Simple and Reliable Fabrication of Bioinspired Mushroom-Shaped Micropillars with Precisely Controlled Tip Geometries

Vacuum-powered soft actuator with oblique air chambers for easy detachment of artificial dry adhesive by coupled contraction and twisting

A gecko foot-inspired, mushroom-shaped artificial dry adhesive exploiting intermolecular forces between microstructure and surface has drawn research attention for its strong adhesive force. However, the high pull-off strength corresponding to the adhesive force matters when detaching fragile substrates. In this study, we report a vacuum-powered soft actuator having oblique air chambers and a dry adhesive. The soft actuator performs coupled contraction and twisting by applying negative pneumatic pressure inward and exhibits not only high pull-off strength but also easy detachment. This effective detachment can be achieved thanks to the twisting motion of the soft actuator. The detachment performances of the actuator models are assessed using a 6-degrees-of-freedom robot arm. Results show that the soft actuators exhibit remarkable pull-off strength decrement from ~20 N cm-2 to ~2 N cm-2 due to the twisting. Finally, to verify a feasible application of this study, we utilize the inherent compliance of the actuators and introduce a glass transfer system for which a glass substrate on a slope is gripped by the flexibility of the soft actuators and delivered to the destination without any fracture.

Bioinspired Microadhesives with Greatly Enhanced Reversible Adhesion Fabricated by Synthesized Silicone Elastomer with Increasing Phenyl Contents

We present a facile chemical method for fabricating bioinspired microadhesives with significant improved reversible adhesion strength. Four kinds of polysiloxane with gradient varying phenyl contents were synthesized and used to fabricate microadhesives. The chemical structures and mechanical properties, as well as surface properties of the four microadhesives, were confirmed and characterized by ATR-FTIR, DSC, XPS, low-field NMR, tensile tests, and SEM, respectively. The macroadhesion test results revealed that phenyl contents showed remarkable and positive impacts on the macroadhesion performance of microadhesives. The pull-off adhesion strength of microadhesives with 90% phenyl content (0.851 N/cm2) was nearly 300% higher than that of pure PDMS (0.309 N/cm2). The macroadhesion mechanism analysis demonstrates that a larger bulk energy dissipation caused by massive π-π interaction, as well as the hydrophobic interaction and van der Waals forces at the interface synergistically resulted in a significant enhancement of the adhesion performance. Our results demonstrate the remarkable impact of chemical structures on the adhesion of microadhesives, and it is conducive to the further improvement of adhesion properties of bioinspired microadhesives.

Core-shell dry adhesives for rough surfaces via electrically responsive self-growing strategy

Bioinspired dry adhesives have an extraordinary impact in the field of robotic manipulation and locomotion. However, there is a considerable difference between artificial structures and biological ones regarding surface adaptability, especially for rough surfaces. This can be attributed to their distinct structural configuration and forming mechanism. Here, we propose a core-shell adhesive structure that is obtained through a growth strategy, i.e., an electrically responsive self-growing core-shell structure. This growth strategy results in a specific mushroom-shaped structure with a rigid core and a soft shell, which exhibits excellent adhesion on typical target surfaces with roughness ranging from the nanoscale to the microscale up to dozens of micrometers. The proposed adhesion strategy extends dry adhesives from smooth surfaces to rough ones, especially for rough surfaces with roughness up to dozens or hundreds of micrometers, opening an avenue for the development of dry adhesive-based devices and systems.

Fresnel Diffraction Strategy Enables the Fabrication of Flexible Superomniphobic Surfaces

Doubly re-entrant surfaces inspired by springtails exhibit excellent repellency to low-surface-tension liquid. However, the flexible doubly re-entrant surfaces are difficult to fabricate, especially for the overhang of the structure. Herein, we demonstrate a simple Fresnel aperture diffraction modulation strategy in microscale lithography coupled with a molding process to obtain the flexible doubly re-entrant superomniphobic surfaces with nanoscale overhangs. The negative nanoscale overhang features were formed in a single-layer photoresist due to the fine-modulation of the optical intensity fluctuation of the Fresnel aperture diffraction. The as-prepared flexible non-fluorinated polydimethylsiloxane (PDMS) doubly re-entrant microstructure based on the Fresnel aperture diffraction (D-BF) surface (without any additional treatments) could repel ethanol droplets (21.8 mN m^-1) in the Cassie-Baxter state. The robust nanoscale overhangs obtained by the molding process enable the maximum breakthrough pressure for the low-surface-tension ethanol droplets on the D-BF surfaces up to about 230 Pa, allowing ethanol liquids with Weber numbers up to 8.7 to fully bounce off. The fabricated non-fluorinated D-BF superomniphobic surface maintains outstanding liquid repellency after the surface wettability modification and deformation test.

