This study investigated the impact of cellulose polymorphs on cellulose nanocrystals (CNCs) preparation. Cellulose samples with different types of polymorphs (cellulose I, cellulose II, and cellulose III) were prepared. Subsequently, CNCs were obtained from the above cellulose samples by sulfuric acid hydrolysis. The initial cellulose and CNCs were characterized by X-ray diffraction (XRD), thermo gravimetric analysis (TGA), scanning electronic microscopy (SEM) and particle size distribution (PSD). A comparative study showed that the properties of CNCs were closely related to the polymorphs of initial cellulose. The original polymorphs of cellulose I and cellulose II were retained, while that of cellulose III was converted back to cellulose I during the sulfuric acid hydrolysis process of CNCs preparation. For cellulose I and cellulose II, the crystallinity of both corresponding CNCs continuously increased with the increase of sulfuric acid concentration, until reaching a maximum at approximately 84.0%, while the crystallinity of CNCs obtained from cellulose III was slightly affected by sulfuric acid concentration. The yields of CNCs obtained from cellulose II and cellulose III with 61 wt% H₂SO₄ were extremely high, reaching 46.8% and 43.4%, respectively. Smaller CNCs particles with short-length were obtained by sulfuric acid hydrolysis of cellulose II. In addition, the properties (yield, dimension and thermal degradation behavior) of CNCs obtained from cellulose III were more susceptible to the acid concentration compared with those of CNCs obtained from cellulose I.

Conductive hydrogels have emerged as promising material candidates for multifunctional strain sensors, attributed to their similarity to biological tissues, good wearability, and high accuracy of information acquisition. However, it is difficult to simultaneously manufacture conductive hydrogel-based multifunctional strain sensors with the synergistic properties of reliable healability for long-term usage and environmental degradability/recyclability for decreasing the electronic waste. This work reports a facile strategy to engineer a self-healing, recyclable and conductive strain sensor by virtue of molecular-level multi-dynamic interactions (MMDIs) including Schiff base complexes, hydrogen bonds, and coordination bonds, which were fabricated using a dialdehyde TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized nanofibrillated cellulose (DATNFC) pre-reinforced gelatin nanocomposite hydrogel (gelatin/DATNFC hydrogel, GDH) followed by dipping in an Fe³⁺ aqueous solution. The MMDI strategy allows synchronous regulation of both bulk and interfacial interactions to obtain exciting properties that outperform those of conventional hydrogels, including extraordinary compressive stress (1310 kPa), intriguing self-healing abilities, and remarkable electrical conductivity. With these outstanding merits, the as-prepared gelatin/DATNFC/Fe³⁺ hydrogel (GDIH) is developed to be a multifunctional strain sensor with appealing strain sensitivity (GF = 2.24 under 6% strain) and compressive sensitivity (S = 1.14 kPa⁻¹ under 15 kPa), which can be utilized to manufacture electronic skin and accurately discern subtle bodily motions, handwriting and personal signatures. Notably, this GDIH-based sensor also exhibited reliable self-healing properties for long-term usage, environmental degradability and complete recyclability for decreasing the electronic waste. In consideration of the extremely facile preparation process, biocompatibility, satisfactory functionalities, remarkable self-healing properties and recyclability, the emergence of the GDIH-based sensor is believed to propose a new strategy for the development of sustainable-multifunctional strain sensors and healthcare monitoring.