Nano-enabled smart and functional materials toward human well-being and sustainable developments.

Fabrication and operation on increasingly smaller dimensions have been highly integrated with the development of smart and functional materials, which are key to many technological innovations to meet economic and societal needs. Along with researchers worldwide, the Waterloo Institute for Nanotechnology (WIN) has long realized the synergetic interplays between nanotechnology and functional materials and designated 'Smart & Functional Materials' as one of its four major research themes. Thus far, WIN researchers have utilized the properties of smart polymers, nanoparticles, and nanocomposites to develop active materials, membranes, films, adhesives, coatings, and devices with novel and improved properties and capabilities. In this review article, we aim to highlight some of the recent developments on the subject, including our own research and key research literature, in the context of the UN Sustainability development goals.

Anisotropic hydrogel scaffold by flow-induced stereolithography 3D printing technique.

Essential organs, such as the heart and liver, contain a unique porous network that allows oxygen and nutrients to be exchanged, with distinct random to ordered regions displaying varying degrees of strength. A novel technique, referred to here as flow-induced lithography, was developed. This technique generates tunable anisotropic three-dimensional (3D) structures. The ink for this bioprinting technique was made of titanium dioxide nanorods (Ti) and kaolinite nanoclay (KLT) dispersed in a GelMA/PEGDA polymeric suspension. By controlling the flow rate, aligned particle microstructures were achieved in the suspensions. The application of UV light to trigger the polymerization of the photoactive prepolymer freezes the oriented particles in the polymer network. Because the viability test was successful in shearing suspensions containing cells, the flow-induced lithography technique can be used with both acellular scaffolds and cell-laden structures. Fabricated hydrogels show outstanding mechanical properties resembling human tissues, as well as significant cell viability (> 95 %) over one week. As a result of this technique and the introduction of bio-ink, a novel approach has been pioneered for developing anisotropic tissue implants utilizing low-viscosity biomaterials.

Anisotropy and Nanomechanics of Cellulose Nanocrystals/Polyethylene Glycol Composite Films.

The influence of shear flow on the nanomechanical properties of cellulose nanocrystal (CNC)/polyethylene glycol (PEG) composite films and the distribution of anisotropic phases are investigated at various CNC/PEG ratios. Here, the drying process of CNC/PEG mixed suspensions is systematically traced by rheology, followed by the spatial mapping of local mechanical properties of CNC/PEG films by nanoindentation. The detailed study of the morphology of CNC/PEG films by polarized optical microscopy (POM) and image analysis revealed the link between the mechanical properties and the influence of shear flow. A comparison of the data obtained for shear-dried films with nonsheared films showed the improved reduced Young's modulus (Er) and hardness (H), and suppression of microphase separation in the shear-dried films. Based on this experimental evidence, a mechanism is proposed to explain the microstructural transition during the shear-drying process leading to the generation of the anisotropic domains containing the shear-induced assembled structure of CNC particles coexisting with the elongated PEG microphases.

Self-healing, stretchable, and highly adhesive hydrogels for epidermal patch electrodes.

Flexible, self-healing and adhesive conductive materials with Young's modulus matching biological tissues are highly desired for applications in bioelectronics. Here, we report self-healing, stretchable, highly adhesive and conductive hydrogels obtained by mixing polyvinyl alcohol, sodium tetraborate and a screen printing paste containing the conducting polymer Poly (3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) and diol additives. The as prepared hydrogels exhibited modelling ability, high adhesion on pig skin (1.96 N/cm2), high plastic stretchability (>10000%), a moderate conductivity, a low compressive modulus (0.3-3.7 KPa), a good strain sensitivity (gauge factor = 3.88 at 500% strain), and remarkable self-healing properties. Epidermal patch electrodes prepared using one of our hydrogels demonstrated high-quality recording of electrocardiography (ECG) and electromyography (EMG) signal. Because of their straightforward fabrication, outstanding mechanical properties and possibility to combine the electrode components in a single material, hydrogels based on PVA, borax and PEDOT:PSS are highly promising for applications in bioelectronics and wearable electronics. STATEMENT OF SIGNIFICANCE: Soft materials with electrical conductivity are investigated for healthcare applications, such as electrodes to measure vital signs that can easily adapt to the shape and the movements of human skin. Conductive hydrogels (i.e. gels containing water) are ideal materials for this purpose due softness and flexibility. In this this work, we report hydrogels obtained mixing an electrically conductive polymer, a water-soluble biocompatible polymer and a salt. These materials show high adhesion on skin, electrical conductivity and ability to self-repair after a mechanical damage. These hydrogels were successfully used to fabricate electrode to measure cardiac and muscular electrical signals.

