Theoretical optimization of micropillar arrays for structurally stable bioinspired dry adhesives

Inspired by the excellent adhesion performances of setae structure from organisms,micro/nano-pillar array has become one of the paradigms for adhesive surfaces.The micropillar arrays are composed of the resin pillars for adhesion and the substrate with different elastic modulus for supporting.The stress singularity at the bi-material corner between the pillars and the substrate can induce the failure of the micropillar-substrate corner and further hinder the fabrication and application of micropillar arrays,yet the design for the stability of the micropillar array lacks systematical and quantitative guides.In this work,we develop a semi-analytical method to provide the full expressions for the stress distribution within the bi-material corner combining analytical derivations and numerical calculations.The predictions for the stress within the singularity field can be obtained based on the full expressions of the stress.The good agreement between the predictions and the FEM results demonstrates the high reliability of our method.By adopting the strain energy density factor approach,the stability of the pillar-substrate corner is assessed by predicting the failure at the corner.For the elastic mismatch between the pillar and substrate given in this paper,the stability can be improved by increasing the ratio of the shear modulus of the substrate to that of the micropillar.Our study provides accurate predictions for the stress distribution at the bi-material corner and can guide the optimization of material combinations of the pillars and the substrate for more stable bioinspired dry adhesives.

PVC gel bio-inspired adhesives with variable modulus and its application in a gripper

Inspired by the microstructure of gecko’s toe,two kinds of polyvinyl chloride(PVC)gels with different modulus were poured on a silicon mold with micropillars,and then a bio-inspired adhesive with variable modulus was manufactured in this study.The adhesions of variable modulus and fixed modulus bio-inspired adhesives were tested,respectively,on a smooth glass and a printed circuit board(PCB)with different surface structures.The results show that PVC gel bio-inspired adhesives with variable modulus have many advantages compared with the fixed modulus bioinspired adhesives.The adhesion of variable modulus bio-inspired adhesives on the rough PCB surface increased by 2−5 times,and due to the use of variable modulus of PVC gel,the surface micropillars can maintain high aspect ratio and flexible tips at the same time.Moreover,the use of PVC gel makes it easier to demold during the bio-inspired adhesives preparation.An adhesion-desorption device was made according to the movement of the gecko toes,and the PCB was successfully grasped.

Switchable shape memory polymer bio-inspired adhesive and its application for unmanned aerial vehicle landing

Controlled and switchable adhesion is commonly observed in biological systems.In recent years,many scholars have focused on making switchable bio-inspired adhesives.However,making a bio-inspired adhesive with high adhesion performance and excellent dynamic switching properties is still a challenge.A Shape Memory Polymer Bio-inspired Adhesive(SMPBA)was successfully developed,well realizing high adhesion(about 337 kPa),relatively low preload(about90 kPa),high adhesion-to-preload ratio(about 3.74),high switching ratio(about 6.74),and easy detachment,which are attributed to the controlled modulus and contact area by regulating temperature and the Shape Memory Effect(SME).Furthermore,SMPBA exhibits adhesion strength of80–337 kPa on various surfaces(silicon,iron,and aluminum)with different roughness(Ra=0.021–10.280)because of the conformal contact,reflecting outstanding surface adaptability.The finite element analysis verifies the bending ability under different temperatures,while the adhesion model analyzes the influence of preload on contact area and adhesion.Furthermore,an Unmanned Aerial Vehicle(UAV)landing device with SMPBA was designed and manufactured to achieve UAV landing on and detaching from various surfaces.This study provides a novel switchable bio-inspired adhesive and UAV landing method.

Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

Transfer printing based on switchable adhesive is essential for developing unconventional systems,including flexible electronics,stretchable electronics,and micro light-emitting diode(LED)displays.Here we report a design of switchable dry adhesive based on shape memory polymer(SMP)with hemispherical indenters,which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP.Experimental and numerical studies reveal the fundamental aspects of design,fabrication,and operation of the switchable dry adhesive.Demonstrations of this adhesive concept in transfer printing of flat objects(e.g.,silicon wafers),three-dimensional(3D)objects(e.g.,stainless steel balls),and rough objects(e.g.,frosted glasses)in two-dimensional(2D)or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

