Introduction

Synthetic resin adhesives (e.g., urea-formaldehyde resin, phenolic resin, and melamine-formaldehyde resin) are widely employed in packaging, construction, electrical equipment, new-energy engineering, et al. at 19.9 million tons in 20221,2,3,4,5,6. However, end-of-life resin adhesives exhibit a lack of recycling or biodegradability, resulting in vast amounts of end-of-use composites. Statistical analysis indicates that the annual generation of wasted materials resulting from the permanent bonding of resin adhesives is estimated to exceed 237 million tons7,8,9,10. The increasing seriousness of environmental concerns motivate researchers to explore the potential of attractive biomass-derived and stimuli-switchable alternatives.

Biobased adhesives are characterized by their wide range of raw material sources, health security, and biodegradability11,12,13. In recent years, considerable progress has been made in the development of advanced strategies for strong biobased adhesives14,15,16. For instance, Shuai et al.17 achieved lignin-based adhesives through the self-crosslinking of uncondensed lignin under hot pressing, meeting the minimum industrial requirement for adhesion. Wilker et al.1 have presented a sustainable adhesive derived from epoxidized soy oil, malic acid, and tannic acid, based on a cross-linked and heterogeneous system. In many cases, the adhesion performance exceeds 6 MPa, comparable to that of a classic resin adhesive. Generally, higher adhesion strength with strong interfacial cohesion signifies greater difficulty for the good reversibility18,19. Commonly-seen cohesion-interface adhesion trade-off poses a great challenge to balance the strongly cross-linked network and interfacial bonding20,21.

Recent progress in dynamic polymer networks demonstrates promising pathways for reusable adhesives by leveraging stimuli-responsive supramolecular motifs22,23,24,25,26,27. For instance, temperature-triggered reversible polymer adhesives, including ionogels, supramolecular elastomer, dynamic resin achieve reversible adhesion through thermo-responsive non-covalent interactions (Diels-Alder, coordinate bonds, hydrogen bonds, etc.) coupled with tunable viscoelastic phase separation. However, single supramolecular network relying on homogeneous bond distributions inherently suffers from crack tip stress intensification28,29, as the concurrent activation of bulk chain disentanglement and interfacial bond rupture leads to uncontrolled crack propagation30,31,32—a phenomenon particularly pronounced in temperature-cycling conditions where thermal expansion mismatch exacerbates interfacial stresses. A fundamental mechanistic conflict arises: while high bond density enhances interfacial cohesion, it elevates the energy barrier for reversible dissociation concurrently, resulting in either residue-prone debonding or sluggish switching rate33,34. Consequently, the imperative lies in formulating methodologies that can effectively address the discordance between adhesion and switchability, while concurrently calibrating internal and external interactions to achieve robust yet intelligent switchable adhesives with large switch ratios35,36,37. This undertaking is of paramount significance yet poses considerable challenges.

Herein, we propose a supramolecularly connected nanoconfined network for strong yet thermo-reversible bio-adhesives, which balance interfacial adhesion strength, cohesive energy, structural stability, and energy dissipation capability well. The obtained adhesive enables bonding on heterogeneous substrates for multilayer composite boards with efficient closed-loop recyclability, achieving a fully sustainable lifecycle that spans from biomass source substitution to high-value recycling. USEtox-based eco-performance assessment indicates their significant contribution on environmental (≈7.52 * 102 PAF m3 d/kgemitted) and health (≈2.04 * 10−4 cases/kgemitted) burdens through the closed-loop circulation of hetero-layered composites. This work pioneers a novel paradigm for advanced intelligent materials for closed-loop engineered composites, addressing waste material-sustainability and environmental pollution dilemma.

