Biomimetic Applications Research Articles

Stepwise Synthesis of Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are a category of porous materials that offer unparalleled control over their surface areas (demonstrated as higher than for any other material), pore characteristics, and functionalization. This allows them to be customized for exceptional performance in a wide variety of applications, most commonly including gas storage and separation, drug delivery, luminescence, or heterogeneous catalysis. In order to optimize biomimicry, controlled separations and storage of small molecules, and detailed testing of structure-property relationships, one major goal of MOF research is "rational design" or "pore engineering", or precise control of the placement of multiple functional groups in pores of chosen sizes and shapes. MOF crystal growth can be controlled through judicious design of stepwise synthetic routes, which can also allow functionalization of MOFs in ways that were previously synthetically inaccessible. Organic chemists have developed a library of powerful techniques over the last century, allowing the total synthesis and detailed customization of complex molecules. Our hypothesis is that total synthesis is also possible for customized porous materials, through the development of similar multistep techniques. This will enable the rational design of MOFs, which is a major goal of many researchers in the field. We have begun developing a library of stepwise synthetic techniques for MOFs, allowing the synthesis of ultrastable MOFs with multiple crystallographically ordered and customizable functional groups at controlled locations within the pores. In order to design MOFs with precise control over pore size and shape, stability, and the placement of multiple different functional groups within the pores at tunable distances from one another, we have concentrated on methods which allow us to circumvent the lack of control inherent to one-pot MOF crystallization. Kinetically tuned dimensional augmentation (KTDA) is an approach using preformed metal clusters as starting materials and monotopic carboxylates as equilibrium shifting agents to make single crystals of ultrastable MOFs. Postsynthetic metathesis and oxidation (PSMO) takes advantage of the fast ligand exchange rate of a metal ion at the low oxidation state as well as the kinetic inertness of the same metal at high oxidation state to make ultrastable and highly crystalline MOFs. Multiple similar strategies have been successful for the metathesis of Fe-based MOFs to Cr3+. Several highly crystalline Ti-MOFs have also been prepared. Kinetically controlled linker installation and cluster metalation methods utilize a stable MOF with inherent coordinatively unsaturated sites as matrix and postsynthetically install linkers or grow clusters on the matrix, so that a robust MOF with precisely placed functionalities is realized. This method has diverse applications especially when specific functional groups or metals having synergistic effects are desired in the proper proximity. Exceptional porosity and stability are required for many potential applications. We have demonstrated several of these, including entrapment of nanoscaled functional moieties such as enzymes. We have developed a series of metal-organic frameworks (PCN-333) with rationally designed ultralarge mesoporous cages as single-molecule traps for enzyme encapsulation. We successfully incorporated metalloporphyrins, well-known biofunctional moieties, into robust MOFs for biomimetic catalytic applications. By rationally tuning the synthetic conditions, we obtained several different porphyrinic Zr-, Fe-, and Ti-MOFs with distinct pore size and concentrated acid or base stability, which offer eligible candidates for different applications. These and other stepwise kinetic tuning and catalyst incorporation methods are small steps toward achieving the grand challenge of detailed control of the placement of matter on an atomic and molecular level.

3D–4D Printed Objects: New Bioactive Material Opportunities

One of the main objectives of 3D printing in health science is to mimic biological functions. To reach this goal, a 4D printing might be added to 3D-printed objects which will be characterized by their abilities to evolve over time and under external stimulus by modifying their shape, properties or composition. Such abilities are the promise of great opportunities for biosensing and biomimetic systems to progress towards more physiological mimicking systems. Herein are presented two 4D printing examples for biosensing and biomimetic applications using 3D-printed enzymes. The first one is based on the printing of the enzymatic couple glucose oxidase/peroxidase for the chemiluminescent detection of glucose, and the second uses printed alkaline phosphatase to generate in situ programmed and localized calcification of the printed object.

