): peak voltage vs load reached 42.4 V at 100 MΩ (~63% higher than pure cellulose); peak power density ~87% higher than pure cellulose. 0.1 µF capacitor charged faster and to higher voltage (~1.3 V) than pure cellulose (~0.6 V). Stable over 2000 cycles. Frequency response peaked at 2 Hz.
• Biodegradation and circularity: Domestic composting reduced strength by up to ~27% and toughness by up to ~60%, demonstrating biodegradability. Protein successfully recovered via DMSO leaching and water precipitation; FTIR confirmed complete extraction. DMSO treatment increased fiber diameter by ~2 µm (~10% porosity increase), suggesting tunable morphology.

The findings demonstrate that integrating biomanufactured, sequence-designed SRT proteins into or onto cellulose yields significant gains in triboelectric output without sacrificing mechanical performance or scalability. Charged amino acids and compatible interactions with cellulose (hydrogen bonding/entanglement) enhance charge accumulation and output, particularly when protein is surface-localized via coating. Continuous wet spinning provides uniform, strong fibers at pilot scale, aligning with industrial practices. Superior voltage density and durability position these fibers for smart textiles and self-powered devices; finer fibers achievable by the process may further boost performance and filtration efficacy. The observed difference between single- and two-electrode configurations (likely ground leakage) highlights an area for further mechanistic study. Additionally, biodegradation in domestic conditions and recoverability of protein support end-of-life circularity, advancing sustainable material design.

The study presents scalable, high-strength cellulose fibers enhanced with biomanufactured SRT proteins via blending and surface coating. At 10 wt% protein, triboelectric voltage increased by ~49–108% depending on substrate (cellulose vs CTA), with coated cellulose fibers achieving the highest voltage density among surveyed sustainable fibrous materials. Devices showed improved power density, faster capacitor charging, and long-cycle durability. Structurally, fibers retained crystallinity and anisotropy; proteins dispersed compatibly with predominantly non-β-sheet conformations in the cellulose matrix. Biodegradation and protein recovery demonstrate circularity potential. Future work indicated includes clarifying the mechanisms underlying configuration-dependent outputs, optimizing protein distribution/content, leveraging finer fiber diameters for enhanced performance, and exploring applications in smart textiles and filtration.

• The mechanism for higher outputs in single-electrode versus two-electrode mode remains unresolved; authors note likely ground leakage and call for further studies.
• Post-biodegradation mechanical testing could not be performed for all fiber types due to sample constraints.
• Protein secondary structure quantification in CTA fibers has higher fitting error near ~1701 cm⁻¹ due to carbonyl overlap, potentially overestimating β-sheet content.
• Triboelectric tests used finger tapping and contact mode to avoid fiber pile-up; while practical, this may introduce variability relative to standardized mechanical actuation (not discussed in detail).