Adhesion Wear on Tool Rake and Flank Faces in Dry Cutting of Ti-6Al-4V

Titanium alloys are very chemically reactive and,therefore,have a tendency to weld to the cutting tool during machining.The deterioration in the tool life caused by adhesion is a serious problem when titanium alloys are cut using carbide tools.The chemical reactivity of titanium alloys with carbide tool materials and their consequent welding by adhesion onto the cutting tool during dry cutting leads to excessive chipping,premature tool failure,and poor surface finish.In the present study,dry turning and milling tests were carried out on Ti-6Al-4V alloys with WC?Co carbide tools.The adhesion on the tool rake and flank face was explored,the adhesive joint interface between the workpiece materials and tools were observed.SEM observation showed that adhesion can be observed both on the rake and the flank face,and was more pronounced in rake face than in flank face.There was evidence of element diffusion from the tool rake face to the adhering layer(vice versa) through the adhesive joint interface,which leads to the tool element loss and microstructure change.While the adhering materials at the flank face can be easily separated from the joint interface owing to the lower temperature and less pressure at the flank face,the adhesive wear attack results in an abrasive wear in the flank face.Moreover,adhesion is more notable in turning than in milling.The proposed research provides references for studying the adhesion between the workpiece materials and the tools,the adhesion mechanisms and their effect on the tool wear.

Dynamic culture improves MSC adhesion on freeze-dried bone as a scaffold for bone engineering

AIM: To investigate the interaction between mesenchymal stem cells (MSCs) and bone grafts using two different cultivation methods: static and dynamic. METHODS: MSCs were isolated from rat bone marrow. MSC culture was analyzed according to the morphology, cell differentiation potential, and surface molecular markers. Before cell culture, freeze-dried bone (FDB) was maintained in culture for 3 d in order to verify culture medium pH. MSCs were co-cultured with FDB using two different cultivation methods: static co-culture (two-dimensional) and dynamic co-culture (three-dimensional). After 24 h of cultivation by dynamic or static methods, histological analysis of Cell adhesion on FDB was performed. Cell viability was assessed by the Trypan Blue exclusion method on days 0, 3 and 6 after dynamic or static culture. Adherent cells were detached from FDB surface, stained with Trypan Blue, and quantified to determine whether the cells remained on the graft surface in prolonged non-dynamic culture. Statistical analyses were performed with SPSS and a P < 0.05 was considered significant. RESULTS: The results showed a clear potential for adipogenic and osteogenic differentiation of MSC cultures. Rat MSCs were positive for CD44, CD90 and CD29 and negative for CD34, CD45 and CD11bc. FDBs were maintained in culture for 3 d and the results showed there was no significant variation in the culture medium pH with FDB compared to pure medium pH (P > 0.05). In histological analysis, there was a significant difference in the amount of adhered cells on FDB between the two cultivation methods (P < 0.05). The MSCs in the dynamic co-culture method demonstrated greater adhesion on the bone surface than in static co-culture method. On day 0, the cell viability in the dynamic system was significantly higher than in the static system (P < 0.05). There was a statistical difference in cell viability between days 0, 3 and 6 after dynamic culture (P < 0.05). In static culture, cell viability on day 6 was significantly lower than on day 3 and 0 (P < 0.05). CONCLUSION: An alternative cultivation method was developed to improve the MSCs adhesion on FDB, demonstrating that dynamic co-culture provides a superior environment over static conditions.

Optimized Bio-inspired Micro-pillar Dry Adhesive and Its Application for an Unmanned Aerial Vehicle Adhering on and Detaching from a Ceiling

Various bio-inspired dry adhesive materials have been investigated.In this work,lithography and silicon deep etching method were used to fabricate bio-inspired micro-pillar dry adhesive materials of which the parameters,such as side length,Height to Side length Aspect Ratio(HSAR),and inter-pillar Distance to Side length Aspect Ratio(DSAR)were comprehensively studied to obtain a dry adhesive with high adhesion.An orthogonal array method was designed and a series of fabrication experiments were conducted to identify optimum parameters,which resulted in a high normal adhesion of 2.98 N·cm^-2 and a shear adhesion of 9.59 N·cm^-2.An adhesion and desorption device was further designed on basis of the optimum dry adhesive,which enables an Unmanned Aerial Vehicle(UAV)to successfully adhere on and detach from a ceiling.This would allow an UAV to stay aloft for prolonged time,which could be invaluable to many applications,such as energy conservation and aerial detection.