Transparency-switchable actuator based on aligned carbon nanotube and paraffin-polydimethylsiloxane composite

High-performance actuators have been studied for many years, as they can be used in the fields of artificial muscles, optical switches and biomimetic applications. However, previous actuators mostly had single colors. Actuators with switchable optical properties are still rare. Here, we propose a transparency-switchable actuator based on single-layer superaligned carbon nanotube sheet and paraffin-polydimethylsiloxane composite. When a voltage is applied, the transmittance of the actuator changes from 0.7% to 67% (at the wavelength of 550 nm). At the same time, it bends obviously with a displacement up to 8.4 mm. The actuator is also durable, demonstrating both reversible actuation phenomenon and repeatable switchable transparency for 100 cycles. In addition, since this new type of actuator demonstrates optical property changing together with actuation process, which is smarter compared to previous conventional actuators, a smart window is fabricated, indicating its application in personal privacy protection and intelligent household devices. In a word, this work opens new perspectives on smart windows, optical switches and so on.

Features of owl wings that promote silent flight.

Owls are an order of birds of prey that are known for the development of a silent flight. We review here the morphological adaptations of owls leading to silent flight and discuss also aerodynamic properties of owl wings. We start with early observations (until 2005), and then turn to recent advances. The large wings of these birds, resulting in low wing loading and a low aspect ratio, contribute to noise reduction by allowing slow flight. The serrations on the leading edge of the wing and the velvet-like surface have an effect on noise reduction and also lead to an improvement of aerodynamic performance. The fringes at the inner feather vanes reduce noise by gliding into the grooves at the lower wing surface that are formed by barb shafts. The fringed trailing edge of the wing has been shown to reduce trailing edge noise. These adaptations to silent flight have been an inspiration for biologists and engineers for the development of devices with reduced noise production. Today several biomimetic applications such as a serrated pantograph or a fringed ventilator are available. Finally, we discuss unresolved questions and possible future directions.

Newton Output Blocking Force under Low-Voltage Stimulation for Carbon Nanotube-Electroactive Polymer Composite Artificial Muscles.

This is a study on the development of carbon nanotube-based composite actuators using a new ionic liquid-doped electroactive ionic polymer. For scalable production purposes, a simple hot-pressing method was used. Carbon nanotube/ionic liquid-Nafion/carbon nanotube composite films were fabricated that exhibited a large output blocking force and a stable cycling life with low alternating voltage stimuli in air. Of particular interest and importance, a blocking force of 1.5 N was achieved at an applied voltage of 6 V. Operational durability was confirmed by testing in air for over 30 000 cycles (or 43 h). The superior actuation performance of the carbon nanotube/ionic liquid-Nafion/carbon nanotube composite, coupled with easy manufacturability, low driving voltage, and reliable operation, promises great potential for artificial muscle and biomimetic applications.

Replication of micro-sized pillars in polypropylene using the extrusion coating process

A recent advancement in nano-pattern replication using roll-to-roll extrusion coating (R2R-EC) shows potential for many biomimetic applications. For further development of the technique a study of the micro-replication regime is carried out. In this study a full and partial replication in polypropylene (PP) of micro-sized pillars has been demonstrated using the extrusion coating process. The replication fidelity of the pillars is investigated in a systematic variation of different process parameters: the line-speed of the rolls, the extruder output, the cooling roll temperature and the pressure on the cooling roll. The parameter making biggest impact on the replication is the temperature of the cooling roll.The micro-sized pillars show replication behavior opposite to what was found previously for the nano-patterns. The larger structures take the time to fill in and replicate best at the lower speeds. In this article a direct comparison of the speed at the same coating thickness is conducted.

Hygroscopic motions of fossil conifer cones

Conifer cones represent natural, woody compliant structures which move their scales as passive responses to changes in environmental humidity. Here we report on water-driven opening and closing motions in coalified conifer cones from the Eemian Interglacial (approx. 126,000–113,000 years BP) and from the Middle Miocene (approx. 16.5 to 11.5 million years BP). These cones represent by far the oldest documented evidence of plant parts showing full functionality of such passive hydraulically actuated motion. The functional resilience of these structures is far beyond the biological purpose of seed dispersal and protection and is because of a low level of mineralization of the fossils. Our analysis emphasizes the functional-morphological integrity of these biological compliant mechanisms which, in addition to their biological fascination, are potentially also role models for resilient and maintenance-free biomimetic applications (e.g., adaptive and autonomously moving structures including passive hydraulic actuators).

Graphene papers: smart architecture and specific functionalization for biomimetics, electrocatalytic sensing and energy storage

We present a review of graphene papers regarding their smart assembly, functionalization and applications in biomimetics, sensors and energy devices.

