Piezoelectric nanoribbons for monitoring cellular deformations

Molecular Engineering of Amino Acid Crystals with Enhanced Piezoelectric Performance for Biodegradable Sensors

Amino acid crystals have emerged as promising piezoelectric materials for biodegradable and biocompatible sensors; however, their relatively low piezoelectric coefficients constrain practical applications. Here, we introduce a fluoro-substitution strategy to overcome this limitation and enhance the piezoelectric performance of amino acid crystals. Specifically, we substituted hydrogen atoms on the aromatic rings of L-tryptophan, L-phenylalanine, and N-Cbz-L-phenylalanine with fluorine, resulting in significantly elevated piezoelectric coefficients. Density functional theory calculations further indicate that fluorination strengthens polarization by modifying molecular dipole moments. Consequently, these fluoro-substituted crystals achieve piezoelectric coefficients of up to 50.36 pm/V, surpassing those of other organic piezoelectric materials such as polyvinylidene fluoride (PVDF), poly(L-lactic acid) (PLLA), and gelatin. When integrated into flexible, biodegradable force sensors, the fluoro-substituted crystals exhibit a broad sensing range, high sensitivity, and stable in vivo operation over extended periods. This work establishes a versatile route for boosting piezoelectricity in biomaterials, thereby broadening their scope in biomedical applications.

Fully biodegradable hierarchically designed high-performance nanocellulose piezo-arrays

While piezoelectric sensing and energy-harvesting devices still largely rely on inorganic components, biocompatible and biodegradable piezoelectric materials, such as cellulose nanocrystals, might constitute optimal and sustainable building blocks for a variety of applications in electronics and transient implants. To this aim, however, effective methods are needed to position cellulose nanocrystals in large and high-performance architectures. Here, we report on scalable assemblies of cellulose nanocrystals in multilayered piezoelectric systems with exceptional response, for various application scopes. The submicrometer patterning with effective-flow topography and multilayer stacking promote piezoelectric performance. Record output power and pressure sensitivity in the gentle touch range are obtained in flexible, fully biodegradable systems with stable piezoelectric properties and demonstrated compatibility with different cell lines and implanted devices. These architectures offer new design principles for piezoelectric sustainable materials and for realizing an innovative class of practical components for mechanical energy harvesting and biologically relevant wearables and implants.

Intelligent sensing for the autonomous manipulation of microrobots toward minimally invasive cell surgery

The physiology and pathogenesis of biological cells have drawn enormous research interest. Benefiting from the rapid development of microfabrication and microelectronics, miniaturized robots with a tool size below micrometers have widely been studied for manipulating biological cells in vitro and in vivo. Traditionally, the complex physiological environment and biological fragility require human labor interference to fulfill these tasks, resulting in high risks of irreversible structural or functional damage and even clinical risk. Intelligent sensing devices and approaches have been recently integrated within robotic systems for environment visualization and interaction force control. As a consequence, microrobots can be autonomously manipulated with visual and interaction force feedback, greatly improving accuracy, efficiency, and damage regulation for minimally invasive cell surgery. This review first explores advanced tactile sensing in the aspects of sensing principles, design methodologies, and underlying physics. It also comprehensively discusses recent progress on visual sensing, where the imaging instruments and processing methods are summarized and analyzed. It then introduces autonomous micromanipulation practices utilizing visual and tactile sensing feedback and their corresponding applications in minimally invasive surgery. Finally, this work highlights and discusses the remaining challenges of current robotic micromanipulation and their future directions in clinical trials, providing valuable references about this field.

Flexible self-powered bioelectronics enables personalized health management from diagnosis to therapy

Flexible self-powered bioelectronics (FSPBs), incorporating flexible electronic features in biomedical applications, have revolutionized the human-machine interface since they hold the potential to offer natural and seamless human interactions while overcoming the limitations of battery-dependent power sources. Furthermore, as biosensors or actuators, FSPBs can dynamically monitor physiological signals to reveal real-time health abnormalities and provide timely and precise treatments. Therefore, FSPBs are increasingly shaping the landscape of health monitoring and disease treatment, weaving a sophisticated and personalized bond between humans and health management. Here, we examine the recent advanced progress of FSPBs in developing working mechanisms, design strategies, and structural configurations toward personalized health management, emphasizing its role in clinical medical scenarios from biophysical/biochemical sensors for sensing diagnosis to robust/biodegradable actuators for intervention therapy. Future perspectives on the challenges and opportunities in emerging multifunctional FSPBs for the next-generation health management systems are also forecasted.

(2+δ)-dimensional theory of the electromechanics of lipid membranes: Electrostatics

The coupling of electric fields to the mechanics of lipid membranes gives rise to intriguing electromechanical behavior, as, for example, evidenced by the deformation of lipid vesicles in external electric fields. Electromechanical effects are relevant for many biological processes, such as the propagation of action potentials in axons and the activation of mechanically gated ion channels. Currently, a theoretical framework describing the electromechanical behavior of arbitrarily curved and deforming lipid membranes does not exist. Purely mechanical models commonly treat lipid membranes as two-dimensional surfaces, ignoring their finite thickness. While holding analytical and numerical merit, this approach cannot describe the coupling of lipid membranes to electric fields and is thus unsuitable for electromechanical models. In a sequence of articles, we derive an effective surface theory of the electromechanics of lipid membranes, called the (2+δ)-dimensional theory, which has the advantages of surface descriptions while accounting for finite thickness effects. The present article proposes a generic dimension reduction procedure relying on low-order spectral expansions. This procedure is applied to the electrostatics of lipid membranes to obtain the (2+δ)-dimensional theory that captures potential differences across and electric fields within lipid membranes. This model is tested on different geometries relevant for lipid membranes, showing good agreement with the corresponding three-dimensional electrostatics theory.

