Patent Application: US-83463107-A

Abstract:
nano - scale or micro - scale adhesive structures comprising an array of nano - fabricated , pillars , the pillars having coated upon , or having disposed on a working surface thereof , a protein - mimetic , marine - adhesive coating . methods of fabricating the nano - scale pillars , synthesis of the protein - mimetic coating or wet adhesive and application of the adhesive to the pillars are described .

Description:
described herein is a new class of hybrid biologically - inspired adhesives comprising an array of nanofabricated polymer columnar pillars coated with a thin layer of a synthetic polymer that mimics the wet adhesive proteins found in mussel holdfasts . wet adhesion of the nanostructured polymer pillar arrays of this invention increased nearly 15 - fold when coated with mussel adhesive protein mimetic polymer . the adhesive can function like a sticky note ( a repositionable partially adherent note structure ), maintaining its adhesive performance for over 1000 contact cycles in both dry and wet environments . this hybrid adhesive , which uniquely combines the salient design elements of both gecko and mussel adhesives , provides a useful reversible attachment means for a variety of surfaces in many environments . “ single use ” or single attachments also are contemplated . the adhesive forces of the gecko have been observed to be on the order of 40 μn or more per seta 11 , 12 and 10 nn per spatula . 13 gecko adhesion has been explained as arising from weak secondary bond forces such as van der waals . 11 however , adhesion of a single spatulae varies as a function of humidity and is dramatically reduced under water , 9 , 10 suggesting some contribution from capillary forces . contact mechanics arguments have been invoked to explain the subdivision of the setal contact surface into multiple independent nanosized spatulae , giving rise to enhancement of the mechanical behavior . 14 for the idealized case of a circular contact area , theory suggests that the adhesion strength scales as √{ square root over ( n )}, where n is the number of independent contacts into which the area is subdivided . the contact splitting theory qualitatively explains the scaling of dry adhesive systems employed by some amphibians and insects , and provides guidance for development and optimization of synthetic gecko mimics . 6 , 15 , 16 synthetic gecko adhesives that exhibit dry adhesion have been fabricated from polymers 2 - 4 as well as multiwalled carbon nanotubes . 5 however , maintenance of adhesion during repetitive contacts has only been demonstrated for a few contact cycles , 2 , 8 and none have been shown to function under water or in high humidity environments . a celebrated biological model for wet adhesion is the mussel , which is well known for its ability to cling to wet surfaces . 17 , 18 mussels secrete specialized adhesive proteins containing a high content of the catecholic amino acid 3 , 4 - dihydroxy - l - phenylalanine ( dopa ). 19 - 21 both natural and synthetic adhesives containing dopa and its derivatives have demonstrated strong interfacial adhesion strength . 22 - 25 using single molecule measurements in aqueous media , we recently demonstrated that dopa formed extraordinarily strong yet reversible bonds with surfaces . 26 in fact , the force necessary to dissociate dopa from an oxide surface (˜ 800 pn ) was the highest ever observed for a reversible interaction between a small molecule and a surface . 26 it was theorized that the incorporation of mussel adhesive protein mimetic polymer onto a gecko - mimetic nanoadhesive structure would yield strong yet reversible wet / dry adhesion — a property that existing materials do not exhibit . arrays of gecko foot - mimetic nanoscale pillars coated with a thin map - mimetic polymer film are shown in fig1 . designs of both the pillar array and the coating polymer were undertaken in view of current knowledge of the respective biological systems . for the pillar array , primary design criteria include the dimensions of the pillars and their spacing , as well as the stiffness of the pillar material . 15 , 16 for flexibility in adapting to rough surfaces , both the supporting substrate and the pillar material were fabricated from poly ( dimethylsiloxane ) ( pdms ) elastomer , which is a well - known organic material with a long history of use in microfabrication . 