One-Step Fabrication of Flexible Bioinspired Superomniphobic Surfaces

Flexible superomniphobic doubly re-entrant (Dual-T) microstructures inspired by springtails have attracted growing attention due to their excellent liquid-repellent properties. However, the simple and practical manufacturing processes of the flexible Dual-T microstructures are urgently needed. Here, we proposed a one-step molding process coupled with the lithography technique to fabricate the elastomeric polydimethylsiloxane (PDMS) Dual-T microstructure surfaces with high uniformity. The angle between the downward overhang and the horizontal direction could reach 90° (vertical overhang). The flexible superomniphobic Dual-T microstructure surfaces, without fluorination treatment and physical treatments, could repel liquids with a surface tension lower than 20 mN m^-1 in the Cassie-Baxter state. Owing to the excellent robustness of the one-step molding downward overhanging, the max breakthrough pressure of this surface could reach up to 164.3 Pa for ethanol droplets. Furthermore, the flexible superomniphobic Dual-T surface allowed impinging ethanol droplets to completely rebound at the Weber number up to 7.1 with an impact velocity of ∼0.32 m s^-1. The Dual-T microstructure surface maintained excellent superomniphobicity even after surface oxygen plasma treatment and exhibited excellent structural robustness and recoverability to various large mechanical deformations.

Smart Mushroom-Inspired Imprintable and Lightly Detachable (MILD) Microneedle Patterns for Effective COVID-19 Vaccination and Decentralized Information Storage

The key to controlling the spread of the coronavirus disease 2019 (COVID-19) and reducing mortality is highly dependent on the safe and effective use of vaccines for the general population. Current COVID-19 vaccination practices (intramuscular injection of solution-based vaccines) are limited by heavy reliance on medical professionals, poor compliance, and laborious vaccination recording procedures, resulting in a waste of health resources and low vaccination coverage, etc. In this study, we developed a smart mushroom-inspired imprintable and lightly detachable (MILD) microneedle platform for the effective and convenient delivery of multidose COVID-19 vaccines and decentralized vaccine information storage. The mushroom-like structure allows the MILD system to be easily pressed into the skin and detached from the patch base, acting as a "tattoo" to record the vaccine counts in situ without any storage equipment, offering quick accessibility and effortless readout, saving a great deal of valuable time and energy for both patients and health professionals. After loading inactivated SARS-CoV-2 virus-based vaccines, MILD system induced a high level of antibodies against the SARS-CoV-2 receptor-binding domain (RBD) in vivo without eliciting systemic toxicity and local damage. Collectively, this smart delivery platform serves as a promising carrier to improve COVID-19 vaccination efficacy through its dual capabilities of vaccine delivery and in situ data storage, thus exhibiting great potential for helping to contain the COVID-19 pandemic or a resurgence.

Contact Printing of Multilayered Thin Films with Shape Memory Polymers

This study describes a method for transfer printing microarrays of multilayered organic-inorganic thin films using shape memory printing stamps and microstructured donor substrates. By applying the films on the microstructured donor substrates during physical vapor deposition and modulating the interfacial adhesion using a shape memory elastomer during printing, this method achieves (1) high lateral and feature-edge resolution and (2) high transfer efficiency from the donor to the receiver substrate. For demonstration, polyurethane-acrylate stamps and silicon/silicon oxide donor substrates were used in the large-area transfer printing of organic-inorganic thin-film stacks with micrometer lateral dimensions and sub-200 nm thickness.