Piezoelectricity, Pyroelectricity, and Ferroelectricity in Biomaterials and Biomedical Applications

Piezoelectric, pyroelectric, and ferroelectric materials are considered unique biomedical materials due to their dielectric crystals and asymmetric centers that allow them to directly convert various primary forms of energy in the environment, such as sunlight, mechanical energy, and thermal energy, into secondary energy, such as electricity and chemical energy. These materials possess exceptional energy conversion ability and excellent catalytic properties, which have led to their widespread usage within biomedical fields. Numerous biomedical applications have demonstrated great potential with these materials, including disease treatment, biosensors, and tissue engineering. For example, piezoelectric materials are used to stimulate cell growth in bone regeneration, while pyroelectric materials are applied in skin cancer detection and imaging. Ferroelectric materials have even found use in neural implants that record and stimulate electrical activity in the brain. This paper reviews the relationship between ferroelectric, piezoelectric, and pyroelectric effects and the fundamental principles of different catalytic reactions. It also highlights the preparation methods of these three materials and the significant progress made in their biomedical applications. The review concludes by presenting key challenges and future prospects for efficient catalysts based on piezoelectric, pyroelectric, and ferroelectric nanomaterials for biomedical applications.

Black Phosphorus-Based Conductive Hydrogels Assisted by Electrical Stimulus for Skin Tissue Engineering

Conductive hydrogels have shown great potential in wound healing and skin tissue engineering, owing to their electroactive, mechanical, and chemical properties. However, it still remains as a challenge to incorporate other functions into conductive hydrogels, such as antibacterial ability, controllable drug release, and biodegradability. In this study, a black phosphorus-based conductive hydrogel (HA-DA@BP) is prepared by an amidation reaction coupled with a coordination of Fe3+ -catechol. The hydrogel could be changed from the sol phase to the gel phase under electrical stimulus (ES). The results show that BP could be released under slight acidity, which is cell compatible but could achieve synergistic electrical antibacterial action and promote wound healing. This study proves that BP is a strong candidate for electroactive materials and provides a new insight for the development of BP-based biomedical materials in skin tissue engineering.

Piezoelectricity of Bi2Se3 Nanosheet

Bi2Se3, one of the most extensively studied topological insulators, has received significant attention, and abundant research has been dedicated to exploring its surface electronic properties. However, little attention has been given to its piezoelectric properties. Herein, we investigate the piezoelectric response in a five-layer Bi2Se3 nanosheet using scanning probe microscopy (SPM) techniques. The piezoelectricity of Bi2Se3 is characterized using both conventional piezoresponse force microscopy (PFM) and a sequential excitation scanning probe microscopy (SE-SPM) technique. To confirm the linear piezoelectricity of Bi2Se3 two-dimensional materials, measurements of point-wise linear and quadratic electromechanical responses are carried out. Furthermore, the presence of polarization and relaxation is confirmed through hysteresis loops. As expected, the Bi2Se3 nanosheet exhibits an electromechanical solid response. Due to the inevitable loss of translational symmetry at the crystal edge, the lattice of the odd-layer Bi2Se3 nanosheet is noncentrosymmetric, indicating its potential for linear piezoelectricity. This research holds promise for nanoelectromechanical systems (NEMS) applications and future nanogenerators.

Stretchable polymer composites with ultrahigh piezoelectric performance

Flexible piezoelectric materials capable of withstanding large deformation play key roles in flexible electronics. Ferroelectric ceramics with a high piezoelectric coefficient are inherently brittle, whereas polar polymers exhibit a low piezoelectric coefficient. Here we report a highly stretchable/compressible piezoelectric composite composed of ferroelectric ceramic skeleton, elastomer matrix and relaxor ferroelectric-based hybrid at the ceramic/matrix interface as dielectric transition layers, exhibiting a giant piezoelectric coefficient of 250 picometers per volt, high electromechanical coupling factor k_eff of 65%, ultralow acoustic impedance of 3MRyl and high cyclic stability under 50% compression strain. The superior flexibility and piezoelectric properties are attributed to the electric polarization and mechanical load transfer paths formed by the ceramic skeleton, and dielectric mismatch mitigation between ceramic fillers and elastomer matrix by the dielectric transition layer. The synergistic fusion of ultrahigh piezoelectric properties and superior flexibility in these polymer composites is expected to drive emerging applications in flexible smart electronics.

Highly piezoelectric, biodegradable, and flexible amino acid nanofibers for medical applications

Amino acid crystals are an attractive piezoelectric material as they have an ultrahigh piezoelectric coefficient and have an appealing safety profile for medical implant applications. Unfortunately, solvent-cast films made from glycine crystals are brittle, quickly dissolve in body fluid, and lack crystal orientation control, reducing the overall piezoelectric effect. Here, we present a material processing strategy to create biodegradable, flexible, and piezoelectric nanofibers of glycine crystals embedded inside polycaprolactone (PCL). The glycine-PCL nanofiber film exhibits stable piezoelectric performance with a high ultrasound output of 334 kPa [under 0.15 voltage root-mean-square (Vrms)], which outperforms the state-of-the-art biodegradable transducers. We use this material to fabricate a biodegradable ultrasound transducer for facilitating the delivery of chemotherapeutic drug to the brain. The device remarkably enhances the animal survival time (twofold) in mice-bearing orthotopic glioblastoma models. The piezoelectric glycine-PCL presented here could offer an excellent platform not only for glioblastoma therapy but also for developing medical implantation fields.

Green Fabrication of Freestanding Piezoceramic Films for Energy Harvesting and Virus Detection

Most electronics such as sensors, actuators and energy harvesters need piezoceramic films to interconvert mechanical and electrical energy. Transferring the ceramic films from their growth substrates for assembling electronic devices commonly requires chemical or physical etching, which comes at the sacrifice of the substrate materials, film cracks, and environmental contamination. Here, we introduce a van der Waals stripping method to fabricate large-area and freestanding piezoceramic thin films in a simple, green, and cost-effective manner. The introduction of the quasi van der Waals epitaxial platinum layer enables the capillary force of water to drive the separation process of the film and substrate interface. The fabricated lead-free film, [Formula: see text] (BCZT), shows a high piezoelectric coefficient d33 = 209 ± 10 pm V-1 and outstanding flexibility of maximum strain 2%. The freestanding feature enables a wide application scenario, including micro energy harvesting, and covid-19 spike protein detection. We further conduct a life cycle analysis and quantify the low energy consumption and low pollution of the water-based stripping film method.