Results

Biobased adhesives with supramolecularly connected nanoconfined network

Synthesis of the lipoic acid (LA)/1,8-Diazabicyclo [5.4.0]undec-7-ene (DBU) adhesive confined by sulfhydrated cellulose nanocrystals (LA-DBU/DCNC-cys) is based on the almost homogeneous reaction. As presented in Fig. 1a, CNC prepared form the sulfuric acid hydrolysis of plants is selectively oxidized with sodium periodate firstly (DCNC) for abundant aldehyde groups. DCNC-cys is obtained through the Schiff base coupling with L-cysteine based on these reactive sites (Supplementary Fig. 1), which exhibits no intrinsic adhesion properties (Supplementary Fig. 2). Heat-induced ring-opening polymerization (ROP) of coenzyme LA forms linear chains based on which dynamic disulfide exchange leads to the poly(LA) branched DCNC-cys (Fig. 1b). It is interesting that the thiol-disulfide exchange proceeds at a very high rate (<180 s), which may be attributed to the existence of DBU. The composite ionic liquid of LA and DBU (LA-DBU) puts the way for a homogeneous system of adhesives, where modified cellulose can accommodate and adjust the multi-dynamic interactions. The polymer chains in the vicinity of the DCNC-cys are confined, thereby establishing a dense localized network, while the distant segments maintain sufficient mobility. The chemical structure of the final biobased adhesive at room temperature is presented in Fig. 1c, accompanied with the illustration of its thermo-reversibility for hetero-layered composites (Fig. 1d, e). Moreover, the adhesive can be applied for a large composite photovoltaic backplane (1 m2), indicating its great potential of industrialization (Supplementary Fig. 3).

Fig. 1: The structure design and closed-loop recycle of engineered composites.
figure 1

a Synthesis process of the sulfhydryl cellulose. b Disulfide bond exchange reaction. c The strong cohesion with multi-dynamic network at room temperature. d Schematic for the dissociation and reconstruction of dynamic bonds. e Thermo-induced structural switching. f Closed-loop recycle of engineered composites.

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Fourier transform infrared (FTIR) analysis (Supplementary Fig. 4) reveals enhanced carbonyl stretching (1730 cm−1) alongside emerging S–H vibrations (2550 cm−1), confirming L-cysteine grafting via Schiff base chemistry. Transmission electron microscopy (TEM) imaging (Supplementary Fig. 5) demonstrates the preserved rod-like morphology with reduced average diameter (from 48 to 33 nm), while zwitterionic L-cysteine induces nanofiber aggregation through charge screening between protonated amines and carboxylates38. ¹H NMR of LA-DBU (Fig. 2a) evidences ionic pair formation via carboxyl proton disappearance and DBU signal downfield shifts, indicative of electron density redistribution39. This ionic penetration disrupts native hydrogen bonds, creating homogeneous nanoconfined DCNC-cys domains40,41. Subsequent thermal processing triggers LA’s ring-opening polymerization via dynamic disulfide exchange, as verified by Raman spectral changes in Fig. 2b: C–S bond shift (from 684 to 690 cm−1) and disulfide vibration splitting (from 511 cm−1 to dual peaks)42,43,44.

Fig. 2: Characterization of the supramolecularly crosslinked network.
figure 2

a 1H NMR spectra of LA, DBU, and LA-DBU with Deuterated chloroform. b Raman spectra of LA and LA–DBU. c Temperature-variable FTIR in 3800–2100 cm−1, d 1800–1500 cm−1, e 1500–1150 cm−1. f–h PCMW2D spectra of LA–DBU/DCNC-cys. i Polarized optical microscopy (POM) images of LA–DBU/DCNC-cys with the content of DBU increasing from 0% to 20%.

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The dynamic thermo-responsive structure is investigated in depth through temperature-variable FTIR spectroscopy (Fig. 2c–e). The broad –OH stretching band (3384 cm−1) exhibits progressive intensity reduction from 25 °C to 125 °C, confirming multi-hydrogen bond coexistence45,46,47,48. Simultaneously, the C═O stretching region transforms from hydrogen-bonded (1707 cm−1) to free carbonyl conversion (1745 cm−1), accompanied by C-N vibration downshifts, demonstrates the formation of thermally activated [DBU]+:[COO−] ion pair—a critical mechanism enabling viscosity reduction through enhanced chain mobility49,50. Perturbation-correlation moving-window 2D (PCMW 2D) spectra (Fig. 2f–h) map these dynamic bond rearrangements graphically, revealing characteristic hysteresis loops diagnostic of supramolecular network reversibility. While disulfide-based networks gain traction in developing dynamic polymers51,52,53, their poly(LA) architectures exhibit metastability, with terminal sulfur radicals predisposed to initiating closed-loop depolymerization54. Superbase-stabilized topological confinement structure preserves the metastable system and dynamic reconfigurability effectively in this work, where zwitterionic DCNC-cys interfaces spatially restrict radical mobility via coulombic screening effects. POM analysis (Fig. 2i) reveals the complete absence of crystalline LA domains with DBU concentrations exceeding 20 wt%, whereas Scanning Electron Microscope (SEM) images show homogeneously morphologies throughout the matrix (Supplementary Fig. 6). These findings verify the effective depolymerization inhibition through ionic microenvironment-mediated radical quenching mechanisms. 5.

