3D Printing Mechanically Robust and Transparent Polyurethane Elastomers for Stretchable Electronic Sensors.

Advanced stretchable electronic sensors with a complex structure place higher requirements on the mechanical properties and manufacturing process of the stretchable substrate materials. Herein, three kinds of polyurethane acrylate oligomers were synthesized successfully and mixed with a commercial acrylate monomer (isobornyl acrylate) to prepare photocurable resins with a low viscosity for a digital light processing three-dimensional (3D) printer without custom equipment. Results showed that the resin containing poly(tetrahydrofuran) units (PPTMGA-40) exhibited optimal mechanical properties and shape recoverability. The tensile strength and elongation at break of PPTMGA-40 were 15.7 MPa and 414.3%, respectively. The unprecedented fatigue resistance of PPTMGA-40 allowed it to withstand 100 compression cycles at 80% strain without fracture. The transmittance of PPTMGA-40 reached 89.4% at 550 nm, showing high transparency. An ionic hydrogel was coated on the surface of 3D-printed structures to fabricate stretchable sensors, and their conductivity, transparency, and mechanical performance were characterized. A robust piezoresistive strain sensor with a high strength (∼6 MPa) and a wearable finger guard sensor were fabricated, demonstrating that this hydrogel-elastomer system can meet the requirements of applications for advanced stretchable electronic sensors and expand the usage scope.

Structure-Property Relationship of Stereolithography Resins Containing Polysiloxane Core-Shell Nanoparticles.

Stereolithography (SL) is an additive manufacturing technique for fabricating bulk and delicate objects layer by layer using UV-curable resin. However, epoxy-based photocurable resins used in SL printers are commonly brittle due to the high cross-linking density, thus restricting the widespread adoption of SL. In an effort to overcome this drawback, this paper details an approach of toughening the resulting workpieces by incorporating polysiloxane core-shell nanoparticles (SCSP) into an epoxy-based, photocurable formulation. This approach attempted to attain both thermal stabilities and transparency qualities comparable to that of resin without SCSP. This work systematically analyzed how the shell thickness of the SCSP impacted the final properties of the printed product. Introducing 5% w/w SCSP with a diameter of approximately 132 nm into the resin improved strain at break measured by tensile and flexural tests by 745.5 and 248.6%, respectively, and increased the fracture toughness by 166.3%. Owing to the advantages of toughness, thermal stabilities, transparency, and high accuracy of epoxy-based photocurable resin with SCSP, the 3D printing nanocomposite developed here is capable of preparing a poly(methyl methacrylate) (PMMA)-like workpiece with a commercial SL 3D printer. These results may expand the scope of the application of 3D printing in a wide variety of industries.

Dynamic Imine Bond-Based Shape Memory Polymers with Permanent Shape Reconfigurability for 4D Printing.

Shape memory polymer (SMP)-based 4D printing combines the advantages of SMP and 3D printing to form active materials with delicate structure. Nowadays, studies of SMP-based 4D printing materials mainly focus on cross-linked (meth)acrylate of which the permanent shape cannot be changed for their covalent linkage, limiting the usage of 4D printing materials. In this paper, a novel (meth)acrylate monomer with an aldehyde group (2-(methacryloyloxy)ethyl 4-formylbenzoate, MEFB) and hyperbranched cross-linker (HPASi) are synthesized to build (meth)acrylate systems (IEMSis) with dynamic imine bonds for 4D printing. The flexible chain structure of HPASi significantly enhances the toughness of IEMSis, which is 33-97-fold higher than that of the one without HPASi (IEM). The addition of HPASi also endows IEMSis good shape memory properties, and the shape fixity and shape recovery ratios of them are 97.5-97.6 and 91.4-93.7%, respectively. At the same time, IEMSis can undergo a stress relaxation process by dynamic exchanges of imine bonds under relatively mild conditions without a catalyst to acquire an ability of permanent shape reconfiguration. The shape retention ratio of IEMSi3 is 84.3%. In addition, the 4D-printed structures displayed here indicate that these 4D printing systems have a myriad of potential applications including aerospace structures, soft robotic grippers, smart electron switches, and intelligent packaging, while the reconfigurability shown by IEMSi3 will expand the scope of application fields of 4D printing materials.

Colorimetric and ratiometric pH responses by the protonation of phenolate within hemicyanine.

Compared to the nitrogen-reaction based pH optical responsive compounds, oxygen-reaction related pH sensors have attracted less attention. In this paper, hemicyanine based pH probes are designed by establishing the equilibrium between phenolate and phenol, and their reversible absorption and emission responses towards pH are evaluated. The indolium-phenol based tetramethylene hemicyanine (1a) has colorimetric responses at 455 and 578 nm due to the protonating and deprotonating processes; its emission spectra shows ratiometric changes at 594 and 654 nm with large Stokes shifts under acidic (139 nm) and basic conditions (76 nm). The bromide substituent of the hemicyanine (1b) has a lower pKa value compared with unsubstituted hemicyanine (1a), which suggests that adjustable pKa can be achieved by the modification of electron withdrawing groups. The theoretical calculations based on the density functional theory (DFT) were also used to explain the optical properties. Moreover, the in cellulo fluorescence imaging shows that the hemicyanine (1a) can be used for the detection of intracellular pH levels.

Optical properties of hemicyanines with terminal amino groups and their applications in near-infrared fluorescent imaging of nucleoli.

Long wavelength chromophores are of interest for material and biological applications. Six hemicyanine dyes (1a-f) constructed with oxazolo-pyridinium, indolium, benzooxazolium, or benzothiazolium groups and terminal anilino groups with multiple methylenes were prepared. They all exhibited fluorescence from 600 to 850 nm with emission maxima in the near-infrared region. The Stokes shifts became larger in more polar solvents, likely as a result of the greater dipole moments and geometry relaxations in the excited states as suggested by (TD)DFT calculations. The hemicyanine dye with the benzothiazolium group (1d) exhibited the best photostability and thermal stability among those tested. The potential application of 1d as a nucleic acid-staining fluorophore was evaluated. Fluorescent responses were observed from 575 to 850 nm when 1d was titrated with commercially available DNA and RNA. Dye 1d was also used in the selective fluorescent staining of RNA in live HeLa, KB and V79 cells. The results indicated that 1d can be used as a near-infrared fluorescent probe for nucleolus imaging in live cells.