Nib-Assisted Coaxial Electrohydrodynamic Jet Printing for Nanowires Deposition

This paper presents the concrete design of nanowires under the precise size and morphology that play a crucial role in the practical operation of the micro/nano devices. A straightforward and operative method termed as nib-assistance coaxial electrohydrodynamic (CEHD) printing technology was proposed. It extracts the essence of a nib-assistance electric field intensity to enhance and lessen the internal fluid reflux of the CEHD jet. The experiments were performed to add microparticles into the inner liquid to indicate the liquid flow consistency within the coaxial jet. The reflux in the coaxial jet was observed for the first time in experiments. The nanowires with a minimum size of 70 nm were printed under optimum experimental conditions. The nanopatterns contained aligned nanowires structures with diameters much smaller than the inner diameter of nozzle, relying on the coaxial nib-assisted technique. The printed results revealed that the nib-assisted CEHD printing technique offers a certain level high quality for application of NEMS system.

Plant nanobionics: Fortifying food security via engineered plant productivity

The world's human population is increasing exponentially, increasing the demand for high-quality food sources. As a result, there is a major global concern over hunger and malnutrition in developing countries with limited food resources. To address this issue, researchers worldwide must focus on developing improved crop varieties with greater productivity to overcome hunger. However, conventional crop breeding methods require extensive periods to develop new varieties with desirable traits. To tackle this challenge, an innovative approach termed plant nanobionics introduces nanomaterials (NMs) into cell organelles to enhance or modify plant function and thus crop productivity and yield. A comprehensive review of nanomaterials affect crop yield is needed to guide nanotechnology research. This article critically reviews nanotechnology applications for engineering plant productivity, seed germination, crop growth, enhancing photosynthesis, and improving crop yield and quality, and discusses nanobionic approaches such as smart drug delivery systems and plant nanobiosensors. Moreover, the review describes NM classification and synthesis and human health-related and plant toxicity hazards. Our findings suggest that nanotechnology application in agricultural production could significantly increase crop yields to alleviate global hunger pressures. However, the environmental risks associated with NMs should be investigated thoroughly before their widespread adoption in agriculture.

Computational Study of Drop-on-Demand Coaxial Electrohydrodynamic Jet and Printing Microdroplets

Currently, coaxial electrohydrodynamic jet (CE-Jet) printing is used as a promising technique for the alternative fabrication of drop-on-demand micro- and nanoscale structures without using a template. Therefore, this paper presents numerical simulation of the DoD CE-Jet process based on a phase field model. Titanium lead zirconate (PZT) and silicone oil were used to verify the numerical simulation and the experiments. The optimized working parameters (i.e., inner liquid flow velocity 150 m/s, pulse voltage 8.0 kV, external fluid velocity 250 m/s, print height 16 cm) were used to control the stability of the CE-Jet, avoiding the bulging effect during experimental study. Consequently, different sized microdroplets with a minimum diameter of ~5.5 µm were directly printed after the removal of the outer solution. The model is considered the easiest to implement and is powerful for the application of flexible printed electronics in advanced manufacturing technology.

Boosting the Piezoelectric Sensitivity of Amino Acid Crystals by Mechanical Annealing for the Engineering of Fully Degradable Force Sensors

Biodegradable piezoelectric force sensors can be used as implantable medical devices for monitoring physiological pressures of impaired organs or providing essential stimuli for drug delivery and tissue regeneration without the need of additional invasive removal surgery or battery power. However, traditional piezoelectric materials, such as inorganic ceramics and organic polymers, show unsatisfactory degradability, and cytotoxicity. Amino acid crystals are biocompatible and exhibit outstanding piezoelectric properties, but their small crystal size makes it difficult to align the crystals for practical applications. Here, a mechanical-annealing strategy is reported for engineering all-organic biodegradable piezoelectric force sensors using natural amino acid crystals as piezoelectric materials. It is shown that the piezoelectric constant of the mechanical-annealed crystals can reach 12 times that of the single crystal powders. Moreover, mechanical annealing results in flat and smooth surfaces, thus improving the contact of the crystal films with the electrodes and leading to high output voltages of the devices. The packaged force sensors can be used to monitor dynamic motions, including muscle contraction and lung respiration, in vivo for 4 weeks and then gradually degrade without causing obvious inflammation or systemic toxicity. This work provides a way to engineer all-organic and biodegradable force sensors for potential clinical applications.

Overview: State-of-the-Art in the Energy Harvesting Based on Piezoelectric Devices for Last Decade

Technologies of energy harvesting have been developed intensively since the beginning of the twenty-first century, presenting themselves as alternatives to traditional energy sources (for instance, batteries) for small-dimensional and low-power electronics. Batteries have numerous shortcomings connected, for example, with restricted service life and the necessity of periodic recharging/replacement that create significant problems for portative and remote devices and for power equipment. Environmental energy covers solar, thermal, and oscillation energy. By this, the vibration energy exists continuously around us due to the operation of numerous artificial structures and mechanisms. Different materials (including piezoelectrics) and conversion mechanisms can transform oscillation energy into electrical energy for use in many devices of energy harvesting. Piezoelectric transducers possessing electric mechanical coupling and demonstrating a high density of power in comparison with electromagnetic and electrostatic sensors are broadly applied for the generation of energy from different oscillation energy sources. For the last decade, novel piezoelectric materials, transformation mechanisms, electrical circuits, and experimental and theoretical approaches with results of computer simulation have been developed for improving different piezoelectric devices of energy harvesting. This overview presents results, obtained in the area of piezoelectric energy harvesting for the last decade, including a wide spectrum of experimental, analytical, and computer simulation investigations.