A Mechanical Model for the Adhesion of Spiders to Nominally Flat Surfaces

In dry attachment systems of spiders and geckos, van der Waals forces mediate attraction between substrate and animal tarsus. In particular, the scopula of Evarcha arcuata spiders allows for reversible attachment and easy detachment to a broad range of surfaces. Hence, reproducing the scopula's roughness compatibility while maintaining anti-bunching features and dirt particle repellence behavior is a central task for a biomimetic transfer to an engineered model. In the present work we model the scopula of E. arcuata from a mechano-elastic point of view analyzing the influence of its hierarchical structure on the attachment behavior. By considering biological data of the gecko and spider, and the simulation results, the adhesive capabilities of the two animals are compared and important confirmations and new directives in order to reproduce the overall structure are found. Moreover, a possible suggestion of how the spider detaches in an easy and fast manner is proposed and supported by the results.

Enhanced Compliant Adhesive Design and Fabrication with Dual-Level Hierarchical Structure

Synthetic dry adhesives inspired by the nano-and micro-scale hairs found on the feet of geckos and some spiders have beendeveloped for almost a decade. Elastomeric single level micro-scale mushroom shaped fibres are currently able to function evenbetter than natural dry adhesives on smooth surfaces under normal loading. However, the adhesion of these single level syntheticdry adhesives on rough surfaces is still not optimal because of the reduced contact surface area. In nature, contact area ismaximized by hierarchically structuring different scales of fibres capable of conforming surface roughness. In this paper, weadapt the nature’s solution arid propose a novel dual-level hierarchical adhesive design using Polydimethylsiloxane (PDMS),which is tested under peel loading at different orientations. A negative macro-scale mold is manufactured by using a laser cutterto define holes in a Poly(methyl methacrylate) (PMMA) plate. After casting PDMS macro-scale fibres by using the obtainedPMMA mold, a previously prepared micro-fibre adhesive is bonded to the macro-scale fibre substrate. Once the bondingpolymer is cured, the micro-fibre adhesive is cut to form macro scale mushroom caps. Each macro-fibre of the resulting hierarchicaladhesive is able to conform to loads applied in different directions. The dual-level structure enhances the peel strengthon smooth surfaces compared to a single-level dry adhesive, but also weakens the shear strength of the adhesive for a given areain contact. The adhesive appears to be very performance sensitive to the specific size of the fibre tips, and experiments indicatethat designing hierarchical structures is not as simple as placing multiple scales of fibres on top of one another, but can requiresignificant design optimization to enhance the contact mechanics and adhesion strength.

First mushroom-shaped adhesive microstructure:A review

This letter reviews the adhesive and frictional properties of the first mushroom-shaped adhesive microstructure （MSAMS）, which has come a long way from inspiration by the attachment devices evolved in beetles to a large-scale industrial production. It was shown to have an that about twice higher pull-off force compared to a smooth control made from the same material measured on smooth substrates. Pull-off forces measured underwater are even higher than those in air. Moreover, it retained adhesive performance over thousands of attachment cycles and initial adhesive capability could be recovered by washing after being contaminated. In shearing, MSAMS exhibits reduced and stabilized friction in comparison with a smooth control, which demonstrated pronounced stick-slip motion, and shows zero pull-off force in a sheared state, allowing the adhesion to be switched on and off. The presence of a fluid in the contact zone showed adhesion enhancement on both smooth and rough substrates. All these features lead us to conclude that MSAMS may have practical potential in a variety of applications.

Review:Resuspension of wall deposits in spray dryers

Wall deposition occurs in spray dryers when dried or partially dried particles contact and adhere to the walls during operation, thus reducing the yield of product collected. Wall deposits also present a product contamination risk and a fire or explosion risk when spray drying products that oxidize exothermically, such as milk powder. Re-entrainment is the resuspension of spray dryer wall deposits into the main gas stream for collection as product. Literature suggests that the process for re-entrainment of particles from spray dryer wall deposits is strongly dependent on particle size and gas velocity.

重组香菇鸭肉脯的配方和加工工艺优化

以鸭肉为主要原料,以香菇为特色风味原料,通过单因素试验和正交试验优化重组香菇鸭肉脯的配方和加工工艺。结果表明:重组香菇鸭肉脯辅料的最佳添加量为食盐0.8%,鱼露4%,料酒6%,香菇4%,此时感官评分为96.8。黏合剂的最佳添加量为鸡蛋清3%、玉米淀粉3%、卡拉胶1.0%,此时重组香菇鸭肉脯的拉伸力为12.8 N。最佳加工工艺参数为烘干时间30 min、烘干温度60℃、烘烤时间6 min、烘烤温度190℃,此时的感官评分为97。

Bio-inspired Superhydrophobic Coating with Low Hydrate Adhesion for Hydrate Mitigation