27 arrays of pdms pillars 200 , 400 , and 600 nm in diameter , 1 - 3 μm center - to - center distance , and 600 - 700 nm in height were successfully fabricated using e - beam lithography ( ebl ) ( see fig1 ). the pillar arrays are supported on a continuous film of pdms 2 - 3 mm in thickness , with each pdms pillar representing a single spatula found at the surface of a gecko foot ( fig2 a , b ). pillar arrays of 400 nm diameter and 600 nm height were tested for adhesion . analysis of mussel adhesive protein compositions gave insight into a rational design for a mussel - mimetic polymer . first , the synthetic polymer should have a high catechol content since dopa accounts for as much as 27 % of amino acids in the adhesive proteins found at the interface between mussel byssal pads and their substrate . 21 second , long - lasting waterproof adhesion requires polymers with low water solubility to prevent their loss into the aqueous medium . 28 poly ( dopaminemethacrylamide - co - methoxyethylacrylate ) ( p ( dma - co - mea ), ( fig2 c ) was synthesized using free - radical polymerization where the adhesive monomer , dma , accounts for 17 % of this copolymer by weight ( 1 h nmr ). p ( dma - co - mea ) has a high molecular weight and is insoluble in water . p ( dma - co - mea ) was applied to the pdms pillar array by dip coating in an ethanol solution of p ( dma - co - mea ). x - ray photoelectron spectroscopy ( xps ) analysis of the coated substrate indicated a thin coating (& lt ; 20 nm ) as demonstrated by the presence of both silicon ( 103 ev , si 2 p ) from the pdms and nitrogen ( 399 ev , n 1 s ) from the p ( dma - co - mea ) ( fig6 ). spin - coating p ( dma - co - mea ) onto pdms resulted in no silicon signals ( 2 s , 153 ev and 2 p 103 ev ) indicating that the coating thickness is more than the x - ray penetration depth , typically around 20 nm ( fig6 a ). dip - coating resulted in both silicon and nitrogen signals , thus indicating that the coated polymer thickness is & lt ; 20 nm . the surface atomic compositions of unmodified and dip - coated pdms substrates are shown in fig6 b . dip - coated sample showed both silicon and nitrogen compositions . a thin coating was desired for minimizing the change in pillar dimensions during coating , which was confirmed by scanning electron microscopy after coating with p ( dma - co - mea ) ( fig2 d ). we refer to the resulting flexible organic nanoadhesive as ‘ geckel ’, reflecting inspiration from both the gecko and the mussel . the performance of geckel adhesive was evaluated using an atomic force microscopy ( afm ) system fully integrated with optical microscopy , which permitted simultaneous measurement of the adhesive contact force along with clear visualization of the nanoscale contact area down to the single pillar level . in a typical adhesion experiment ( fig3 ), the afm piezo was used to bring a tipless cantilever ( si 3 n 4 ) into contact with the geckel pillar array , and upon retraction the force necessary to separate the cantilever from the pillar array was measured . furthermore , independently changing the spacing ( d ) between pillars ( d = 1 , 2 , and 3 μm ; fig3 a ) and the angle of orientation ( θ ) between the pillar array and the cantilever axis ( fig3 b ) allowed us to control the number of pillars contacting the cantilever precisely from one to six . for example , a geckel adhesive with d = 3 μm and θ = 45 ° resulted in a single pillar contact ( fig3 c ), whereas d = 1 μm and θ = 0 ° resulted in six pillars interacting with the cantilever simultaneously ( fig3 d , movie 1 ). adhesion experiments were performed both in air and under water for uncoated ( hereafter ‘ gecko ’) and p ( dma - co - mea ) coated (‘ geckel ’) pillar arrays ( fig4 ). pillar - resolved ( i . e . area - defined ) force measurements showed strong adhesive forces when the cantilever was pulled away from the pillar surface . fig4 a and 4b show typical force - distance ( f - d ) curves , with each curve representing a specific number ( 1 - 6 ) of 400 nm diameter pillars interacting with the si 3 n 4 cantilever surface . the pull - off force was determined from each f - d curve , and mean values from multiple experiments were plotted in fig4 d as a function of the number of contacting pillars . the observed linear increase in force with pillar number indicates constructive force accumulation , i . e . simultaneous detachment of individual pillars from the cantilever . the adhesive force per pillar ( nn / pillar ) was calculated from the individual slopes ( fig4 e ): 39 . 