Reversible adhesion

The binding energy between LA-DBU (both CNC-cys modified and unmodified) and Fe substrate is examined through Materials Studio (MS) in Fig. 3a. Additionally, the mechanism by which the incorporation of CNC-cys contributes to the increased cohesion of the system is investigated. Isothermo heterophase molecular dynamics simulations of LA–DBU and LA–DBU/DCNC-cys models (with Fe substrate) are conducted at 298 K for energy-minimized configurations, enabling binding energy calculation through:

$${{{\rm{E}}}}_{{\rm{bind}}}={{{\rm{E}}}}_{{{\rm{polymer}}}}+{{{\rm{E}}}}_{{\rm{filler}}}-\,{{{\rm{E}}}}_{{\rm{total}}}$$
(1)

where Ebind represents the binding energy between LA–DBU/DCNC-cys and Fe substrate; Epolymer and Efiller denote the corresponding energies of the polymer matrix and the Fe layer in the optimized conformation, while Etotal signifies the total energy of the system. Remarkably, the introduction of DCNC-cys doubles the binding energy (from 610.53 to 1209.31 kcal/mol), resulted from the densification of supramolecularly crosslinked network. Thiolated-induced multivalent hydrogen bonding restructures ion-rich conformations, establishing a self-reinforcing supramolecular architecture at the metal-polymer interface.

Fig. 3: Adhesion properties of the bio-adhesive.
figure 3

a Binding energy simulation models of LA-DBU and LA-DBU/DCNC-cys. b Synergistic interplay of multiple interfacial interactions c Display of the bond strength across various surfaces. d Tensile strength bond of the bio-adhesive with 4 cm2 contact enduring a 65 kg adult. e Comparison with many reported biobased counterparts (green) and reversible petroleum-based competitors (red). f Cyclic Thermo-dependent complex viscosity (η) of the bio-adhesive. g Switchable adhesion upon alternating temperatures from 25 to 100 °C.

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LA-DBU/DCNC-cys demonstrates universal adhesion that can support a 7.5 kg dumbbell with 4 cm2 contact across various surfaces through adaptive interfacial energy dissipation mechanism (Fig. 3c). The adhesion strength on metal, plastic, glass, and wood is 6.02, 7.70, 2.19, and 4.35 MPa, respectively (Supplementary Fig. 7). Notably, its energy-delocalized metamaterial characteristic achieves extraordinary load distribution capacity—a lap shear configuration (4 cm2 overlap) can support an adult weighing 65 kg easily (Fig. 3d). This adhesive demonstrates outstanding bonding strength that reaches a very high level in both biobased counterparts and recyclable petroleum-based competitors (Fig. 3e, and Supplementary Information Table 1)27,55,56,57,58,59,60,61,62,63,64,65,66. As illustrated in Fig. 3b, the remarkable interfacial adhesion property of LA-DBU/DCNC-cys stems from a synergistic interplay of interfacial interactions, encompassing multi-hydrogen bonds, metal-coordination bonds, electrostatic interactions, hydrophobic effects, and physical absorption67.