Piezoelectric nanogenerators for personalized healthcare

The development of flexible piezoelectric nanogenerators has experienced rapid progress in the past decade and is serving as the technological foundation of future state-of-the-art personalized healthcare. Due to their highly efficient mechanical-to-electrical energy conversion, easy implementation, and self-powering nature, these devices permit a plethora of innovative healthcare applications in the space of active sensing, electrical stimulation therapy, as well as passive human biomechanical energy harvesting to third party power on-body devices. This article gives a comprehensive review of the piezoelectric nanogenerators for personalized healthcare. After a brief introduction to the fundamental physical science of the piezoelectric effect, material engineering strategies, device structural designs, and human-body centered energy harvesting, sensing, and therapeutics applications are also systematically discussed. In addition, the challenges and opportunities of utilizing piezoelectric nanogenerators for self-powered bioelectronics and personalized healthcare are outlined in detail.

Numerical modeling and analysis of coaxial electrohydrodynamic jet printing

Coaxial electrohydrodynamic jet (CE-Jet) printing is an encouraging method for fabrication of high-resolution micro and nanostructures in MEMS systems. This paper presents a novel simulation work based on phase field method which is considered as a precise technique in fluid dynamics. The study explores influence of various parameters such as applied voltage, needle-substrate distance, dynamic viscosity, relative permittivity, needle size and flow rate on stability and resolution of CE-Jet morphologies. The morphology of CE-Jet exhibits that width of cone-jet profile and printed structures on substrate were directly proportional to relative permittivity and flow rate. In addition, it was inversely proportional to dynamic viscosity and applied voltage. The study examine that CE-Jet length of inner liquid is inversely proportional to needle-substrate distance in same time. It was later verified in experimental study by producing stable CE-Jet morphology with 300 μm diameter using optimized parameters (i.e., DC voltage 7.0 kV and inner liquid flow rate 400 nl/min) as compared to other validation studies such as 400 μm and 500 μm. The CE-Jet printing technique investigates significant changes in consistency and stability of CE-Jet morphologies and makes Jet unique and comparable when adjustment accuracy reaches 0.01 mm. PZT sol line structures with a diameter of 1 µm were printed directly on substrate using inner needle (diameter of 120 µm). Therefore, it is considered as a powerful tool for nano constructs production in M/NEMS devices.

On the Coupling between Mechanical Properties and Electrostatics in Biological Membranes

Cell membrane structure is proposed as a lipid matrix with embedded proteins, and thus, their emerging mechanical and electrostatic properties are commanded by lipid behavior and their interconnection with the included and absorbed proteins, cytoskeleton, extracellular matrix and ionic media. Structures formed by lipids are soft, dynamic and viscoelastic, and their properties depend on the lipid composition and on the general conditions, such as temperature, pH, ionic strength and electrostatic potentials. The dielectric constant of the apolar region of the lipid bilayer contrasts with that of the polar region, which also differs from the aqueous milieu, and these changes happen in the nanometer scale. Besides, an important percentage of the lipids are anionic, and the rest are dipoles or higher multipoles, and the polar regions are highly hydrated, with these water molecules forming an active part of the membrane. Therefore, electric fields (both, internal and external) affects membrane thickness, density, tension and curvature, and conversely, mechanical deformations modify membrane electrostatics. As a consequence, interfacial electrostatics appears as a highly important parameter, affecting the membrane properties in general and mechanical features in particular. In this review we focus on the electromechanical behavior of lipid and cell membranes, the physicochemical origin and the biological implications, with emphasis in signal propagation in nerve cells.

Nanotechnology Facilitated Cultured Neuronal Network and Its Applications

The development of a biomimetic neuronal network from neural cells is a big challenge for researchers. Recent advances in nanotechnology, on the other hand, have enabled unprecedented tools and techniques for guiding and directing neural stem cell proliferation and differentiation in vitro to construct an in vivo-like neuronal network. Nanotechnology allows control over neural stem cells by means of scaffolds that guide neurons to reform synaptic networks in suitable directions in 3D architecture, surface modification/nanopatterning to decide cell fate and stimulate/record signals from neurons to find out the relationships between neuronal circuit connectivity and their pathophysiological functions. Overall, nanotechnology-mediated methods facilitate precise physiochemical controls essential to develop tools appropriate for applications in neuroscience. This review emphasizes the newest applications of nanotechnology for examining central nervous system (CNS) roles and, therefore, provides an insight into how these technologies can be tested in vitro before being used in preclinical and clinical research and their potential role in regenerative medicine and tissue engineering.

Fundamental Definitions for Axially-Strained Piezo-Semiconductive Nanostructures

Piezoelectric nanotransducers may offer key advantages in comparison with conventional piezoelectrics, including more choices for types of mechanical input, positions of the contacts, dimensionalities and shapes. However, since most piezoelectric nanostructures are also semiconductive, modeling becomes significantly more intricate and, therefore, the effects of free charges have been considered only in a few studies. Moreover, the available reports are complicated by the absence of proper nomenclature and figures of merit. Besides, some of the previous analyses are incomplete. For instance, the local piezopotential and free charges within axially strained conical piezo-semiconductive nanowires have only been systematically investigated for very low doping (10¹⁶ cm⁻³) and under compression. Here we give the definitions for the enhancement, depletion, base and tip piezopotentials, their characteristic lengths and both the tip-to-base and the depletion-to-enhancement piezopotential-ratios. As an example, we use these definitions for analyzing the local piezopotential and free charges in n-type ZnO truncated conical nanostructures with different doping levels (intrinsic, 10¹⁶ cm⁻³, 10¹⁷ cm⁻³) for both axial compression and traction. The definitions and concepts presented here may offer insight for designing high performance piezosemiconductive nanotransducers.

