Physically and chemically gelling hydrogel formulations based on poly(ethylene glycol) diacrylate and Poloxamer 407

Development of novel 3D printable inks for protein delivery

The application of 3D printing technology in the delivery of macromolecules, such as proteins and enzymes, is limited by the lack of suitable inks. In this study, we report the development of novel inks for 3D printing of constructs containing proteins while maintaining the activity of the proteins during and after printing. Different ink formulations containing Pluronic F-127 (20-35 %, w/v), trehalose (2-10 %, w/v) or mannitol, poly (ethylene glycol) diacrylate (PEGDA) (0 or 10 %, w/w), and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO, 0 or 0.2 mg/mL) were prepared for 3D-microextrusion printing. The F2 formulation that contained β-galactosidase (β-gal) as a model enzyme, Pluronic F-127 (30 %), and trehalose (10 %) demonstrated the desired viscosity, printability, and dose flexibility. The shear-thinning property of the F2 formulation enabled the printing of β-gal containing constructs with a good peak force during extrusion. After 3D printing, the enzymatic activity of the β-gal in the constructs was maintained for an extended period, depending on the construct design and storage conditions. For instance, there was a 50 % reduction in β-gal activity in the two-layer constructs, but only a 20 % reduction in the four-layer construct (i.e., 54.5 ± 1.2 % and 82.7 ± 9.9 %, respectively), after 4 days of storage. The β-gal activity in constructs printed from the F2 formulation was maintained for up to 20 days when stored in sealed bags at room temperatures (21 ± 2 °C), but not when stored unsealed in the same conditions (e.g., ∼60 % activity loss within 7 days). The β-gal from constructs printed from F2 started to release within 5 min and reached 100 % after 20 min. With the design flexibility offered by the 3D printing, the β-gal release from the constructs was delayed to 3 h by printing a backing layer of β-gal-free F5 ink on the constructs printed from the F2 ink. Finally, ovalbumin as an alternative protein was also incorporated in similar ink compositions. Ovalbumin exhibited a release profile like that of the β-gal, and the release can also be modified with different shape design and/or ink composition. In conclusion, ink formulations that possess desirable properties for 3D printing of protein-containing constructs while maintaining the protein activity during and after printing were developed.

Soft Sub-Structured Multi-Material Biosensor Hydrogels with Enzymes Retained by Plant Viral Scaffolds

An all-soft multi-material combination consisting of a hydrogel based on poly(ethylene glycol) (PEG) coated with spatially defined spots of gelatin methacryloyl (GM) containing selectively addressable viral nanorods is presented, and its basic application as a qualitative biosensor with reporter enzymes displayed on the tobacco mosaic virus (TMV) bioscaffolds within the GM is demonstrated. Biologically inert PEG supports are equipped with GM spots serving as biological matrix for enzymes clustered on TMV particles preventing diffusion out of the gel. For this multi-material combination, i) the PEG-based hydrogel surface is modified to achieve a clear boundary between coated and non-coated regions by introducing either isothiouronium or thiol groups. ii) Cross-linking of the GM spots is studied to achieve anchoring to the hydrogel surface. iii) The enzymes horseradish peroxidase or penicillinase (Pen) are conjugated to TMV and integrated into the GM matrix. In contrast to free enzymes, enzyme-decorated TMVs persist in GM spots and show sustained enzyme activity as evidenced by specific color reaction after 7 days of washing, and for Pen after 22 months after dry storage. Therefore, the integration of enzyme-coupled TMV into hydrogel matrices is a promising and versatile approach to obtaining reusable and analyte-specific sensor components.

Bacteria cellulose framework-supported solid composite polymer electrolytes for ambient-temperature lithium metal batteries

Composite polymer electrolyte (CPE) films with high room temperature ionic conductivity are urgently needed for the practical application of high-safety solid-state batteries (SSBs). Here, a flexible polymer-polymer CPE thin film reinforced by a three-dimensional (3D) bacterial cellulose (BC) framework derived from natural BC hydrogel was prepared via thein situphoto-polymerization method. The BC film was utilized as the supporting matrix to ensure high flexibility and mechanical strength. The BC-CPE attained a high room temperature ionic conductivity of 1.3 × 10^-4S cm^-1. The Li∣BC-CPE∣Li symmetric cell manifested stable cycles of more than 1200 h. The LCO∣BC-CPE∣Li full cell attained an initial discharge specific capacity of 128.7 mAh g^-1with 82.6% discharge capacity retention after 150 cycles at 0.2 C under room temperature. The proposed polymer-polymer CPE configuration represents a promising route for manufacturing environmental SSBs, especially since cellulose biomaterials are abundant in nature.