Gecko-Inspired Biomimetic Surfaces with Annular Wedge Structures Fabricated by Ultraprecision Machining and Replica Molding

The current research on gecko-inspired dry adhesives is focused on micropillar arrays with different terminal shapes, such as flat, spherical, mushroom, and spatula tips. The corresponding processing methods are mostly chemical methods, including lithography, etching, and deposition, which not only are complex, expensive, and environmentally unfriendly, but also cannot completely ensure microstructural integrity or performance stability. The present study demonstrates a high-precision, high-efficiency, and green method for the fabrication of a gecko-inspired surface, which can promote its application in dexterous robot hands and mechanical grippers. Based on the bendable lamellar structures of the gecko, annular wedge adhesive surfaces that stick to the finger surfaces of dexterous robot hands to improve their load capacity are proposed and fabricated via a suitable combined processing method of ultraprecision machining and replica molding. The greater the width, the higher the replication integrity, and when the minimum width is 20 μm, the replication error is less than 5.5% due to the superior processing performance of the nickel-phosphorus (Ni-P) plating of the master mold. The fabricated annular wedge structures with an optimized width of 20 μm not only exhibit a strong friction force of up to 35.48 mN under a preload of 20 mN in the GCr15/poly(dimethylsiloxane) (PDMS) friction pair but also demonstrate an obviously improved anisotropic friction characteristic of up to λ = 1.36, as the molecular force exhibits a stronger increase as compared to the decrease of the mechanical force of the structure with a small width.

A Pressure-Insensitive Self-Attachable Flexible Strain Sensor with Bioinspired Adhesive and Active CNT Layers

Flexible tactile sensors are required to maintain conformal contact with target objects and to differentiate different tactile stimuli such as strain and pressure to achieve high sensing performance. However, many existing tactile sensors do not have the ability to distinguish strain from pressure. Moreover, because they lack intrinsic adhesion capability, they require additional adhesive tapes for surface attachment. Herein, we present a self-attachable, pressure-insensitive strain sensor that can firmly adhere to target objects and selectively perceive tensile strain with high sensitivity. The proposed strain sensor is mainly composed of a bioinspired micropillar adhesive layer and a selectively coated active carbon nanotube (CNT) layer. We show that the bioinspired adhesive layer enables strong self-attachment of the sensor to diverse planar and nonplanar surfaces with a maximum adhesion strength of 257 kPa, while the thin film configuration of the patterned CNT layer enables high strain sensitivity (gauge factor (GF) of 2.26) and pressure insensitivity.

Reliable and Robust Fabrication Rules for Springtail-Inspired Superomniphobic Surfaces

We report a reliable and robust method for the fabrication of bioinspired superomniphobic surfaces with precise concave-cap-shaped micropillar arrays. This method includes silicon-based conventional microelectromechanical systems (MEMS) and polymer replication processes. We have elucidated two critical cases of fabrication rules for precise micromachining of a negative-shaped bioinspired silicon master. The fabricated polymeric structure replicated from the semipermanent silicon master based on the design rules exhibited high structural fidelity and robustness. Finally, we validated the superomniphobic properties, structural durability, and long-term stability of the fabricated bioinspired surfaces.

Strong and Reversible Adhesion of Interlocked 3D-Microarchitectures

Diverse physical interlocking devices have recently been developed based on one-dimensional (1D), high-aspect-ratio inorganic and organic nanomaterials. Although these 1D nanomaterial-based interlocking devices can provide reliable and repeatable shear adhesion, their adhesion in the normal direction is typically very weak. In addition, the high-aspect-ratio, slender structures are mechanically less durable. In this study, we demonstrate a highly flexible and robust interlocking system that exhibits strong and reversible adhesion based on physical interlocking between three-dimensional (3D) microscale architectures. The 3D microstructures have protruding tips on their cylindrical stems, which enable tight mechanical binding between the microstructures. Based on the unique 3D architectures, the interlocking adhesives exhibit remarkable adhesion strengths in both the normal and shear directions. In addition, their adhesion is highly reversible due to the robust mechanical and structural stability of the microstructures. An analytical model is proposed to explain the measured adhesion behavior, which is in good agreement with the experimental results.