Trichomes as a natural biophysical barrier for plants and their bioinspired applications.

Nature has inspired mankind to create novel inventions with biomimetic structures and materials, where plants provide a significant source of inspiration. Plants have evolved a range of effective appendages, among which trichomes have attracted extensive research interest due to their enormous functions. It is important to understand trichome functions and corresponding mechanisms for their bioinspired applications. In this review, we provide a comprehensive overview of the diverse functions of trichomes, with emphasis placed upon their roles as biophysical barriers that can create a complex three-dimensional (3D) network to help the plant adapt to severe environments. Moreover, we also summarize the bioinspired applications of four typical trichomes, including needle-like, hook-like, foliar-like, and antenna-like trichomes. This review offers a new perspective of interdisciplinary research on both trichome functions and their biomimetic applications.

Ionic polymer-metal composites (IPMCs) and ionic polymer-polymer composites (IP<sup>2</sup>Cs): Effects of electrode on mechanical, thermal and electromechanical behaviour

Electro-active polymers (EAPs) are soft polymers that develop mechanical strain if subjected to electrical stimuli or, on the contrary, produce voltages if mechanically deformed. Their discovery has led to take into account their possible use as sensor-actuator for both bio-mimetic applications and traditional robotics. Ionic polymer-metal composites (IPMCs) and ionic polymer-polymer composites (IP 2 Cs) belong to EAPs. Both of them are fabricated starting from an ionic polymer (typically Nafion ® 117) but while in IPMCs the electrode is based on a metal (Ag, Au or Pt), IP 2 Cs are all-organic devices: in fact a conducting polymer (e.g., poly(3,4-ethylendioxytiophene)-polystyrenesulfonate, PEDOT/PSS) is used as the electrode. In this work, a comparison has been done among the fabrication processes and the mechanical, thermal and electromechanical behaviour of IPMCs and IP 2 Cs. The objective is to investigate the influence of the kind of electrode on the final properties of the devices, taking, also, into account the effect of different solvents, used for realizing the actuation process (H 2 O or Ethylene Glycol). Reported results show that the performance of the obtained devices are function of manufacture parameters, the kind of electrode and solvent; consequently, taking into account the possible application, it is possible to choose the device that better meets specific requirements.

Biomimetic superwettable materials with structural colours.

Structural colours and superwettability are of great interest due to their unique characteristics. However, the application of materials with either structural colours or superwettability is limited. Moreover, materials possessing both structural colours and superwettability are crucial for many practical applications. The combination of structural colours and superwettability can result in materials for use various applications, such as in sensors, detectors, bioassays, anti-counterfeiting, and liquid actuators, by controlling surfaces to repel or absorb liquids. Regarding superwettability and structural colours, surface texture and chemical composition are two factors for the construction of materials with superwettable structural colours. This review aims at offering a comprehensive elaboration of the mechanism, recent biomimetic research, and applications of biomimetic superwettable materials with structural colours. Furthermore, this review provides significant insight into the design, fabrication, and application of biomimetic superwettable materials with structural colours.

Physicochemical Property Variation in Spider Silk: Ecology, Evolution, and Synthetic Production.

The unique combination of great stiffness, strength, and extensibility makes spider major ampullate (MA) silk desirable for various biomimetic and synthetic applications. Intensive research on the genetics, biochemistry, and biomechanics of this material has facilitated a thorough understanding of its properties at various levels. Nevertheless, methods such as cloning, recombination, and electrospinning have not successfully produced materials with properties as impressive as those of spider silk. It is nevertheless becoming clear that silk properties are a consequence of whole-organism interactions with the environment in addition to genetic expression, gland biochemistry, and spinning processes. Here we assimilate the research done and assess the techniques used to determine distinct forms of spider silk chemical and physical property variability. We suggest that more research should focus on testing hypotheses that explain spider silk property variations in ecological and evolutionary contexts.