8 ± 2 ( gecko in air ), 5 . 9 ± 0 . 2 ( gecko in water ), 120 ± 6 ( geckel in air ), and 86 . 3 ± 5 ( geckel in water ). although the addition of p ( dma - co - mea ) coating on the pillars significantly increased dry adhesion , the enhancement of wet adhesion was particularly dramatic , as the wet adhesive force per pillar increased nearly 15 times ( 5 . 9 → 86 . 3 nn / pillar , si 3 n 4 ) when coated with p ( dma - co - mea ). the geckel wet - adhesion strength was also high when tested against other surfaces : titanium oxide ( 130 . 7 ± 14 . 3 nn / pillar ) and gold ( 74 . 3 ± 4 . 1 nn / pillar ) ( fig7 ). the versatility of geckel is not surprising given recent single molecule force experiments showing the ability of dopa to interact strongly with both organic and inorganic surfaces . 26 these interactions can take many forms , including metal coordination bonds , pi electron interactions , and covalent bonds . the lower adhesion strength of geckel on gold is in qualitative agreement with our earlier single molecule pull - off and polymer adsorption studies that indicated dopa interacts less strongly with gold than with titanium oxide . 26 - 29 the ability of the bond between dopa and a metal oxide surface to rupture upon pulling , and then re - form when brought back into contact with the surface , 26 is an important aspect of this invention . repetitive afm measurements showed that geckel adhesive &# 39 ; s wet - and dry - adhesion power was only slightly diminished during many cycles of adhesion , maintaining 85 % in wet ( red ) and 98 % in dry ( black ) conditions after 1100 contact cycles ( fig5 ). to our knowledge no other gecko - mimetic adhesive has demonstrated efficacy for more than a few contact cycles , 2 , 8 and none have been shown to work under water . this surprising and unexpected advantage of the present invention suggests many possible applications . control experiments involving pillar arrays coated with catechol - free polymer , p ( mea ), showed lower adhesion strength ( 26 nn / pillar for the first contact cycle ) as well as rapid decay in the adhesion performance under cyclic testing occurred over 5 adhesive contacts ( fig8 a ). from xps spectra shown in fig8 b , carbonyl peak for the p ( mea )- coated surface disappeared over 18 hours of incubation suggesting the detachment of the polymer . although repeatable adhesion can be achieved underwater using a dhpd - free polymer , the adhesive performance is significantly reduced emphasizing the importance of the mussel - mimetic catechol groups in enhancing wet adhesion as well as anchoring the p ( dma - co - mea ) polymer to the pillar array . at the same time , it appears that the nanostructured surface is essential to the observed geckel adhesive behavior . force measurements on flat substrates coated with p ( dma - co - mea ) indicated a complex peeling behavior initiating at low adhesive strength ( fig4 c ), which is in distinct contrast to the linear force accumulation behavior exhibited by the geckel adhesive ( fig4 d ). the geckel nanoadhesive was shown to be highly effective at adhering reversibly to surfaces under water , and with functional performance resembling that of a sticky note . although we must be cautious in extrapolating our results to larger areas because of the challenges associated with maintaining equal load sharing among a large number of pillars , in its current form ( 400 nm pillar diameter and 1 μm spacing ) a 1 cm 2 surface area of geckel adhesive would transmit 9 n of force under water ( 90 kpa ). it is interesting to note that this value is similar to estimates for the strength of gecko dry adhesion , 9 , 11 , 12 suggesting that under wet conditions our hybrid geckel adhesive may perform as well as gecko adhesives do under dry conditions . further refinement of the pillar geometry and spacing , the pillar material , and mussel mimetic polymer may lead to even greater improvements in performance of this nanostructured adhesive . we believe geckel type adhesives will prove useful in a great variety of medical , industrial , consumer and military settings . for the fabrication of gecko - mimetic adhesive arrays , e - beam lithography was used to create