Continuous Fabrication of Wide-Tip Microstructures for Bio-Inspired Dry Adhesives via Tip Inking Process

In this paper, we report a new method for continuous fabrication of dry adhesives composed of microstructures with mushroom-shaped ends. Conventional mushroom microstructure fabrication is performed with a simple molding technique using a reversed phase master. In a typical fabrication process, thin- and wide-tip portions may be ripped during demolding, making it difficult to use in a continuous process. It is also difficult to apply the mushroom structure master to a continuous process system in roll form. Here, a continuous fabrication process was developed by applying the method of fabricating a wide tip using a tip inking method after forming a micropillar. Through the continuous process, the dry adhesive was successfully fabricated and the durability was measured with a reasonable pull-off strength (13 N/cm2). In addition to the reasonable adhesion, high durability is guaranteed, and fabricated dry adhesives are expected to be used in various fields.

Multilayered inorganic-organic microdisks as ideal carriers for high magnetothermal actuation: assembling ferrimagnetic nanoparticles devoid of dipolar interactions

The two major limitations for nanoparticle based magnetic hyperthermia in theranostics are the delivery of a sufficient number of magnetic nanoparticles (MNPs) with high heating power to specific target cells and the residence time of the MNPs at the target location. Ferromagnetic or Ferrimagnetic single domain nanoparticles (F-MNPs), with a permanent magnetic dipole, produce larger magnetic and thermal responses than superparamagnetic nanoparticles (SP-MNPs) but also agglomerate more. MNP agglomeration degrades their heating potential due to dipolar interaction effects and interferes with specific targeting. Additionally, MNPs bound to cells are often endocytosed by the cells or, in vivo, cleared out by the immune system via uptake in macrophages. Here, we present a versatile approach to engineer inorganic-polymeric microdisks, loaded with biomolecules, fluorophores and Fe3O4 F-MNPs that solves both challenges. These microdisks deliver the F-MNPs efficiently, while controlling any undesirable agglomeration and dipolar interaction, while also rendering the F-MNPs endocytosis resistant. We show that these micro-devices are suitable carriers to transport a flat assembly of F-MNPs to the cell membrane unchanged, preserving the magnetic response of the MNPs in any biological environment. The F-MNPs concentration per microdisk and degree of MNP interaction are tunable. We demonstrate that the local heat generated in microdisks is proportional to the surface density of F-MNPs when attached to the cell membrane. The key innovation in the production of these microdisks is the fabrication of a mushroom-shaped photolithographic template that enables easy assembly of the inorganic film, polymeric multilayers, and MNP cargo while permitting highly efficient lift-off of the completed microdisks. During the harvesting of the flat microdisks, the supporting mushroom-shaped templates are sacrificed. These resulting magnetic hybrid microdisks are tunable and efficient devices for magnetothermal actuation and hyperthermia.

Continuous Tip Widening Technique for Roll-to-Roll Fabrication of Dry Adhesives

In this study, we reported continuous partial curing and tip-shaped modification methods for continuous production of dry adhesive with microscale mushroom-shaped structures. Typical fabrication methods of dry adhesive with mushroom-shaped structures are less productive due to the failure of large tips on pillar during demolding. To solve this problem, a typical pillar structure was fabricated through partial curing, and tip widening was realized through applying the proper pressure. Polyurethane acrylate was used in making the mushroom structure using two-step UV-assisted capillary force lithography (CFL). To make the mushroom structure, partial curing was performed on the micropillar, followed by tip widening. Dry adhesives with properties similar to those of typical mushroom-shaped dry adhesives were fabricated with reasonable adhesion force using the two-step UV-assisted CFL. This production technology was applied to the roll-to-roll process to improve productivity, thereby realizing continuous production without any defects. Such a technology is expected to be applied to various fields by achieving the productivity improvement of dry adhesives, which is essential for various applications.