In this paper,we fabricate a biomimetic superhydrophobic coating on an X90 pipeline steel substrate by electrodeposition of copper,hydrothermal treatment to form a copper oxide layer,and subsequent surface modification with Stearic Acid(SA).The coating exhibits static contact angles of water of 160°±-3.1°in the air and 170.7°±2.5°in cyclopentane,indicating the strongly water-repelling nature of the coating.The morphologies of the cyclopentane hydrate formed on steel substrates with and without the superhydrophobic coating are investigated.The results show that the hydrate particle on the coating exhibits spherical morphology and herein the interfacial contact area and adhesion force to the solid surface can be essentially reduced.The adhesion force reduction may be resulted by the decrease in the contact area between the hydrate and the solid surface.

Magnetic-field-driven switchable adhesion of NdFeB/PDMS composite with gecko-like surface

Switchable adhesives have attracted widespread attention due to their strong reusability and adaptability to operate stably in complex environments.However,the simple fabrication of adhesive structures and reliable control of adhesion remain challenging.Here,we developed a neodymium iron boron/polydimethylsiloxane(NdFeB/PDMS)magnetic composite with optimal mechanical and magnetic performance.Then we fabricated lamellar structures and setal arrays using a molding and magnetic field-induced process,imitating the multi-level adhesion system of gecko feet.The lamellar can be deformed under the action of a magnetic field to control the adhesion,the setal array is used to enhance adhesion and provide self-cleanability to the adhering surface.Switchable adhesion was realized by applying an external magnetic field,where the maximum adhesion strength was 5.1 kPa,the switchable range was within 40%.Through finite element analysis simulations and experimental verification,it was proved that the adhesion force variation was ascribed to the magnetic field-induced surface deformation.Finally,we installed the adhesive on the end of the robotic arm,realizing the transfer of the target object.This work provides a simple method to fabricate a gecko-like surface and a practical strategy to realize switchable adhesion,which sheds light on broad application potential in production lines,medical products,more.

Cleaning properties of dry adhesives

In this paper we present a study into the cleaning properties of synthetic dry adhesives. We have manufactured the adhesive micro-fibres through a low-cost, high yield fabrication method using Sylgard 184 Polydimethylsiloxane (PDMS) as the structural material. We deliberately contaminated the adhesive samples with different sized particles in the micro and macro scales and tested different cleaning methods for their efficacy with respect to each particle size. We investigated different cleaning methods, which included the use of wax moulding, vibration and pressure sensitive adhesives. For adhesion testing we used a custom system with a linear stage and a force sensor indenting a hemispherical probe into the adhesive surface and measuring the pull-off force. To characterize the cleaning efficacy we visually inspected each sample in a microscope and weighed the samples with a microgram-accuracy analytical balance. Results showed that the moulding method induced adhesion recovery in a greater percentage than the other cleaning methods and even helped with the recovery of collapsed posts in some cases. On the other hand pressure sensitive adhesives seem to have the upper hand with regards to certain particle sizes that can potentially pose problems with the moulding method.

Fabrication and Testing of Self Cleaning Dry Adhesives Utilizing Hydrophobicity Gradient

In this paper we present a method to create a hydrophobicity gradient on the surface of a Polydimethylsiloxane （PDMS） dry adhesive. The method consists of the partial silanization of the surface of the dry adhesive by Chemical Vapour Deposition （CVD） of octadecyltrichlorosilane （OTS）. The partial silanization of the surface of the sample results in a hydrophobic to hy- drophilic gradient across the surface of the dry adhesive. The resulting change in hydrophobicity across the surface of the dry adhesive results in the uphill motion of a droplet of water, which appears to be directly proportional to the area of contact between the droplet and the adhesive. Normal adhesion testing is performed to quantify the effect of the hydrophobic gradient across the surface of the sample. While a variation in adhesion are only minimally affected by the silanization, and the motion strength across the sample is measured, the adhesive properties of the droplet of water doesn＇t cause any loss of adhesion.