Progress in the Applications of Smart Piezoelectric Materials for Medical Devices

Smart piezoelectric materials are of great interest due to their unique properties. Piezoelectric materials can transform mechanical energy into electricity and vice versa. There are mono and polycrystals (piezoceramics), polymers, and composites in the group of piezoelectric materials. Recent years show progress in the applications of piezoelectric materials in biomedical devices due to their biocompatibility and biodegradability. Medical devices such as actuators and sensors, energy harvesting devices, and active scaffolds for neural tissue engineering are continually explored. Sensors and actuators from piezoelectric materials can convert flow rate, pressure, etc., to generate energy or consume it. This paper consists of using smart materials to design medical devices and provide a greater understanding of the piezoelectric effect in the medical industry presently. A greater understanding of piezoelectricity is necessary regarding the future development and industry challenges.

High-speed interferometric imaging reveals dynamics of neuronal deformation during the action potential

Neurons undergo nanometer-scale deformations during action potentials, and the underlying mechanism has been actively debated for decades. Previous observations were limited to a single spot or the cell boundary, while movement across the entire neuron during the action potential remained unclear. Here we report full-field imaging of cellular deformations accompanying the action potential in mammalian neuron somas (-1.8 to 1.4 nm) and neurites (-0.7 to 0.9 nm), using high-speed quantitative phase imaging with a temporal resolution of 0.1 ms and an optical path length sensitivity of <4 pm per pixel. The spike-triggered average, synchronized to electrical recording, demonstrates that the time course of the optical phase changes closely matches the dynamics of the electrical signal. Utilizing the spatial and temporal correlations of the phase signals across the cell, we enhance the detection and segmentation of spiking cells compared to the shot-noise-limited performance of single pixels. Using three-dimensional (3D) cellular morphology extracted via confocal microscopy, we demonstrate that the voltage-dependent changes in the membrane tension induced by ionic repulsion can explain the magnitude, time course, and spatial features of the phase imaging. Our full-field observations of the spike-induced deformations shed light upon the electromechanical coupling mechanism in electrogenic cells and open the door to noninvasive label-free imaging of neural signaling.

Biomimetic Nanostructures with Compositional Gradient Grown by Combinatorial Matrix-Assisted Pulsed Laser Evaporation for Tissue Engineering

There is permanent progress with the fabrication of smart bioactive surfaces that could govern tissue regeneration. Thin coatings of two or more materials with compositional gradient allow the construction of arrays with different chemical and physical features on a solid substrate. With such intelligent bio-platforms, cells can be exposed to a tissue-like biomimetic micro-environment with precise characteristics that directs cells fate towards specific phenotypes. We have introduced combinatorial matrix-assisted pulsed laser evaporation (C-MAPLE) as an alternative approach for the fabrication in a single-step process of either organic or inorganic thin and nanostructured coatings with variable composition. A continuous reciprocal gradient of two biomolecules can be achieved by C-MAPLE with discrete areas exhibiting physicochemical specificity that modulates intracellular signaling events. Herein, we present a review of the current combinatorial laser strategies and methods for fabricating thin organic and inorganic films with compositional gradient with emphasis on the surface influence on cell responsiveness. In particular, the specific biological potential of surface functionalization with thin coatings of biopolymers, proteins and drugs will be discussed. Laser deposition combinatorial processes are considered an emerging unconventional technology that can be widely applied to produce composite multilayers and micro-patterns for faster cell colonization and tissue engineering.

Biodegradable nanofiber-based piezoelectric transducer

Piezoelectric materials, a type of "smart" material that generates electricity while deforming and vice versa, have been used extensively for many important implantable medical devices such as sensors, transducers, and actuators. However, commonly utilized piezoelectric materials are either toxic or nondegradable. Thus, implanted devices employing these materials raise a significant concern in terms of safety issues and often require an invasive removal surgery, which can damage directly interfaced tissues/organs. Here, we present a strategy for materials processing, device assembly, and electronic integration to 1) create biodegradable and biocompatible piezoelectric PLLA [poly(l-lactic acid)] nanofibers with a highly controllable, efficient, and stable piezoelectric performance, and 2) demonstrate device applications of this nanomaterial, including a highly sensitive biodegradable pressure sensor for monitoring vital physiological pressures and a biodegradable ultrasonic transducer for blood-brain barrier opening that can be used to facilitate the delivery of drugs into the brain. These significant applications, which have not been achieved so far by conventional piezoelectric materials and bulk piezoelectric PLLA, demonstrate the PLLA nanofibers as a powerful material platform that offers a profound impact on various medical fields including drug delivery, tissue engineering, and implanted medical devices.

Advancing Versatile Ferroelectric Materials Toward Biomedical Applications

Ferroelectric materials (FEMs), possessing piezoelectric, pyroelectric, inverse piezoelectric, nonlinear optic, ferroelectric-photovoltaic, and many other properties, are attracting increasing attention in the field of biomedicine in recent years. Because of their versatile ability of interacting with force, heat, electricity, and light to generate electrical, mechanical, and optical signals, FEMs are demonstrating their unique advantages for biosensing, acoustics tweezer, bioimaging, therapeutics, tissue engineering, as well as stimulating biological functions. This review summarizes the current-available FEMs and their state-of-the-art fabrication techniques, as well as provides an overview of FEMs-based applications in the field of biomedicine. Challenges and prospects for future development of FEMs for biomedical applications are also outlined.