Structures for biomimetic, fluidic, and biological applications

Abstract

Selenium-Decorated Graphene for Peroxynitrite Detection

Peroxynitrite (PON, ONOO−) is a member of the reactive oxygen-nitrogen species family. PON is generated in vivo by the diffusion-limited reaction (~2×1010 M−1S−1) of nitric oxide ( · NO) and superoxide (O2 · −) radicals. Elevated peroxynitrite levels are associated with several human pathologies, such as arthritis, inflammation, and carcinogenesis, as well as ageing-associated diseases. Thus, the precise detection of this species in biological systems is crucial, not only to understand the genesis and causes of ailments at the tissue/cellular level, but also to suggest and design potential therapies. The essential trace element selenium (Se) is the catalytic cofactor of important endogenous antioxidative systems of the human body. In the past decade, selenium attracted the attention of many research groups for the understanding of fundamental biological functions and biomimetic applications. Selenium is found in several human proteins (selenoproteins), many of them involved in anti-oxidant defense systems. Therefore, selenium plays a key role in redox regulation as a modulator of reactive oxygen species (ROS). Recently, a number of novel synthetic organoselenium compounds has been prepared and used as antioxidants in medicinal chemistry such as ebselen. In this work, we prepare and investigate interfaces based on the electrochemical deposition of selenium nanoparticles. We prepared surfaces of electrodeposited elemental selenium (Se) and selenium-decorated graphene-based nanostructures on glassy carbon electrodes and carbon fiber microelectrodes. We have used several physicochemical methods to characterize these PON-sensitive interfaces, including Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDX), X-ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, and UV-vis on ITO transparent electrodes. We tested these interfaces for electrocatalytic sensing of peroxynitrite. We found that selenium-modified graphene platform is sensitive to peroxynitrite in solution, and it provides a viable interface for electrochemical sensing of this analyte. We observed that there is a synergistic effect of the presence of graphene as a substrate for selenium nanoparticles in the electrocatalytic detection of PON. In this paper, we will present our findings using this new PON-sensitive interface. We will compare and contrast the performance of the various modified carbon electrodes in terms of PON sensitivity using cyclic voltammetry and dose-response chronoamperometry.

ChemInform Abstract: Metal Organic Frameworks Mimicking Natural Enzymes: A Structural and Functional Analogy

AbstractReview: 288 refs.

Ugi Reaction of Natural Amino Acids: A General Route toward Facile Synthesis of Polypeptoids for Bioapplications.

Polypeptoids represent a significant class of synthetic analogues of natural polypeptides with potential biomimetic applications in materials, catalysis, and pharmaceuticals, but their simple and general synthesis still remains a key challenge. Herein, we demonstrate that Ugi reaction of natural amino acids leads to structurally diverse polypeptoids, including γ- and δ-, as well as poly(ε-peptoid)s, under mild conditions (open to air, room temperature, and catalyst free). Moreover, this strategy also offers manifold opportunities to introduce functional groups such as fluorescent and clickable alkenes groups into polypeptoids. Such poly(ε-peptoid)s not only exhibit good biocompatibility and antibacterial activity, but perform very effectively as a drug-delivery system. The bacterial inhibition rate of poly(ε-peptoid) was up to 88.8% at concentration of 20 μg mL-1 in comparison to 61.8% of the poly(ε-lysine) control. Overall, this study offers us a general methodology toward facile preparation of polypeptoids for bioapplications.

Convection roll-driven generation of supra-wavelength periodic surface structures on dielectrics upon irradiation with femtosecond pulsed lasers

The significance of the magnitude of Prandtl number of a fluid in the propagation direction of induced convection rolls is elucidated. Specifically, we report on the physical mechanism to account for the formation and orientation of previously unexplored supra-wavelength periodic surface structures in dielectrics, following melting and subsequent capillary effects induced upon irradiation with ultrashort laser pulses. Counterintuitively, it is found that such structures exhibit periodicities, which are markedly, even multiple times, higher than the laser excitation wavelength. It turns out that the extent to which the hydrothermal waves relax depends upon the laser beam energy, produced electron densities upon excitation with femtosecond pulsed lasers, magnitude of the induced initial local roll disturbances and the magnitude of the Prandtl number with direct consequences on the orientation and size of the induced structures. It is envisaged that this elucidation may be useful for the interpretation of similar, albeit large-scale periodic or quasi-periodic structures formed in other natural systems due to thermal gradients, while it can also be of great importance for potential applications in biomimetics.

Screening of Osteogenic-Enhancing Short Peptides from BMPs for Biomimetic Material Applications.