Thermo-activated interfacial adhesion energy and cohesive fracture toughness based on these dynamically-reconfigurable architectures can be achieved via supramolecularly crosslinked energy-dissipative nanonetwork68,69,70. During the thermal transition process, the strongly correlated network undergoes entropy-driven controlled disintegration and spontaneous reconfiguration71. On the macro level, this process is accompanied with a phase-switching behavior between rich-ion liquid domain and solid phase. The rheological behavior dependent on frequency, strain and temperature are shown in Supplementary Fig. 8a, b and and Supplementary Fig. 9. The storage modulus (G’) is higher than loss modulus (G”) throughout the strain and frequency range, suggesting the existence of confined enhanced network. The gradual increase of G’ and G” under frequency scanning as well as the relatively stable state within 100% strain further confirms the strong network structure that enables robust adhesion via the high cohesion at room temperature. The dynamic rheological behavior dependent on frequency and strain are systematically investigated through oscillatory shear measurements. The storage modulus (G’) consistently surpasses the loss modulus (G”) across wide strain (0.1–1000%) and frequency (0.1–100 rad/s) regimes, confirming a multiple dynamic architecture stabilized by synergistic physico-chemical crosslinking72. In the temperature-dependent rheological figure, the continuously increasing storage modulus (G’) and loss modulus (G”) of the adhesive indicate the improved transition temperature induced by the content of DCNC-cys. Particularly, the characteristic frequency-dependent hardening behavior (monotonic augmentation of both moduli with ascending frequency) coupled with remarkable strain tolerance (quasi-invariant modulus below 100% strain amplitude) validates the formation of energy-dissipative network73. The confined supramolecular architecture enables temperature-triggered reversible cohesion, as demonstrated by cyclic rheological tests (Fig. 3f and Supplementary Fig. 10). The initial viscosity and transition temperature point of the adhesives exhibited a continuous rise as the DCNC-cys content increased, indicating the progressive enhancement of the supramolecular-linked nanoconfined network. The complex viscosity (η) decreases over a large span upon thermal stimulation (from 30 to 80 °C), attributable to the collapse of thermally responsive supramolecular crosslinked networks. Confined adhesives demonstrate near-quantitative viscosity recovery during multiple thermal cycles, governed by the thermodynamically favored supramolecular reassembly. Such robust reversibility is leveraged to engineer switchable adhesion in lap-shear configurations. Supplementary Fig. 11 further elucidates the role of the supramolecular confined network in adhesive strength under different temperatures, and indicates a satisfactory operating temperature range. Quantitative lap-shear analysis (Fig. 3g) reveals dramatical switching span in interfacial strength from 6.02 to ≈0 MPa (<10 s), which achieves a high on-off ratio over 2000. Supramolecular nanoconfined adhesive achieves undiminished bonding performance after 50-cycle endurance testing, achieving an exceptional combination of ultrahigh bond strength and scatheless cyclability.

As demonstrated in Fig. 4a–c and Supplementary Fig. 12, pristine LA-DBU sample exhibits limited interfacial adhesion capability with the shear strength peaking at 1.72 MPa (1:1 ratio). Strikingly, systematic incorporation of DCNC-cys induces a remarkable enhancement in interfacial adhesion, reaching peak performance (6.02 MPa) at the content of 100 wt%. Interestingly, we observe characteristic three-stage deformation behavior—linear elasticity, yield plateau, and strain hardening in tensile curves of DCNC-cys adhesives (Fig. 4b), in contrast to pure LA-DBU’s brittle failure (single-stage fracture). This phenomenon stems from the synergistic effect between topological constraint and progressive rupture of supramolecular crosslinking74. Furthermore, we conducted in-depth research on the impact of critical engineering parameters, including bonding time, bonding film size, area, thickness, and contact pressure (Supplementary Fig. 13a-e). The bonding strength of adhesives with different thicknesses, sizes, and overlapping areas remained equivalent, and the increasing bonding time (when exceeding 1 min) and bonding pressure (when exceeding 7.5 kPa) had limited effects. It is mainly attributed to the excellent fluidity and wettability enabled by the destructible confined network under a high temperature, along with the outstanding curing rate that can synchronize with rapid cooling.