低成本快干型淀粉胶的研究

选用新型的催化剂,探讨了各种因素对淀粉氧化和糊化的影响,结果表明,制备淀粉胶的最佳条件为:40%的淀粉乳中加入5%的氧化剂,在催化剂存在时于58℃下氧化30min,加入10%的NaOH糊化,加填料、加水调节固含量至12%(均以淀粉质量为基准),合成的玉米淀粉胶粘剂成本低、干燥快,有望彻底取代泡花碱。

重组水蛭素-2鼻腔给药纳米粒冻干粉制备工艺及体外渗透特性研究

目的:优选重组水蛭素-2(rHV2)鼻腔给药纳米粒冻干粉最佳制备工艺;考察rHV2纳米粒冻干粉稳定性及经家兔鼻黏膜的体外渗透特性。方法:选用不同冻干保护剂制备水蛭素纳米粒冻干粉末,以外观、再分散性及包封率作为评价指标,对冻干保护剂及制备工艺进行优选,并考察了光照、高温等因素对冻干粉的影响;采用直立式扩散池法,以家兔鼻中隔黏膜为渗透屏障,测定rHV2壳聚糖溶液、rHV2纳米粒溶液及冻干粉的渗透系数(P),并计算与rHV2生理盐水溶液的促渗比(ER)。结果:rHV2鼻腔给药纳米粒冻干粉最佳制备工艺为:用5%甘露醇与葡萄糖(1∶1)作为冻干保护剂,于-70℃预冻24h后,置真空冻干箱中冷冻干燥,24h后取出,该冻干粉中rHV2的含量为1.1μg/mg。光照对rHV2纳米粒冻干粉的外观、再分散性以及包封率影响均较小,而高温对rHV2纳米粒冻干粉的影响较大;rHV2壳聚糖纳米粒具有较高的渗透系数,是生理盐水组的5倍多,冻干粉仍然保持了良好的渗透性。结论:壳聚糖鼻腔给药纳米粒冻干粉有可能成为rHV2鼻腔给药的良好给药剂型。

玉米酒精粕基木材胶黏剂的制备及其性能

以玉米酒精粕(DDGS)为原料、聚酰胺环氧氯丙烷树脂(PAE)为交联剂、水溶性豆粕(SM)为增强剂制备玉米酒精粕基木材胶黏剂,对比了DDGS与SM蛋白质含量及其氨基酸种类与含量,测试了胶黏剂黏度与制备胶合板的胶合强度,表征了固化胶黏剂功能性基团、热稳定性、断面形态,解析了DDGS基胶黏剂组分对性能的影响规律与增强作用机制。结果表明:与SM相比较,DDGS中蛋白含量低46.3%,蛋白质亲水侧链氨基酸含量低22.0%,疏水侧链氨基酸高43.4%,DDGS蛋白溶解性和反应活性低于SM蛋白;当原料m(DDGS)∶m(SM)=3∶1时,胶黏剂制备胶合板耐水胶合强度为0.91 MPa,较DDGS胶黏剂提高727.3%,达到国家Ⅱ类胶合板标准要求,同时原料成本降低29.2%;相比SM胶黏剂,m(DDGS)∶m(SM)=1∶3时,胶黏剂制备胶合板耐水胶合强度提高7.5%,达1.43 MPa,其原因是SM的加入提高了胶黏剂交联密度;胶黏剂黏度降低了32.4%,更容易渗透到木材孔隙中形成机械结合力;DDGS中含有7%纤维,可以对胶黏剂产生物理增强作用,阻碍裂隙延伸,提高内聚力。因此,利用DDGS为原料可有效制备耐水木材胶黏剂并用于胶接人造板,有利于推动蛋白基木材胶黏剂的工业化应用。

Multi-Scale Compliant Foot Designs and Fabrication for Use with a Spider-Inspired Climbing Robot

Climbing robots are of potential use for surveillance, inspection and exploration in different environments. In particular, the use of climbing robots for space exploration can allow scientists to explore environments too challenging for traditional wheeled designs. To adhere to surfaces, biomimetic dry adhesives based on gecko feet have been proposed. These biomimetic dry adhesives work by using multi-scale compliant mechanisms to make intimate contact with different surfaces and adhere by using Van der Waals forces. Fabrication of these adhesives has frequently been challenging however, due to the difficulty in combining macro, micro and nanoscale compliance. We present an all polymer foot design for use with a hexapod climbing robot and a fabrication method to improve reliability and yield. A high strength, low-modulus silicone, TC-5005, is used to form the foot base and microscale fibres in one piece by using a two part mold. A macroscale foot design is produced using a 3D printer to produce a base mold, while lithographic definition of microscale fibres in a thick photoresist forms the ＇hairs＇ of the polymer foot. The adhesion of the silicone fibres by themselves or attached to the macro foot is examined to determine best strategies for placement and removal of feet to maximize adhesion. Results demonstrate the successful integration of micro and macro compliant feet for use in climbing on a variety of surfaces.