Multifunctional Smart Skin Adhesive Patches for Advanced Health Care

A skin adhesive patch is the most fundamental and widely used medical device for diverse health-care purposes. Conventional skin adhesive patches have been mainly utilized for routine medical purposes such as wound management, fixation of medical devices, and simple drug release. In contrast to traditional skin adhesive patches, recently developed patches incorporate multiple key functions of bulky medical devices into a thin, flexible patch based on emerging nanomaterials and flexible electronic technologies. Consequently, the meaning of the term "skin adhesive patch" becomes broader and smarter compared to the traditional term. This review summarizes recent efforts undertaken in the development of multifunctional advanced skin adhesive patches, and briefly describes future directions and challenges toward the next generation of smart skin adhesive patches for ubiquitous personalized health care.

Slanted Functional Gradient Micropillars for Optimal Bioinspired Dry Adhesion

For biologically inspired dry adhesives, the fibrillar structure of the surface requires sufficient flexibility to form contacts and meanwhile high rigidity to maintain stability. This fundamental conflict has greatly hindered the advance of synthetic adhesives toward mass-scale and practical applications, where adhesion is desired to be simultaneously strong, durable, directional, and roughness-adaptive. In this work, we overcome such a long-term challenge by developing fibrillar structures that combine both slanted geometry and gradient material of micropillars. The termed slanted functional gradient pillars (s-FGPs), fabricated by a magnetically assisted mold replication technique, exhibit flexible tips for contacts, gradually stiffened stalks for reinforcement, slanted structure to give rise to anisotropy, and high aspect ratio (AR) to facilitate surface adaptation. We demonstrate that the material and structure of the s-FGPs complement each other, synergetic effects of which result in a multifunctional combination of adhesion properties including high strength (∼9 N/cm² in shear), ultradurability (over 200 cycles of attachment/detachment without adhesion degradation), super anisotropy (anisotropic ratio of ∼7), and good adaptability to rough surfaces. The s-FGPs not only step forward the bioinspired adhesion toward optimized designs and performances for practical applications but may also open up other concepts for various high-AR and structurally stable fibrillar surfaces with emerging functionalities and applications in the fields of self-cleaning, superhydrophobicity, biosensors, energy harvesting, etc.

Bioinspired reversible hydrogel adhesives for wet and underwater surfaces

A hydrogel-based wet adhesive with bioinspired microstructures can exhibit strong and reversible adhesion to wet and underwater surfaces.

Flexible and Shape-Reconfigurable Hydrogel Interlocking Adhesives for High Adhesion in Wet Environments Based on Anisotropic Swelling of Hydrogel Microstructures

This study presents wet-responsive, shape-reconfigurable, and flexible hydrogel adhesives that exhibit strong adhesion under wet environments based on reversible interlocking between reconfigurable microhook arrays. The experimental investigation on the swelling behavior and structural characterization of the hydrogel microstructures reveal that the microhook arrays undergo anisotropic swelling and shape transformation upon contact with water. The adhesion between the interlocked microhook arrays is greatly enhanced under wet conditions because of the hydration-triggered shape reconfiguration of the hydrogel microstructures. Furthermore, wet adhesion monotonically increases with water-exposure time. A maximum adhesion force of 79.9 N cm^-2 in the shear direction is obtained with the hydrogel microhook array after 20 h of swelling, which is 732.3% greater than that under dry conditions (i.e., 9.6 N cm^-2). A simple theoretical model is developed to describe the measured adhesion forces. The results are in good agreement with the experimental data.

A Review of the State of Dry Adhesives: Biomimetic Structures and the Alternative Designs They Inspire

Robust and inexpensive dry adhesives would have a multitude of potential applications, but replicating the impressive adhesive organs of many small animals has proved challenging. A substantial body of work has been produced in recent years which has illuminated the many mechanical processes influencing a dry adhesive interface. The especially potent footpads of the tokay gecko have inspired researchers to develop and examine an impressive and diverse collection of artificial fibrillar dry adhesives, though study of tree frogs and insects demonstrate that successful adhesive designs come in many forms. This review discusses the current theoretical understanding of dry adhesive mechanics, including the observations from biological systems and the lessons learned by recent attempts to mimic them. Attention is drawn in particular to the growing contingent of work exploring ideas which are complimentary to or an alternative for fibrillar designs. The fundamentals of compliance control form a basis for dry adhesives made of composite and “smart,” stimuli-responsive materials including shape memory polymers. An overview of fabrication and test techniques, with a sampling of performance results, is provided.