Electrophysiological-mechanical coupling in the neuronal membrane and its role in ultrasound neuromodulation and general anaesthesia

The current understanding of the role of the cell membrane is in a state of flux. Recent experiments show that conventional models, considering only electrophysiological properties of a passive membrane, are incomplete. The neuronal membrane is an active structure with mechanical properties that modulate electrophysiology. Protein transport, lipid bilayer phase, membrane pressure and stiffness can all influence membrane capacitance and action potential propagation. A mounting body of evidence indicates that neuronal mechanics and electrophysiology are coupled, and together shape the membrane potential in tight coordination with other physical properties. In this review, we summarise recent updates concerning electrophysiological-mechanical coupling in neuronal function. In particular, we aim at making the link with two relevant yet often disconnected fields with strong clinical potential: the use of mechanical vibrations-ultrasound-to alter the electrophysiogical state of neurons, e.g., in neuromodulation, and the theories attempting to explain the action of general anaesthetics. STATEMENT OF SIGNIFICANCE: General anaesthetics revolutionised medical practice; now an apparently unrelated technique, ultrasound neuromodulation-aimed at controlling neuronal activity by means of ultrasound-is poised to achieve a similar level of impact. While both technologies are known to alter the electrophysiology of neurons, the way they achieve it is still largely unknown. In this review, we argue that in order to explain their mechanisms/effects, the neuronal membrane must be considered as a coupled mechano-electrophysiological system that consists of multiple physical processes occurring concurrently and collaboratively, as opposed to sequentially and independently. In this framework the behaviour of the cell membrane is not the result of stereotypical mechanisms in isolation but instead emerges from the integrative behaviour of a complexly coupled multiphysics system.

Computational model of the mechanoelectrophysiological coupling in axons with application to neuromodulation

For more than half a century, the action potential (AP) has been considered a purely electrical phenomenon. However, experimental observations of membrane deformations occurring during APs have revealed that this process also involves mechanical features. This discovery has recently fuelled a controversy on the real nature of APs: whether they are mechanical or electrical. In order to examine some of the modern hypotheses regarding APs, we propose here a coupled mechanoelectrophysiological membrane finite-element model for neuronal axons. The axon is modeled as an axisymmetric thin-wall cylindrical tube. The electrophysiology of the membrane is modeled using the classic Hodgkin-Huxley (H-H) equations for the Nodes of Ranvier or unmyelinated axons and the cable theory for the internodal regions, whereas the axonal mechanics is modeled by means of viscoelasticity theory. Membrane potential changes induce a strain gradient field via reverse flexoelectricity, whereas mechanical pulses result in an electrical self-polarization field following the direct flexoelectric effect, in turn influencing the membrane potential. Moreover, membrane deformation also alters the values of membrane capacitance and resistance in the H-H equation. These three effects serve as the fundamental coupling mechanisms between the APs and mechanical pulses in the model. A series of numerical studies was systematically conducted to investigate the consequences of interaction between the APs and mechanical waves on both myelinated and unmyelinated axons. Simulation results illustrate that the AP is always accompanied by an in-phase propagating membrane displacement of ≈1nm, whereas mechanical pulses with enough magnitude can also trigger APs. The model demonstrates that mechanical vibrations, such as the ones arising from ultrasound stimulations, can either annihilate or enhance axonal electrophysiology depending on their respective directionality and frequency. It also shows that frequency of pulse repetition can also enhance signal propagation independently of the amplitude of the signal. This result not only reconciles the mechanical and electrical natures of the APs but also provides an explanation for the experimentally observed mechanoelectrophysiological phenomena in axons, especially in the context of ultrasound neuromodulation.

Probing electromechanical behaviors by datacube piezoresponse force microscopy in ambient and aqueous environments

For assisting the in-depth investigations of widespread electromechanical phenomena in functional materials, piezoresponse force microscopy (PFM) has gradually evolved to realize full information-flow acquisition and fit the conductive liquid working environments. Here, we designed data cube (DCUBE) based PFM to collect the electromechanical effect into a high-dimensional array of piezoresponse by adding ac bias with a wide range of frequencies to the probe. The electromechanical and mechanical spectra can be consecutively extracted at each pixel in the intermittent-contact mode. High-resolution ferroelectric domains of the poled LiNbO₃ were mapped, corresponding to the ideal phase contrasts of about 180° in air, decane, and deionized water. Rich information detection and non-contact mode in DCUBE-PFM bring many merits on the electromechanical characterizations, especially for elastic-inhomogeneous surfaces and soft materials. Moreover, we systematically reveal the Debye screening effect and time-resolved field-oriented ion dynamics, which play crucial roles in the reduction of PFM spatial resolution in electrolytes. These physical discussions provide strategies to further realize high-resolution electromechanical imaging in highly conductive liquid environments.

Plasmonic imaging of subcellular electromechanical deformation in mammalian cells

A membrane potential change in cells is accompanied with mechanical deformation. This electromechanical response can play a significant role in regulating action potential in neurons and in controlling voltage-gated ion channels. However, measuring this subtle deformation in mammalian cells has been a difficult task. We show a plasmonic imaging method to image mechanical deformation in single cells upon a change in the membrane potential. Using this method, we have studied the electromechanical response in mammalian cells and have observed the local deformation within the cells that are associated with cell-substrate interactions. By analyzing frequency dependence of the response, we have further examined the electromechanical deformation in terms of mechanical properties of cytoplasm and cytoskeleton. We demonstrate a plasmonic imaging approach to quantify the electromechanical responses of single mammalian cells and determine local variability related to cell-substrate interactions.

Piezoelectric Biomaterials for Sensors and Actuators

Recent advances in materials, manufacturing, biotechnology, and microelectromechanical systems (MEMS) have fostered many exciting biosensors and bioactuators that are based on biocompatible piezoelectric materials. These biodevices can be safely integrated with biological systems for applications such as sensing biological forces, stimulating tissue growth and healing, as well as diagnosing medical problems. Herein, the principles, applications, future opportunities, and challenges of piezoelectric biomaterials for medical uses are reviewed thoroughly. Modern piezoelectric biosensors/bioactuators are developed with new materials and advanced methods in microfabrication/encapsulation to avoid the toxicity of conventional lead-based piezoelectric materials. Intriguingly, some piezoelectric materials are biodegradable in nature, which eliminates the need for invasive implant extraction. Together, these advancements in the field of piezoelectric materials and microsystems can spark a new age in the field of medicine.

