Biomedical Applications of Recombinant Silk-Based Materials

Abstract

Silk is mostly known as a luxurious textile, which originates from silkworms first cultivated in China. A deeper look into the variety of silk reveals that it can be used for much more, in nature and by humanity. For medical purposes, natural silks were recognized early as a potential biomaterial for surgical threads or wound dressings; however, as biomedical engineering advances, the demand for high-performance, naturally derived biomaterials becomes more pressing and stringent. A common problem of natural materials is their large batch-to-batch variation, the quantity available, their potentially high immunogenicity, and their fast biodegradation. Some of these common problems also apply to silk; therefore, recombinant approaches for producing silk proteins have been developed. There are several research groups which study and utilize various recombinantly produced silk proteins, and many of these have also investigated their products for biomedical applications. This review gives a critical overview over of the results for applications of recombinant silk proteins in biomedical engineering.

Recombinant silk proteins are an interesting class of biopolymers that are well-suited for biomedical applications. In this review, studies on the use of recombinant silk proteins are summarized, and important conclusions are given. The review also highlights the best features of recombinant silk proteins as a biomaterial, and exciting future directions.

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Transient SHG Imaging on Ultrafast Carrier Dynamics of MoS2 Nanosheets

Abstract

Understanding the collaborative behaviors of the excitons and phonons that result from light–matter interactions is important for interpreting and optimizing the underlying fundamental physics at work in devices made from atomically thin materials. In this study, the generation of exciton-coupled phonon vibration from molybdenum disulfide (MoS₂) nanosheets in a pre-excitonic resonance condition is reported. A strong rise-to-decay profile for the transient second-harmonic generation (TSHG) of the probe pulse is achieved by applying substantial (20%) beam polarization normal to the nanosheet plane, and tuning the wavelength of the pump beam to the absorption of the A-exciton. The time-dependent TSHG signals clearly exhibit acoustic phonon generation at vibration modes below 10 cm⁻¹ (close to the Γ point) after the photoinduced energy is transferred from exciton to phonon in a nonradiative fashion. Interestingly, by observing the TSHG signal oscillation period from MoS₂ samples of varying thicknesses, the speed of the supersonic waves generated in the out-of-plane direction (Mach 8.6) is generated. Additionally, TSHG microscopy reveals critical information about the phase and amplitude of the acoustic phonons from different edge chiralities (armchair and zigzag) of the MoS₂ monolayers. This suggests that the technique could be used more broadly to study ultrafast physics and chemistry in low-dimensional materials and their hybrids with ultrahigh fidelity.

Imaging transient second-harmonic generation (TSHG) enables spatially resolved analysis of exciton-coupled phonon-vibration from atomically layered MoS₂. The TSHG microscopy reveals critical information about frequency, phase, and amplitude of acoustic phonons in a monolayer and multilayer MoS₂ sample simultaneously, thus uncovering the speed of sounds generated in the materials and chiral-dependent acoustic phonon vibrations.

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Tunable Tribotronic Dual-Gate Logic Devices Based on 2D MoS2 and Black Phosphorus

Abstract

With the Moore's law hitting the bottleneck of scaling-down in size (below 10 nm), personalized and multifunctional electronics with an integration of 2D materials and self-powering technology emerge as a new direction of scientific research. Here, a tunable tribotronic dual-gate logic device based on a MoS₂ field-effect transistor (FET), a black phosphorus FET and a sliding mode triboelectric nanogenerator (TENG) is reported. The triboelectric potential produced from the TENG can efficiently drive the transistors and logic devices without applying gate voltages. High performance tribotronic transistors are achieved with on/off ratio exceeding 106 and cutoff current below 1 pA μm^–1. Tunable electrical behaviors of the logic device are also realized, including tunable gains (improved to ≈13.8) and power consumptions (≈1 nW). This work offers an active, low-power-consuming, and universal approach to modulate semiconductor devices and logic circuits based on 2D materials with TENG, which can be used in microelectromechanical systems, human–machine interfacing, data processing and transmission.

A tunable tribotronic dual-gate logic device based on a MoS₂ field-effect transistor, black phosphorus transistors, and a sliding mode triboelectric nanogenerator (TENG) is reported. Triboelectric potential produced from the TENG can efficiently drive the transistors and inverters without applying gate voltages. Tunable electrical behaviors of the inverters are also realized, including tunable gains and power consumptions.

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A Magnetofluorescent Carbon Dot Assembly as an Acidic H2O2-Driven Oxygenerator to Regulate Tumor Hypoxia for Simultaneous Bimodal Imaging and Enhanced Photodynamic Therapy

Abstract

Recent studies indicate that carbon dots (CDs) can efficiently generate singlet oxygen (¹O₂) for photodynamic therapy (PDT) of cancer. However, the hypoxic tumor microenvironment and rapid consumption of oxygen in the PDT process will severely limit therapeutic effects of CDs due to the oxygen-dependent PDT. Thus, it is becoming particularly important to develop a novel CD as an in situ tumor oxygenerator for overcoming hypoxia and substantially enhancing the PDT efficacy. Herein, for the first time, magnetofluorescent Mn-CDs are successfully prepared using manganese(II) phthalocyanine as a precursor. After cooperative self-assembly with DSPE-PEG, the obtained Mn-CD assembly can be applied as a smart contrast agent for both near-infrared fluorescence (FL) (maximum peak at 745 nm) and T₁-weighted magnetic resonance (MR) (relaxivity value of 6.97 mM⁻¹ s⁻¹) imaging. More interestingly, the Mn-CD assembly can not only effectively produce ¹O₂ (quantum yield of 0.40) but also highly catalyze H₂O₂ to generate oxygen. These collective properties of the Mn-CD assembly enable it to be utilized as an acidic H₂O₂-driven oxygenerator to increase the oxygen concentration in hypoxic solid tumors for simultaneous bimodal FL/MR imaging and enhanced PDT. This work explores a new biomedical use of CDs and provides a versatile carbon nanomaterial candidate for multifunctional nanotheranostic applications.