Fig. 4: Structural mechanism for adhesion properties of the bio-adhesive.
figure 4

a Shear-tensile curves of bio-adhesives as a function of the DCNC-cys content. b Illustration of stress yield behavior. c Adhesion strength at different mixing concentrations. d Strain-dependent FEA simulations of LA-DBU adhesive and e LA-DBU/DCNC-cys adhesive. f Local time-dependent stress variation of g LA-DBU and h LA-DBU/DCNC-cys. i Schematic illustration of the nanoconfined structure. j Exchange and rearrangement processes of confined chains under external stress.

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We develop a multiscale finite element analysis (FEA) simulating shear-induced interfacial failure in multilayer composites, in order to mechanistically decode nanoconfinement-driven cohesive enhancement. In Fig. 4d, e, supramolecular confined adhesives have significantly improved energy dissipation and deformation-resistant capacity than contrast samples under the same shear condition (the total deformation is only 21.8% of the contrast). The strain-time curves of four loaded units (Fig. 4f–h) further corroborate the nanoconfinement-resulted exceptional cohesive energy, which enables the desired interfacial debonding rather than bulk fracture-induced detachment75,76,77,78. As illustrated in Fig. 4i, g, polymer chains under confinement (red) can dissociate from nanoparticle surfaces, releasing stored elastic energy. This detachment mechanism enhances adhesion durability, as the energy absorbed during stress-driven desorption significantly exceeds the thermodynamic binding energy between the nanofillers and the polymer network. Additionally, a dynamic equilibrium exists where confined chains detach while neighboring segments (gray) from the surrounding matrix adsorb onto the nanoparticles79. This continuous exchange process allows the adhesive interface to accommodate substantial strain and effectively dissipate mechanical energy. Consequently, crack advancement is suppressed, resulting in improved fracture resistance and robust adhesion performance.

Application in multilayer composites recycling

The bio-adhesive is applied to composite backplanes as a photovoltaic application for electrical engineering to demonstrate its extensive prospect. As shown in Fig. 5b, the multilayered structure assembled with PET and fluoroplastic films are bonded together by adhesives. As is known to all, the inherent low surface energy of fluoropolymers imposes critical challenges in interfacial adhesion for bio-adhesives. While in this work, the homogeneous backplane with a large scale (1 m2) can be prepared through hot-pressing (5 MPa, 100 °C) for 5 min. The filling effect is satisfying at 100 °C owing to its qualified flowability. Besides, the prepared backplane is able to be bended discretionarily and exhibits no separation (Fig. 5c), indicating the prospective engineering application. Films/panels can be well recycled from scrapped hetero-layered composites into high-value-added products, prompting the economic benefit significantly. These materials maintain structural integrity comparable to conventional adhesives-prepared composites during their practical service. Moreover, LA-DBU/DCNC-cys adhesives exhibit excellent closed-loop recyclability for various hetero-layered composites (Fig. 5d) through the reversible supramolecular nanoconfined structure. At the end of their lifecycle, thermal stimuli trigger adhesive dissociation, enabling the almost-undamaged recovery of high-value constituents with minimal energy input. We evaluated the removability of the adhesive through weight change. As presented in Supplementary Fig. 14, the mass change (from 5.63 g to 5.67 g) indicates only 0.71% mass deviation. Moreover, we further corroborated the excellent removability of the adhesive by the change in glass transmittance (86.7%) presented in Supplementary Fig. 15. We attribute the removal efficiency to the high switching ratio (≥2000) that minimizes residual adhesive contamination on separated substrates, guaranteeing the purity of materials for high-value recycling. Moreover, the retained original properties in maximum for direct reintegration into new products or secondary applications, such as integrating electronic elements, lightweight construction materials, and aerospace, where multi-material components necessitate selective disassembly for material recovery. To evaluate the reuse potential and pathways of recovered substrates, debonded PVDF films are reprocessed via melt-spinning, demonstrating the excellent Supplementary Fig. 16. The full-reversible bio-adhesive achieves a sustainable lifecycle displaces existing linear “produce-use-dispose” models, offering a creative approach to facilitating efficient recovery and reuse of end-of-life materials78,80,81.