Intertwined Nanosponge Solid-State Polymer Electrolyte for Rollable and Foldable Lithium-Ion Batteries

Herein, we report a straigthforward procedure to prepare an excellent intertwined nanosponge solid-state polymer electrolyte (INSPE) for highly bendable, rollable, and foldable lithium-ion batteries (LIBs). The mechanically reliable and electrochemically superior INSPE is conjugated with intertwined nanosponge (IN) poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) and ion-conducting polymer electrolyte (PE) containing poly(ethylene glycol) diacrylate (PEGDA), succinonitrile (SCN) plasticizer, and lithium bis(trifluoromethanesilfonyl)imide (LiTFSI). The conjugated INSPE has both high strength with great flexibility (tensile strength of 2.1 MPa, elongation of 36.7%), and excellent ionic conductivity (1.04 × 10^-3 S·cm^-1, similar to the values of liquid electrolytes). As a result of such special combination, the as-prepared INSPE retains almost 100% of its ionic conductivity when subjected to many types of severe mechanical deformations. Therefore, the INSPE is successfully applied to bendable, rollable, and foldable LIBs that show excellent energy storage performance despite the intense mechanical deformations.

Extrusion-Based 3D Printing of Poly(ethylene glycol) Diacrylate Hydrogels Containing Positively and Negatively Charged Groups

Hydrogels are an interesting class of materials used in extrusion-based 3D printing, e.g., for drug delivery or tissue engineering. However, new hydrogel formulations for 3D printing as well as a detailed understanding of crucial formulation properties for 3D printing are needed. In this contribution, hydrogels based on poly(ethylene glycol) diacrylate (PEG-DA) and the charged monomers 3-sulfopropyl acrylate and [2-(acryloyloxy)ethyl]trimethylammonium chloride are formulated for 3D printing, together with Poloxamer 407 (P407). Chemical curing of formulations with PEG-DA and up to 5% (w/w) of the charged monomers was possible without difficulty. Through careful examination of the rheological properties of the non-cured formulations, it was found that flow properties of formulations with a high P407 concentration of 22.5% (w/w) possessed yield stresses well above 100 Pa together with pronounced shear thinning behavior. Thus, those formulations could be processed by 3D printing, as demonstrated by the generation of pyramidal objects. Modelling of the flow profile during 3D printing suggests that a plug-like laminar flow is prevalent inside the printer capillary. Under such circumstances, fast recovery of a high vicosity after material deposition might not be necessary to guarantee shape fidelity because the majority of the 3D printed volume does not face any relevant shear stress during printing.

Covalent incorporation of tobacco mosaic virus increases the stiffness of poly(ethylene glycol) diacrylate hydrogels

Hydrogels are versatile materials, finding applications as adsorbers, supports for biosensors and biocatalysts or as scaffolds for tissue engineering. A frequently used building block for chemically cross-linked hydrogels is poly(ethylene glycol) diacrylate (PEG-DA). However, after curing, PEG-DA hydrogels cannot be functionalized easily. In this contribution, the stiff, rod-like tobacco mosaic virus (TMV) is investigated as a functional additive to PEG-DA hydrogels. TMV consists of more than 2000 identical coat proteins and can therefore present more than 2000 functional sites per TMV available for coupling, and thus has been used as a template or building block for nano-scaled hybrid materials for many years. Here, PEG-DA (M n = 700 g mol-1) hydrogels are combined with a thiol-group presenting TMV mutant (TMVCys). By covalent coupling of TMVCys into the hydrogel matrix via the thiol-Michael reaction, the storage modulus of the hydrogels is increased compared to pure PEG-DA hydrogels and to hydrogels containing wildtype TMV (wt-TMV) which is not coupled covalently into the hydrogel matrix. In contrast, the swelling behaviour of the hydrogels is not altered by TMVCys or wt-TMV. Transmission electron microscopy reveals that the TMV particles are well dispersed in the hydrogels without any large aggregates. These findings give rise to the conclusion that well-defined hydrogels were obtained which offer the possibility to use the incorporated TMV as multivalent carrier templates e.g. for enzymes in future studies.

Real Time-NIR/MIR-Photorheology: A Versatile Tool for the in Situ Characterization of Photopolymerization Reactions

In photopolymerization reactions, mostly multifunctional monomers are employed, as they ensure fast reaction times and good final mechanical properties of the cured materials. Drawing conclusions about the influence of the components and curing conditions on the mechanical properties of the subsequently formed insoluble networks is challenging. Therefore, an in situ observation of chemical and mechanical characteristics during the photopolymerization reaction is desired. By coupling of an infrared spectrometer with a photorheometer, a broad spectrum of different photopolymerizable formulations can be analyzed during the curing reaction. The rheological information (i.e., time to gelation, final modulus, shrinkage force) can be derived from a parallel plate rheometer equipped with a UV- and IR-translucent window (glass for NIR and CaF₂ window for MIR). Chemical information (i.e., conversion at the gel point and final conversion) is gained by monitoring the decrease of the corresponding IR-peak for the reactive monomer unit (e.g., C═C double bond peak for (meth)acrylates, H-S thiol and C═C double bond peak in thiol-ene systems, C-O epoxy peak for epoxy resins). Depending on the relative concentration of reactive functional groups in the sample volume and the intensity of the IR signal, the conversion can be monitored in the near-infrared region (e.g., acrylate double bonds, epoxy groups) or the MIR region (e.g., thiol signal). Moreover, an integrated Peltier element and external heating hood enable the characterization of photopolymerization reactions at elevated temperatures, which also widens the window of application to resins that are waxy or solid at ambient conditions. By switching from water to heavy water, the chemical conversion during photopolymerization of hydrogel precursor formulations can also be examined. Moreover, this device could also represent an analytical tool for a variety of thermally and redox initiated systems.