Tunable metacrylated hyaluronic acid-based hybrid bioinks for stereolithography 3D bioprinting.

Hyaluronic acid is a native extra-cellular matrix derivative that promises unique properties, such as anti-inflammatory response and cell-signaling with tissue-specific applications under its bioactive properties. Here, we investigate the importance of the duration of synthesis to obtain photocrosslinkable methacrylated hyaluronic acid (MeHA) with high degree of substitution. MeHA with high degree of substitution can result in rapid photocrosslinking and can be used as a bioink for stereolithographic (SLA) three dimensional 3D bioprinting. Increased degree of substitution results Our findings show that a ten-day synthesis results in an 88% degree of methacrylation (DM), whereas three-day and five-day syntheses result in 32% and 42% DM, respectively. The rheological characterization revealed an increased rate of photopolymerization with increasing DM. Further, we developed a hybrid bioink to overcome the non-cell-adhesive nature of MeHA by combining it with gelatin methacryloyl (GelMA) to fabricate 3D cell-laden hydrogel scaffolds. The hybrid bioink exhibited a 55% enhancement in stiffness compared to MeHA only and enabled cell-adhesion while maintaining high cell viability. Investigations also revealed that the hybrid bioink was a more suitable candidate for stereolithography (SLA) 3D bioprinting than MeHA because of its mechanical strength, printability, and cell-adhesive nature. This research lays out a firm foundation for the development of a stable hybrid bioink with MeHA and GelMA for first-ever use with SLA 3D bioprinting.

Colloidal Particle-Induced Microstructural Transition in Cellulose/Ionic Liquid/Water Mixtures.

The role of colloidal particles in enhancing the mechanical and thermal properties of liquid crystalline (LC) gels formed in microcrystalline cellulose/1-allyl-3-methylimidazolium chloride/water mixtures is experimentally investigated by means of rheology and polarized optical microscopy (POM). The overshoot in loss modulus and increase in the melting temperature of LC domains as observed in differential scanning calorimetry signal a stronger interaction of cellulose with both hydrophobic polystyrene and hydrophilic silica nanoparticles which in turn point to considerable amphiphilic nature of cellulose. The aggregation of nanoparticles observed by POM and the rheological behavior point to the development of a sample-spanning network of cellulose-nanoparticle clusters during the sol-gel transition with an increase in concentration of water. Furthermore, the LC gels obey Chambon-Winter (CW) criterion, indicating a self-similar gel network, except at very high particle loadings. Moreover, the LC domains show a temporal evolution into a space-spanning network of cellulose spherulites. The evolution process largely depends on the particle concentration, with highly loaded samples showing quicker evolution, which leads to a violation of the CW criterion. Furthermore, the temperature-induced microstructural transition (with and without shear) is also examined.

Rheology and microstructure of concentrated microcrystalline cellulose (MCC)/1-allyl-3-methylimidazolium chloride (AmimCl)/water mixtures.

Water added to a solution of microcrystalline cellulose (MCC) in 1-allyl-3-methylimidazolium chloride (AmimCl) reduces the solvent quality and causes significant changes in the flow properties and microstructure due to restructuring and aggregation of cellulose molecules. We report an experimental investigation by means of polarization optical microscopy (POM) and rheology of the distinct phases formed in 5-20 wt% MCC/AmimCl solutions due to the addition of water. With increase in the cellulose concentration, the MCC/AmimCl/water mixtures showed different morphologies such as the non-aligned cholesteric liquid crystalline (LC) domain, the coexistence of spherulite-like structures within the LC domain and a space-spanning network of spherulite-like structures at high concentrations of water. In situ microscopy during shear and POM observations pre and post shear revealed a significant increase in the size of the birefringent domains as the shear rate is increased, which continued to exist even after the cessation of shear. With an increase in the concentration of water, the zero shear viscosity of the MCC/AmimCl/water mixtures was found to go through a minimum, beyond which the aggregation of cellulose commenced. The corresponding oscillatory shear response showed a sol-gel transition with an increase in water concentration. Moreover, at high cellulose concentrations (12-20 wt%), the MCC/AmimCl/water gels exhibited self-similarity and followed the Chambon-Winter (CW) criterion. The similar phase behavior and rheological response observed for MCC dissolved in 1-butyl-3 methylimidazolium chloride (BmimCl) indicated the generality of the presented results.