Fig. 5: LA-DBU/DCNC-cys adhesive-based composites and the closed-loop circulation.
figure 5

a Schematic illustration of closed-loop sustainable lifecycle. b Composition structure of the photovoltaic backplane. c Photos of the bonded photovoltaic backplane bending. d Process of recycling end-of-life materials into a renewable feedstock for sustainable manufacturing. e CCK-8 assay (error bars are the mean ± standard deviation of three individual experiments) and f CLSM images of LA-DBU/DCNC-cys adhesive extraction. g Human toxicity potentials analysis and h ecosystem toxicity analysis at the midpoint level.

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Biobased adhesives demonstrate superior sustainability metrics versus petrochemical counterparts, particularly in biocompatibility and toxicological safety. Cytocompatibility assessment reveals exceptional cell viability (>80% by CCK-8 assay) of LA-DBU/DCNC-cys extraction after 48 h L929 fibroblast exposure (Fig. 5e). Moreover, a Live/Dead assay observed by confocal laser scanning microscope (CLSM) is further conducted with the co-incubation of extracted solution and L929 cells to investigate the security (Fig. 5f). The overwhelming majority of L929 cells are alive (green fluorescence) even under extraction solution treatment for 48 h. The non-poisonous is attributed to the high biomass content and the supramolecular-densified nanoconfined network.

Adhesive-based hetero-layered composites, such as foam composite boards, consist of complicated components and will release aromatic and ester compounds. In contrast, reversible and repeatable LA-DBU/DCNC-cys adhesives achieve the almost-complete recovery of various components of hetero-layered composites, with minimal environmental impact. We carry out USEtox, a widely recognized tool for assessing human and ecological impacts, to evaluate the toxicity of chemical emissions82. We carry out life cycle impact assessment of the chemical emissions derived from products with USEtox, which is recommended as the best evaluation tool to assess the human toxicological and ecotoxicological impacts. The impact score (IS) is calculated as:

$${IS}=\mathop{\sum}\limits_{i}\mathop{\sum}\limits_{x}{{CF}}_{x,i}*{m}_{x,j}$$
(2)

where IS indicates the impact score, CFx,i represents the characterization factor for substance x released into compartment i (measured in disease cases/kg or DALY/kg), while mx,j denotes the emitted mass (kg). USEtox evaluates human toxicity by considering carcinogenic, non-carcinogenic, and total impacts across multiple environmental compartments, including indoor/outdoor air, freshwater, seawater, and soils. Midpoint results are expressed in disease cases per kg emission, whereas endpoint outcomes use Disability-Adjusted Life Years (DALY). For ecotoxicity, the characterization factors reflect the potentially affected fraction of species (PAF) at midpoint and the potentially disappeared fraction (PDF) at endpoint, normalized per kg emission (PAF m3 d/kg or PDF m3 d/kg). Key pollutants like aromatic compounds, esters, and fillers were analyzed using Comparative Toxicity Units for human health (CTUh). As shown in Fig. 5g, h and Supplementary Fig 17, traditional petrochemical-derived alkane byproducts in engineered composites exhibit substantial toxicity, with IS values of 0.287 (human health) and 2.50 * 105 (ecosystem).

Scalable intelligent bio-adhesives enable closed-loop circulation of engineered composites, transforming end-of-life materials into a renewable feedstock for sustainable manufacturing. The existing lifecycle of engineered composites can be reversed from linear disposal to closed-loop circulation through selective disassembly of complex heterogenous components under controlled heating, avoiding their inherent environmental/human health impacts. For instance, when applied to engineered Nomex honeycomb-based insulating structures (a cornerstone material in HV equipment), these adhesives allow industrial-scale recovery and reassembly of heterogeneous components via controllable thermo-activated debonding, eliminating inherent environmental/human health impacts (about 7.52 × 102 PAF m3 d/kg and 2.04 × 10−4 cases/kg quantified through USEtox-based eco-performance assessment). Moreover, these adhesives can enable high-performance electrical composites with closed-loop circulation, which is critical for sustainable grid infrastructure, transformer housings, and recyclable PCB substrates. This converted lifecycle ensures that end-of-life insulating barriers, bushing supports, and other engineered electrical composites can be systematically disassembled, reprocessed, and reintroduced into manufacturing—transforming the electrical engineering sector from linear consumption to a circular materials economy (Fig. 5a).