Bone regeneration is an important issue in many situations, such as bone fracture and surgery. Umbilical cord mesenchymal stem cells (UC-MSCs) are promising cell sources for bone regeneration. Bone morphogenetic proteins and their bioactive peptides are biomolecules known to enhance the osteogenic differentiation of MSCs. However, fibrosis can arise during the development of implantable biomaterials. Therefore, it is important to control cell organization by enhancing osteogenic proliferation and differentiation and inhibiting fibroblast proliferation. Thus, we focused on the screening of such osteogenic-enhancing peptides. In the present study, we developed new peptide array screening platforms to evaluate cell proliferation and alkaline phosphatase activity in osteoblasts, UC-MSCs and fibroblasts. The conditions for the screening platform were first defined using UC-MSCs and an osteogenic differentiation peptide known as W9. Next, in silico screening to define the candidate peptides was carried out to evaluate the homology of 19 bone morphogenetic proteins. Twenty-five candidate 9-mer peptides were selected for screening. Finally, the screening of osteogenic-enhancing (osteogenic cell-selective proliferation and osteogenic differentiation) short peptide was carried out using the peptide array method, and three osteogenic-enhancing peptides were identified, confirming the validity of this screening.

Cocowood Fibrovascular Tissue System-Another Wonder of Plant Evolution.

The coconut palm (Cocos nucifera L.) stem tissue (referred to as cocowood in this study) is a complex fibrovascular system that is made up of fibrovascular bundles embedded into a parenchymatous ground tissue. The complex configuration of fibrovascular bundles along with the non-uniform distribution of the material properties likely allow senile coconut stems to optimize their biomechanical performance per unit mass (i.e., mechanical efficiency) and grow into tall, slender, and very flexible plants with minimum resources of biomass and water. For the first time, to the best of the authors' knowledge, this paper examines, from the integral (i.e., stem structure) and macroscopic (i.e., tissue structure) levels of hierarchy, the characteristic triple helix formation depicted by the fibrovascular bundles within the monocotyledon cocowood. The natural course of the tangential orientation of the axial fibrovascular bundles is mapped for the whole cocowood structure by quantifying 264 cocowood discs, corresponding to 41 senile coconut palms estimated to be >70 years old. The observed variations were modeled in this paper by simple equations that partially enabled characterization of the cocowood fibrovascular tissue system. Furthermore, 11 finite element analyses (FEA) were performed over a three dimensional (3D) finite element (FE) model resembling a characteristic coconut palm stem of 25 m in height to analyze the biomaterial reactions produced by the progressive deviation of the tangential fibrovascular bundles on the cocowood mechanical response (i.e., on the material compressive strength and the bending stiffness). The analyses in this study were carried out for the critical wind speed of 23 m/s (i.e., Gale tornado according to the Fujita tornado scale). For each analysis, the characteristic average maxima degree of orientation of the cocowood fibrovascular bundles was varied from 0° to 51°. The acquired results provided a deep understanding of the cocowood optimum fibrovascular tissue system that denotes the natural evolution of the material through millions of years. The knowledge advanced from this study may also serve as concept generators for innovative biomimetic applications to improve current engineered wood products.

Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications.

A comprehensive survey of the construction principles and occurrences of superhydrophobic surfaces in plants, animals and other organisms is provided and is based on our own scanning electron microscopic examinations of almost 20 000 different species and the existing literature. Properties such as self-cleaning (lotus effect), fluid drag reduction (Salvinia effect) and the introduction of new functions (air layers as sensory systems) are described and biomimetic applications are discussed: self-cleaning is established, drag reduction becomes increasingly important, and novel air-retaining grid technology is introduced. Surprisingly, no evidence for lasting superhydrophobicity in non-biological surfaces exists (except technical materials). Phylogenetic trees indicate that superhydrophobicity evolved as a consequence of the conquest of land about 450 million years ago and may be a key innovation in the evolution of terrestrial life. The approximate 10 million extant species exhibit a stunning diversity of materials and structures, many of which are formed by self-assembly, and are solely based on a limited number of molecules. A short historical survey shows that bionics (today often called biomimetics) dates back more than 100 years. Statistical data illustrate that the interest in biomimetic surfaces is much younger still. Superhydrophobicity caught the attention of scientists only after the extreme superhydrophobicity of lotus leaves was published in 1997. Regrettably, parabionic products play an increasing role in marketing.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.