a pattern of holes in a pmma film supported on a silicon wafer ( negative mold ). solid phase pdms was then cast onto the negative mold , thermally solidified , and then lifted off from the substrate to yield a positive array of pdms pillars (˜ 400 nm in diameter and 600 nm in height ) supported on by a continuous pdms film . mussel - mimetic polymer , p ( dma - co - mea ), was synthesized by radical copolymerization of dopamine methacrylamide ( dma ) and methoxyethylacrylate ( mea ) monomers . finally , the geckel adhesive was prepared by dip - coating pdms pillar arrays into an ethanol solution of p ( dma - co - mea ) for 3 hrs . surface chemical compositions were analyzed by x - ray photoelectron spectroscopy ( xps ) and time - of - flight secondary ion mass spectrometry ( tof - sims ). pillar arrays were imaged by afm and scanning electron microscopy ( sem ). adhesive forces under dry and wet conditions were determined by afm equipped with tipless cantilevers . the contact area between tip and the pillar array was precisely controlled by the distance between pillars ( d = 1 , 2 , and 3 μm ) and the angle between cantilever and pillar axis ( θ ), and was determined by optical imaging using a 40 × objective and fiber - optic illumination . 20 g of sodium borate and 8 g of nahco 3 were dissolved in 200 ml of deionized water and bubbled with ar for 20 min . 10 g of dopamine - hcl ( 52 . 8 mmol ) was then added followed by the dropwise addition of 9 . 4 ml of methacrylate anhydride ( 58 . 1 mmol ) in 50 ml of thf , during which the ph of solution was kept above 8 with addition of 1n naoh as necessary . the reaction mixture was stirred overnight at room temperature with ar bubbling . the aqueous mixture was washed twice with 100 ml of ethyl acetate two times and then the ph of the aqueous solution was reduced to less than 2 and the solution extracted with 100 ml of ethyl acetate 3 times . the final three washes were combined and dried over mgso 4 to reduce the volume to around 50 ml . 450 ml of hexane was added with vigorous stirring and the suspension was held at 4 ° c . overnight . the product was recrystallized from hexane and dried to yield 9 . 1 g of grey solid . 1 h nmr ( 400 mhz , dmso - d / tms ): δ 6 . 64 − 6 . 57 ( m , 2h , c 6 hh 2 ( oh ) 2 —), 6 . 42 ( d , 1h , c 6 h 2 h ( oh ) 2 —), 5 . 61 ( s , 1h , — c (═ o )— c (— ch 3 )═ chh ), 5 . 30 ( s , 1h , — c (═ o )— c (— ch 3 )═ chh ), 3 . 21 ( m , 2h , c 6 h 3 ( oh ) 2 — ch 2 — ch 2 ( nh )— c (═ o )—), 2 . 55 ( t , 2h , c 6 h 3 ( oh ) 2 — ch 2 — ch 2 ( nh )— c (═ o )—), 1 . 84 ( s , 3h , — c (═ o )— c (— ch 3 )═ ch 2 ). 13 c nmr ( 400 mhz , dmso - d / tms ): δ167 . 3 ( s , 1c , — nh — c (═ o )— c ( ch 3 )═ ch 2 ), 145 . 0 ( s , 1c , — nh — c (═ o )— c ( ch 3 )═ ch 2 ), 143 . 5 − 115 . 5 ( 6c , c 6 h 3 ( 0 — c (═ o )— ch 3 ) 2 ), 130 . 3 ( s , 1c , — nh — c (═ o )— c ( ch 3 )═ ch 2 ), 41 . 0 ( s , 1c , c 6 h 3 ( oh ) 2 — ch 2 — ch 2 ( nh )— c (═ o )—), 34 . 6 ( s , 1c , c 6 h 3 ( oh ) 2 — ch 2 — ch 2 ( nh )— c (═ o )—), 18 . 7 ( s , 1c , — c (═ o )— c (— ch 3 )═ ch 2 ). 12 . 5 ml of mea was passed through a column packed with 30 g of al 2 o 3 to remove inhibitors . 7 . 5 g of purified mea ( 57 . 9 mmol ), 1 . 7 g of dma ( 7 . 4 mmol ), and 106 mg of aibn ( 0 . 64 mmol ) were added to 20 ml of dmf in an airfree ® flask . the solution mixture was degassed through pump - freeze - thaw cycles 3 times . while sealed under vacuum , the solution was heated to 60 ° c . and stirred overnight . the reaction mixture was diluted with 50 ml of methanol and added to 400 ml of et 2 o to precipitate the polymer . after precipitating in dcm / ethyl ether two more times and drying in a vacuum desicator , 5 . 7 g of white , sticky solid was obtained . 1 h nmr ( 400 mhz , cdcl 3 / tms ): δ6 . 81 − 6 . 70 ( d , br , 2h , c 6 hh 2 ( oh ) 2 —), 6 . 58 ( s , br , 1h , c 6 h 2 h ( oh ) 2 —), 4 . 20 ( s , br , 2h , ch 3 — o — ch 2 — ch 2 — o — c (═ o )—), 3 . 57 ( s , br , 2h , ch 3 — o — ch 2 — ch 2 — o — c (═ o )—), 3 . 36 ( s , br , 3h , ch 3 — o — ch 2 — ch 2 — o — c (═ o )—), 2 . 69 ( s , br , 2h , c 6 h 3 ( oh ) 2 — ch 2 — ch 2 ( nh )— c (═ o )—), 2 . 39 ( s , br , 1h , — o — c (═ o )— ch ( ch 2 —)— ch 2 —), 2 . 14 ( s , br , 2h , c 6 h 3 ( oh ) 2 — ch 2 — ch 2 ( nh )— c (═ o )—), 1 . 93 ( s , 3h , — nh — c (═ o )— c ( ch 3 )( ch 2 —)— ch 2 —), 1 . 68 ( m , br , — o — c (═ o )— ch ( ch 2 —)— ch 2 —), 0 . 