Discussion

In conclusion, we develop strong yet thermo-reversible bio-adhesives for closed-loop engineered composites applied in electrical equipment, 3D printing, aerospace application, etc. Supramolecularly connected nanoconfined structure results in a significant enhancement of cohesive strength and, consequently, a strong adhesion property accompanied with multiple interface interactions. Confined bio-adhesives can switch between strong adhesion strength threshold (6.02 MPa) and ultralow adhesion strength threshold (≈0 MPa), benefited from the thermo-responsive dissociation-reconstruction of crosslinking networks. Supramolecular nanoconfined bio-adhesive-based materials can be selectively disassembled into pristine constituent layers, achieving full material recycling without mechanical or chemical degradation. Life cycle assessment confirms its significant role in eliminating inherent environmental burden (7.52 * 102 PAF m3 d/kgemitted) and health risks (2.04 * 10−4 cases/kgemitted) across the sustainable lifecycle. This work pioneers eco-conscious materials that decouple industrial development from ecological damage, transforming the lifecycle of engineered composite materials—from linear disposal to circular reuse.

Methods

Materials

Cellulose nanocrystalline aqueous dispersion was purchased from Zhejiang Yuewei Advanced Materials Company (China). Sodium periodate (NaIO4, > 99.8%), sodium hydroxide, L-cysteine, 1,8-Diazabicyclo[5.4.0]-7-Undecene (DBU) were all purchased from Shanghai Titan Technology Company (China). (±)−α-Lipoic acid (LA, 99%) was purchased from Shanghai Adamas-beta Reagent Company (China). Ultrapure water was purified using an ultra-pure water purifiers (Pincheng PCDX-JB-10).

Preparation of DCNC

Two hundred milliliters CNC suspension (10.0 wt%) was prepared through ultrasonic dispersion and then reacted with 8.25 g NaIO4 (1.3 times equivalent) at room temperature for 3 h under violent stirring. The oxidation reaction was performed in the dark throughout the preparation procedure to avoid the decomposition of sodium periodate and photo oxidation. At the end the oxidation period, 10 mL ethylene glycol was added into the reaction mixture to remove the residual sodium periodate. Following this, the suspension underwent dialysis and was subsequently stored at 4 °C in liquid form.

Preparation of DCNC-cys

1.67 g of L-cysteine was introduced into the DCNC suspension (200 mL), and then the Schiff base coupling reaction was completed after a continuous stirring process at 40 °C for 5 h. The final product was filtered by a microfiltration filter and then and purified by dialysis. Throughout the progress, DI water was changed in succession. A part of the DCNC-cysteine adsorbent product was freeze-dried for analysis.

Synthesis of LA-DBU/DCNC-cys

0.675 g LA was heated into molten state at 100 °C, and then mixed with DBU at the same molar ratio. The mixture was stirred continuously for 10 min to obtain an even fluid. Next, a certain amount of DCNC-cys powder was added lot by lot under a continuous stirring for the homogeneous dissolution of modified cellulose, followed by a slow temperature declining process to yield LA-DBU/DCNC-cys-x, where x represents the mass ratio of DCNC-cys to LA.

In vitro cytotoxicity of LA-DBU/DCNC-cys adhesive

In vitro cytotoxicity test was performed to evaluate the biocompatibility of LA-DBU/DCNC-cys adhesive against L929 cells. In short, L929 cells were seeded into 96-well plates and cultured for 24 h to become adherent. Next, the cell culture medium was taken away, and LA-DBU/DCNC-cys adhesive was incorporated into wells with 24 and 48 h incubation. Then, the culture medium containing CCK-8 reagents was introduced into wells and incubated for 4 h at 37 °C. Finally, absorption at 450 nm was recorded through a Spectra Max 190 microplate reader (Molecular Devices.USA). The cell viability was calculated according to the following equation:

$${{\rm{Cell}}} \, {{\rm{viability}}}\,\left(\%\right)=\left[\left({A}_{x}-{{\rm{A}}}_{b}\right)/\left({A}_{c}-{{\rm{A}}}_{b}\right)\right]\times 100\%$$
(3)

where Ab is the absorption value of wells with culture medium, CCK-8 reagents, and without cells; Ax is the absorption value of wells with culture medium, CCK-8 reagents, and LA-DBU/DCNC-cys adhesive; Ac is the absorption value of wells with cells, culture medium, CCK-8 reagents, and without LA-DBU/DCNC-cys adhesive.

