No abstract

3866 www.MaterialsViews.com wileyonlinelibrary.com upon oxidation due to anion insertion and contract upon reduction due to anion expulsion or 2) contract on oxidation due to cation expulsion and expand on reduction due to cation insertion. Actuators based on CPs can be electrically controlled at low operating voltages (typically 1-3 V), continuously switched between expanded/ contracted states, and operate well in liquid electrolytes. Interfacing CPs with biological systems is also possible due to the demonstrated biocompatibility in vitro and in vivo . [ 5,6 ] To date, record-breaking CP actuators have been demonstrated to generate stresses as large as 100 MPa [ 7 ] and strains up to 40%, [ 8 ] although the generation of both high stress and high strain has yet to be achieved. Typical CP actuators can generate smaller, yet still notable, stresses of 1-5 MPa with strains on the order of 2%. [ 9 ] These impressive values have led to commercial interest in the development of several types of biomedical devices utilizing CP actuators. [ 1,9 ] However, current optimized device designs are not ideal for applications requiring implantation in vivo . Major diffi culties encountered when fabricating CP-based actuators arise from the fact that the bulk polymers are brittle and insoluble due to the extended conjugated backbone, which restricts the molding or processing of these materials into 3D structures. Therefore, the majority of CP-based actuators are synthesized via electropolymerization directly onto metal foils, [ 1 ] where the metal is often retained in the fi nal device. While metal incorporation helps minimize the iR drop across CP fi lms, these devices are typically limited to 2D fi lm architectures and have signifi cant problems with delamination. [ 10,11 ] While useful for surgical and external biomedical applications, incorporation of non-degradable or rigid components [ 1,12 ] that are incompatible with soft tissues severely limit the possible applications of CP actuators. In addition, device performance in a biologicallyrelevant environment is still unclear as the majority of studies utilize optimized electrolyte systems that employ toxic salts or organic solvents.To avoid the use of metals or rigid inorganic components in the fi nal device, all-polymeric actuators have been constructed by depositing CPs in situ during chemical polymerization onto several types of synthetic backing materials such as PVDF, [13][14][15] crosslinked PEO-based copolymers, [16][17][18][19][20] and polyurethane. [ 21 ] Single-component, metal-free, biocompatible, electromechanical actuator devices are fabricated using a composite material composed of silk fi broin and poly(pyrrole) (PPy). Chemical modifi cation techniques are developed to produce free-standing fi lms with a bilayer-type structure, with unmodifi ed silk on one side and an interpenetrating network (IPN) of silk and PPy on the other. The IPN formed between the silk and PPy prohibits delamination, resulting in a durable and fully biocompatible device. The electroch...

2D covalent organic frameworks (2D COFs) are structurally regular and synthetically versatile (>400 reported structures) layered 2D polymers with high surface areas (>2000 m 2 g-1) and robust covalent bonds. [1,2] This combination of properties makes 2D COFs promising for a variety of applications, 2D covalent organic frameworks (2D COFs) are a unique materials platform that combines covalent connectivity, structural regularity, and molecularly precise porosity. However, 2D COFs typically form insoluble aggregates, thus limiting their processing via additive manufacturing techniques. In this work, colloidal suspensions of boronate-ester-linked 2D COFs are used as a spray-coating ink to produce large-area 2D COF thin films. This method is synthetically general, with five different 2D COFs prepared as colloidal inks and subsequently spray-coated onto a diverse range of substrates. Moreover, this approach enables the deposition of multiple 2D COF materials simultaneously, which is not possible by polymerizing COFs on substrates directly. When combined with stencil masks, spray-coated 2D COFs are rapidly deposited as thin films larger than 200 cm 2 with line resolutions below 50 µm. To demonstrate that this deposition scheme preserves the desirable attributes of 2D COFs, spray-coated 2D COF thin films are incorporated as the active material in acoustic sensors. These 2D-COF-based sensors have a 10 ppb limit-of-quantification for trimethylamine, which places them among the most sensitive sensors for meat and seafood spoilage. Overall, this work establishes a scalable additive manufacturing technique that enables the integration of 2D COFs into thin-film device architectures.

Phase inversion is demonstrated as an effective method for engineering the microstructure of graphene films by exploiting the well-defined solubility characteristics of polymer dispersants. Drying of a tailored phase inversion ink containing a nonvolatile nonsolvent leads to gelation and subsequent pore formation, providing a promising strategy to tailor the porosity of the resulting graphene films. Graphene films with tunable porosity and electrical conductivity ranging from ∼1000 to ∼22 000 S/m are fabricated by this method. Moreover, this dry phase inversion technique is compatible with conventional coating and printing methods, allowing direct ink writing of porous graphene microsupercapacitor electrodes for energy storage applications. Overall, this method provides a straightforward and versatile strategy for engineering the microstructure of solution-processed nanomaterials.

Melanin is a natural pigment with a high refractive index and strong light absorption across the visible spectrum, making it an ideal material for producing structural colors. Here, we report non-iridescent structural color control via inkjet printing of self-assembled synthetic melanin nanoparticles (SMNPs). Adding silica shells to SMNPs allows for further tuning of both the hue and brightness of the resulting structural colors. The peak wavelengths show a linear dependence with the diameter of the nanoparticles, allowing correlation between ink composition and structural color using the Bragg–Snell law. Additionally, mixtures of SMNPs of different sizes result in colors with peak wavelengths that vary linearly with the mixing ratio in the ink, leading to diverse and predictable colors from one type of material. The morphology of the self-assembled SMNP structures is further controlled by the hydrophilicity of the substrate, providing another means for tailoring the structure and properties. Since structural colors are less susceptible to degradation than organic dyes, this work has implications for emerging sensing, display, and security applications.