98 ( m , br , — nh — c (═ o )— c ( ch 3 )( ch 2 —)— ch 2 —). gpc - malls ( wyatt technology , santa barbara , calif . with mobile phase of 20 mm libr in dmf and shodex - oh pak columns ): m n = 252 kda , pd = 1 . 73 . for control experiments , a catechol - free p ( mea ) homopolymer ( m w = 100 kda , scientific polymer products , ontario , n . y .) was used . e - beam resist ( 950pmma a3 , microchem ) was spin - coated ( 4000 rpm , 40 sec ) on silicon wafer several times until the resist thickness , as measured by ellipsometry ( woolam co . lincoln , nebr . ), reached 600 ˜ 700 nm . the resist was patterned at 30 kv with an area dose between 650 - 800 μc / cm 2 using quanta 600f ( fei co . hillsboro , oreg .). resist development was performed for 1 min with a solution of methyl isobutyl ketone / isopropanol ( ⅓ , v / v ), followed by rinsing with water . the patterned substrates were treated with oxygen plasma ( harrick , pleasantville , n . y .) for 30 sec and repeated 2 - 3 times to completely remove residual resist from the exposed si regions . the patterned substrates were then exposed to a triethoxyoctylsilane vapor for 30 min . pdms was prepared as follows : 4 μl of pt - catalyst ( platinum - divinyl tetramethyl - disiloxane in xylene ) and 4 μl of modulator ( 2 , 4 , 6 , 8 - tetramethyl - 2 , 4 , 6 , 8 - tetravinylcyclotetrasioxane ) were added to a 7 - 8 % vinylmethylsiloxane solution ( 3 . 5 g ). the solution was subsequently mixed with a 25 - 30 % methylhydrosiloxane ( 1 g ) solution . finally the solution was cured ( 80 ° c .) after spin - coating ( 1000 rpm for 1 min ) onto the pmma / si master . the spin - coated substrate was covered either by thin cover glass for force measurements or sylgard - 184 pdms for other experiments such as optical imaging or x - ray photoelectron spectroscopy ( xps ). gecko adhesive was obtained by pdms pattern lift - off and brief exposure to oxygen plasma ( 100 w , 30 sec ) and used within 2 - 3 hrs after plasma treatment . geckel adhesive was prepared by dip - coating gecko adhesive in a 1 mg / ml solution of p ( dma - co - mea ) in ethanol at 70 ° c . unstructured controls were fabricated in the same manner using flat pdms . the presence of p ( dma - co - mea ) and p ( mea ) on pdms surfaces was confirmed by x - ray photoelectron spectroscopy ( xps ) ( omicron , taunusstein germany ) equipped with a monochromatic al kα ( 1486 . 8 ev ) 300 w x - ray source and an electron gun to eliminate charge build - up . all force data were collected on an asylum mfp - 1d afm instrument ( asylum research , santa barbara , calif .) installed on a nikon te2000 microscope . spring constants of individual cantilevers ( veecoprobes , np - 20 tipless si 3 n 4 tips , santa barbara , calif .) were calibrated by applying the equipartition theorem to the thermal noise spectrum . 30 due to the large forces exhibited by the adhesive , only tips exhibiting high spring constants ( 280 - 370 pn / nm ) were used . metal and metal oxide coated cantilevers were formed by sputter coating ˜ 10 nm of au or ti ( a native oxide formed at the ti surface , tio x ) using a denton vacuum desk iii ( moorestown , n . j .). the surface composition of each cantilever was confirmed by time - of - flight secondary ion mass spectrometry ( tof - sims ), using a phi - trift iii ( ga + , 15 kev , physical electronics , eden prairie , minn .). cantilevers were treated by oxygen plasma ( 100 w , 150 mtorr ) for 3 min before use . force measurements were conducted either in deionized water or ambient ( air ) conditions at a cantilever pulling speed of 2 μm / sec . in wet experiments , optical microscopic examination of the contact region indicated the absence of air bubbles trapped between nanopillars and on the nanopillar surface ( not shown ). tapping mode afm images were obtained using a multimode veeco digital instrument ( san diego , calif .) with a si cantilever ( resonance frequency of 230 - 280 khz ). contact area was imaged by an inverted optical microscope using a 40 × objective illuminated by a fiber - optic white light source perpendicular to the objective . the following list of references , including the references themselves , is incorporated by reference herein . 1 . ruibal , r . & amp ; ernst , v . the structure of the digital setae of lizards . j . morphology 117 , 271 - 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