The cells were stained with Calcein-AM/PI Double Stain Kit per well for further evaluating the toxicity of LA-DBU/DCNC-cys adhesive via STELLARIS confocal laser scanning microscopy (Leica, Germany), and the images of live/dead fluorescent-based cells were taken.

Bind energy simulation

Molecular dynamic simulations of material systems were performed using Material Studio 2018 software. In this simulation, the force fields were chosen to be a COMPASS II Forcite field, periodic boundary conditions, and Forcite module was adopted. First, the Fe cell was exported from the inventory of the software, followed by Supercell (A:4,B:4,C:4), and cleave surface to obtain Fe(1 1 0), and geometric optimization was carried out after fixing the polymer contact layer. To simplify the calculation, a polylipoic acid molecule with three repeating units and a sulfhydrated modified cellulose molecule with one repeating unit were constructed, and structural optimization was utilized to eliminate the unstable conformation of the crystal structure. Upon modeling each individual component, the entire system is created with minimal initial energy. During the simulation, the position of the Fe layer is fixed, but other molecular chains can adjust their conformation. Subsequently, the Forcite Dynamics of the NVE ensemble was conducted using a time step of 1 fs MD and an initial temperature of 298 K. Finally, the binding energy can be calculated using Forcite Energy based on the final conformational model.

Characterization

A Fourier transform spectrometer (Nicolet iS50, USA) equipped with a deuterated triglycine sulfate detector was used for the FTIR and temperature-dependent FTIR experiments. LA-DBU/DCNC-cys (0.02 g) was spread uniformly on a CaF2 window and then transferred into a homemade sample pool (programmable heating device). The sample protected by high-purity nitrogen gas (200 mL min−1) was heated from 25 to 125 °C at 5 °C min−1.TEM (JEOL JEM100CX, Japan) was employed for observing the morphology of CNCs and DCNC-cys. Scanning electron microscopy (JSM-5900LV, Japan) was employed to observe the surface of LA-DBU/DCNC-cys adhesives, which were sputter-coated with gold before imaging. Samples were diluted in CDCl3 as a solvent with tetramethyl-silane as an internal standard as solvents, and then the relevant 1H NMR spectra were obtained by a Bruker AV III HD NMR spectrometer (400 MHz) with Deuterated chloroform at room temperature. The laser confocal microscopy Raman spectroscopy (HORIBA, HR Evolution, Japan) was used to record Raman spectra of LA and LA-DBU with a 785 nm excitation wavelength. POM images were obtained through a POM (Phenix, XSP-24, China), which was equipped with a hand-made stretching device and crossed polarizers. Adhesion properties of the LA-DBU/DCNC-cys were measured at 50 mm/min at room temperature with a universal testing machine (Instron-5560, USA). Lap joints were made by placing adhesives with 2 × 2 cm square and 1 mm thick between two overlapping substrates (2 × 7.5 cm), including steel, wood, bone, and plastic. While the bonding test on glass controlled the overlapping area to 1 × 1 cm2, given brittle glass is unable to withstand high loads. Unless otherwise specified, the bonding substrate was chosen as a metal sheet (iron), the debonding temperature was set at 100 °C, and the contact pressure was maintained under a weight of 500 g for 1 min. The strain, frequency, and temperature-dependent rheological properties of the adhesive were investigated by a rotational rheometer (Anton Paar, MCR 302, Austria) with parallel plate with diameter of 25 mm. The dynamic strain scanning range of 0.1–100 % (0.1 Hz, room temperature) and the dynamic frequency scanning range of 0.1–100 Hz (0.1% strain, room temperature) were applied. Temperature sweep was conducted circularly between 30 °C and 80 °C at a rate of 10 °C min−1 (1 Hz, 0.1 % strain). We have an informed consent signed by the participant depicted in Fig. 3d.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.