A magnetofluorescent Mn-CD assembly is designed and prepared as an acidic H₂O₂-driven oxygenerator to regulate the hypoxic tumor microenvironment for simultaneous bimodal fluorescence/T₁-weighted magnetic resonance imaging and enhanced photodynamic therapy of solid tumors. This work demonstrates a new biomedical use of CDs and provides a versatile carbon nanomaterial candidate for multifunctional nanotheranostic applications.

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Breaking the Current-Retention Dilemma in Cation-Based Resistive Switching Devices Utilizing Graphene with Controlled Defects

Abstract

Cation-based resistive switching (RS) devices, dominated by conductive filaments (CF) formation/dissolution, are widely considered for the ultrahigh density nonvolatile memory application. However, the current-retention dilemma that the CF stability deteriorates greatly with decreasing compliance current makes it hard to decrease operating current for memory application and increase driving current for selector application. By centralizing/decentralizing the CF distribution, this current-retention dilemma of cation-based RS devices is broken for the first time. Utilizing the graphene impermeability, the cation injecting path to the RS layer can be well modulated by structure-defective graphene, leading to control of the CF quantity and size. By graphene defect engineering, a low operating current (≈1 µA) memory and a high driving current (≈1 mA) selector are successfully realized in the same material system. Based on systematically materials analysis, the diameter of CF, modulated by graphene defect size, is the major factor for CF stability. Breakthrough in addressing the current-retention dilemma will instruct the future implementation of high-density 3D integration of RS memory immune to crosstalk issues.

Cation-based resistive switching (RS) devices are widely considered for memory and selector application in the one-selector-one-resistor (1S1R) scheme for the next-generation nonvolatile memory application. Here, centralizing/decentralizing the conductive filament distribution is proposed to break the current-retention dilemma of cation-based RS device by modulating the filament stability. Ultimately, this method helps to realize low-current memory and high On-state current selector for 1S1R array application.

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In Situ Grown Epitaxial Heterojunction Exhibits High-Performance Electrocatalytic Water Splitting

Abstract

Electrocatalytic performance can be enhanced by engineering a purposely designed nanoheterojunction and fine-tuning the interface electronic structure. Herein a new approach of developing atomic epitaxial in-growth in Co-Ni₃N nanowires array is devised, where a nanoconfinement effect is reinforced at the interface. The Co-Ni₃N heterostructure array is formed by thermal annealing NiCo₂O₄ precursor nanowires under an optimized condition, during which the nanowire morphology is retained. The epitaxial in-growth structure of Co-Ni₃N at nanometer scale facilitates the electron transfer between the two different domains at the epitaxial interface, leading to a significant enhancement in catalytic activities for both hydrogen and oxygen evolution reactions (10 and 16 times higher in the respective turn-over frequency compared to Ni₃N-alone nanorods). The interface transfer effect is verified by electronic binding energy shift and density functional theory (DFT) calculations. This nanoconfinement effect occurring during in situ atomic epitaxial in-growth of the two compatible materials shows an effective pathway toward high-performance electrocatalysis and energy storages.

Epitaxial interface boosts performance Co-Ni₃N nanorod arrays with an atomic epitaxial interface are synthesized which exhibit significant enhancement in catalytic activities for both hydrogen and oxygen evolution reactions. A nanoconfinement effect is proposed to facilitate the interface charge transfer.

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Graphene–Graphene Interactions: Friction, Superlubricity, and Exfoliation

Abstract

Graphite's lubricating properties due to the "weak" interactions between individual layers have long been known. However, these interactions are not weak enough to allow graphite to readily exfoliate into graphene on a large scale. Separating graphite layers down to a single sheet is an intense area of research as scientists attempt to utilize graphene's superlative properties. The exfoliation and processing of layered materials is governed by the friction between layers. Friction on the macroscale can be intuitively understood, but there is little understanding of the mechanisms involved in nanolayered materials. Using molecular dynamics and a new forcefield, graphene's unusual behavior in a superlubric state is examined, and the energy dissipated between two such surfaces sliding past each other is shown. The dependence of friction on temperature and surface roughness is described, and agreement with experiment is reported. The accuracy of the simulated behavior enables the processes that drive exfoliation of graphite into individual graphene sheets to be described. Taking into account the friction between layers, a peeling mechanism of exfoliation is predicted to be of lower energy cost than shearing.

Graphite's lubricating properties due to the "weak" interactions between individual layers, have long been known. However, graphene still cannot be exfoliated on a large scale. A new classical molecular dynamics forcefield is applied to unravel the remarkable interactions between graphene sheets. It is found that peeling graphene sheets apart, rather than shearing, is an easier route to exfoliation.

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