Recent Progress of Janus 2D Transition Metal Chalcogenides: From Theory to Experiments

Since the discovery of graphene, 2D materials with various properties have gained increasing attention in fields such as novel electronic, optic, spintronic, and valleytronic devices. As an important derivative of 2D materials, Janus 2D materials, such as Janus transition metal chalcogenides (TMDs), have become a research hot spot in recent years. Janus 2D materials with mirror asymmetry display novel properties, such as the Rashba effect and normal piezoelectric polarization, providing great promise for their application in sensors, actuators, and other electromechanical devices. Here, the current theoretical and experimental progresses made in the development of Janus 2D TMDs, including their structure and stability, electronic properties, fabrication, and the results of their characterization are reported. Finally, the future prospects for the further development of Janus 2D materials are considered.

Morphological and mechanical determinants of cellular uptake of deformable nanoparticles

Understanding the interactions of nanoparticles (NPs) with cell membranes and regulating their cellular uptake processes are of fundamental importance to the design of drug delivery systems with minimum toxicity, high efficiency and long circulation time. Employing the procedure of coarse-graining, we built an elastically deformable NP model with tunable morphological and mechanical properties. We found that the cellular uptake of deformable NPs depends on their shape: an increase in the particle elasticity significantly slows the uptake rate of spherical NPs, slightly retards that of prolate NPs, and promotes the uptake of oblate NPs. The intrinsic mechanisms have been carefully investigated through analysis of the endocytic mechanisms and free energy calculations. These findings provide unique insights into how deformable NPs penetrate across cell membranes and offer novel possibilities for designing effective NP-based carriers for drug delivery.

Nanoscale coaxial focused electrohydrodynamic jet printing

Controlled patterning of nanostructures at desired positions is of great importance for high-performance M/NEMS devices. Here, we demonstrate a high-resolution, high-speed and cost-effective fabrication method, named coaxial focused electrohydrodynamic jet printing, to print functional nanostructures. A coaxial needle was designed and developed; a functional ink and high viscosity liquid are applied in the inner and outer needle, respectively. Under optimised conditions, a stable coaxial jet is formed; then, the electrical shearing force and electrical field induce viscous shearing force and internal pressure that are jointly applied on the inner functional ink, focusing the inner jet on the nanoscale. Using this stable coaxial jet with a nano-jet inside it, nanostructures with highly aligned nanowire arrays, nano-freebeams and nano-cantilever beams down to the scale of 40 nm were directly printed. The needle size was 130 μm, and the ratio of the sizes of the needle and the printed structure was as high as 3250/1. This technique realizes the controllable printing of nanoscale structures with the use of a one hundred micrometer-sized needle. The printed PZT nanostructures exhibit pure perovskite structures and distinct piezoelectric responses.

Nongenetic Optical Methods for Measuring and Modulating Neuronal Response

The ability to probe and modulate electrical signals sensitively at cellular length scales is a key challenge in the field of electrophysiology. Electrical signals play integral roles in regulating cellular behavior and in controlling biological function. From cardiac arrhythmias to neurodegenerative disorders, maladaptive phenotypes in electrophysiology can result in serious and potentially deadly medical conditions. Understanding how to monitor and to control these behaviors precisely and noninvasively represents an important step in developing next-generation therapeutic devices. As we develop a deeper understanding of neural network formation, electrophysiology has the potential to offer fundamental insights into the inner working of the brain. In this Perspective, we explore traditional methods for examining neural function, discuss recent genetic advances in electrophysiology, and then focus on the latest innovations in optical sensing and stimulation of action potentials in neurons. We emphasize nongenetic optical methods, as these provide high spatiotemporal resolution and can be achieved with minimal invasiveness.

Imaging Action Potential in Single Mammalian Neurons by Tracking the Accompanying Sub-Nanometer Mechanical Motion

Action potentials in neurons have been studied traditionally by intracellular electrophysiological recordings and more recently by the fluorescence detection methods. Here we describe a label-free optical imaging method that can measure mechanical motion in single cells with a sub-nanometer detection limit. Using the method, we have observed sub-nanometer mechanical motion accompanying the action potential in single mammalian neurons by averaging the repeated action potential spikes. The shape and width of the transient displacement are similar to those of the electrically recorded action potential, but the amplitude varies from neuron to neuron, and from one region of a neuron to another, ranging from 0.2-0.4 nm. The work indicates that action potentials may be studied noninvasively in single mammalian neurons by label-free imaging of the accompanying sub-nanometer mechanical motion.

Ultrathin Piezotronic Transistors with 2 nm Channel Lengths

Because silicon transistors are rapidly approaching their scaling limit due to short-channel effects, alternative technologies are urgently needed for next-generation electronics. Here, we demonstrate ultrathin ZnO piezotronic transistors with a ∼2 nm channel length using inner-crystal self-generated out-of-plane piezopotential as the gate voltage to control the carrier transport. This design removes the need for external gate electrodes that are challenging at nanometer scale. These ultrathin devices exhibit a strong piezotronic effect and excellent pressure-switching characteristics. By directly converting mechanical drives into electrical control signals, ultrathin piezotronic devices could be used as active nanodevices to construct the next generation of electromechanical devices for human-machine interfacing, energy harvesting, and self-powered nanosystems.

Talking to cells: semiconductor nanomaterials at the cellular interface

The interface of biological components with semiconductors is a growing field with numerous applications. For example, the interfaces can be used to sense and modulate the electrical activity of single cells and tissues. From the materials point of view, silicon is the ideal option for such studies due to its controlled chemical synthesis, scalable lithography for functional devices, excellent electronic and optical properties, biocompatibility and biodegradability. Recent advances in this area are pushing the bio-interfaces from the tissue and organ level to the single cell and sub-cellular regimes. In this progress report, we will describe some fundamental studies focusing on miniaturizing the bioelectric and biomechanical interfaces. Additionally, many of our highlighted examples involve freestanding silicon-based nanoscale systems, in addition to substrate-bound structures or devices; the former offers new promise for basic research and clinical application. In this report, we will describe recent developments in the interfacing of neuronal and cardiac cells and their networks. Moreover, we will briefly discuss the incorporation of semiconductor nanostructures for interfacing non-excitable cells in applications such as probing intracellular force dynamics and drug delivery. Finally, we will suggest several directions for future exploration.

Biodegradable Piezoelectric Force Sensor

Measuring vital physiological pressures is important for monitoring health status, preventing the buildup of dangerous internal forces in impaired organs, and enabling novel approaches of using mechanical stimulation for tissue regeneration. Pressure sensors are often required to be implanted and directly integrated with native soft biological systems. Therefore, the devices should be flexible and at the same time biodegradable to avoid invasive removal surgery that can damage directly interfaced tissues. Despite recent achievements in degradable electronic devices, there is still a tremendous need to develop a force sensor which only relies on safe medical materials and requires no complex fabrication process to provide accurate information on important biophysiological forces. Here, we present a strategy for material processing, electromechanical analysis, device fabrication, and assessment of a piezoelectric Poly-l-lactide (PLLA) polymer to create a biodegradable, biocompatible piezoelectric force sensor, which only employs medical materials used commonly in Food and Drug Administration-approved implants, for the monitoring of biological forces. We show the sensor can precisely measure pressures in a wide range of 0-18 kPa and sustain a reliable performance for a period of 4 d in an aqueous environment. We also demonstrate this PLLA piezoelectric sensor can be implanted inside the abdominal cavity of a mouse to monitor the pressure of diaphragmatic contraction. This piezoelectric sensor offers an appealing alternative to present biodegradable electronic devices for the monitoring of intraorgan pressures. The sensor can be integrated with tissues and organs, forming self-sensing bionic systems to enable many exciting applications in regenerative medicine, drug delivery, and medical devices.

Full-field interferometric imaging of propagating action potentials

Currently, cellular action potentials are detected using either electrical recordings or exogenous fluorescent probes that sense the calcium concentration or transmembrane voltage. Ca imaging has a low temporal resolution, while voltage indicators are vulnerable to phototoxicity, photobleaching, and heating. Here, we report full-field interferometric imaging of individual action potentials by detecting movement across the entire cell membrane. Using spike-triggered averaging of movies synchronized with electrical recordings, we demonstrate deformations up to 3 nm (0.9 mrad) during the action potential in spiking HEK-293 cells, with a rise time of 4 ms. The time course of the optically recorded spikes matches the electrical waveforms. Since the shot noise limit of the camera (~2 mrad/pix) precludes detection of the action potential in a single frame, for all-optical spike detection, images are acquired at 50 kHz, and 50 frames are binned into 1 ms steps to achieve a sensitivity of 0.3 mrad in a single pixel. Using a self-reinforcing sensitivity enhancement algorithm based on iteratively expanding the region of interest for spatial averaging, individual spikes can be detected by matching the previously extracted template of the action potential with the optical recording. This allows all-optical full-field imaging of the propagating action potentials without exogeneous labels or electrodes.

New monolayer ternary In-containing sesquichalcogenides BiInSe3, SbInSe3, BiInTe3, and SbInTe3 with high stability and extraordinary piezoelectric properties

We predicted several novel monolayers consisting of ternary sesquichalcogenides with extraordinary piezoelectric properties, which may be alternatives to the conventional piezoelectric materials such as PZT.

Plasmonic-Based Electrochemical Impedance Imaging of Electrical Activities in Single Cells

Studying electrical activities in cells, such as action potential and its propagation in neurons, requires a sensitive and non-invasive analytical tool that can image local electrical signals with high spatial and temporal resolutions. Here we report a plasmonic-based electrochemical impedance imaging technique to study transient electrical activities in single cells. The technique is based on the conversion of the electrical signal into a plasmonic signal, which is imaged optically without labels. We demonstrate imaging of the fast initiation and propagation of action potential within single neurons, and validate the imaging technique with the traditional patch clamp technique. We anticipate that the plasmonic imaging technique will contribute to the study of electrical activities in various cellular processes.

Programmable Nano-Bio Interfaces for Functional Biointegrated Devices

A large amount of evidence has demonstrated the revolutionary role of nanosystems in the screening and shielding of biological systems. The explosive development of interfacing bioentities with programmable nanomaterials has conveyed the intriguing concept of nano-bio interfaces. Here, recent advances in functional biointegrated devices through the precise programming of nano-bio interactions are outlined, especially with regard to the rational assembly of constituent nanomaterials on multiple dimension scales (e.g., nanoparticles, nanowires, layered nanomaterials, and 3D-architectured nanomaterials), in order to leverage their respective intrinsic merits for different functions. Emerging nanotechnological strategies at nano-bio interfaces are also highlighted, such as multimodal diagnosis or "theragnostics", synergistic and sequential therapeutics delivery, and stretchable and flexible nanoelectronic devices, and their implementation into a broad range of biointegrated devices (e.g., implantable, minimally invasive, and wearable devices). When utilized as functional modules of biointegrated devices, these programmable nano-bio interfaces will open up a new chapter for precision nanomedicine.

Nanosensor Technology Applied to Living Plant Systems

An understanding of plant biology is essential to solving many long-standing global challenges, including sustainable and secure food production and the generation of renewable fuel sources. Nanosensor platforms, sensors with a characteristic dimension that is nanometer in scale, have emerged as important tools for monitoring plant signaling pathways and metabolism that are nondestructive, minimally invasive, and capable of real-time analysis. This review outlines the recent advances in nanotechnology that enable these platforms, including the measurement of chemical fluxes even at the single-molecule level. Applications of nanosensors to plant biology are discussed in the context of nutrient management, disease assessment, food production, detection of DNA proteins, and the regulation of plant hormones. Current trends and future needs are discussed with respect to the emerging trends of precision agriculture, urban farming, and plant nanobionics.