FIELD OF THE INVENTION:
The present invention relates to adhesive chemistry. More particularly, the present
invention relates to highly efficient adhesive surfaces with hierarchical structures having
tendency to be reused after repeated 5 applications.
BACKGROUND AND THE PRIOR ART:
Inspired by natural adhesive pads present at the feet of many animals, e.g. insects,
geckos and frogs, there have been several attempts to manufacture adhesive surfaces
with hierarchical structures. Compared to conventional viscoelastic glues present in
10 most man-made adhesive materials, the bio-inspired ones have the advantage that they
remain usable even after repeated applications, they allow easy removal of particulate
contaminants, they can also be made of materials which are bio-compatible and
environmentally benign. The important challenge, however, is to generate nano- to
microscopic structures over a large surface area and in economically viable process. It is
15 now well-known that nano to microscopic patterns on a soft and hard surface are useful
for variety of applications. For example, patter an adhesive surface can enhance
adhesion via crack arrest and crack initiation mechanism. Such patterns, especially the
ones with hierarchical structures, can enhance also the compliance of the adhesive, thus
enhancing its ability to adhere strongly to rough adherents. However, generation of such
20 patterns on a surface over a large area is a problem. Conventional top down approach,
e.g. lithography methods have been effective for proof of principle, but have not been
good enough for scale up. In general lithography methods are severely limited by shape
and size of the templates, generation of which, in itself, is an expensive and time
consuming affair. Many a time, these methods involve use of toxic and corrosive
25 chemicals which too limit their applications.
Template-assisted fabrication methods exist, which are based on deposition or
introduction of a desired material into arrays of micro- and nano-channels, which is a
method of forming one-dimensional micro- and nanostructures. Considerable efforts
3
have been made in designing and controlling fabrication of templates for forming
gecko-like micro/nanostructures, including electron beam lithography, intermediate film
molding or photolithographic methods followed by silicon micromachining to construct
templates for casting or hot embossing polymeric structures.
Among potential templates, nano-porous anodic alumina (NPAA) has 5 drawn attention
because it provides a self-assembled array of relatively uniformly-sized parallel pores or
channels with large depths, in large scale and low cost. Availability of NPAA films has
triggered investigations into their utilization as templates or masks to fabricate various
nanostructures including nanotubues, nanowires, nanoporous films and nanodot arrays
10 for wide ranging applications such as catalysis, electronics, optics, and biosensing. It
has been reported that low viscosity polymeric solutions were used to cast polymer in
NPAA templates in attempts to fabricate nanofibrils. However, long solvent evaporation
times (e.g. 24 hours) make this an unsuitable method for use in mass production.
Furthermore, the long and thin nanotube fibrils are apt to bend, entangle or clump
15 together especially during removal of the NPAA template by wet etching, thereby
impairing adhesive efficacy.
WO 2014025793 A1 provides a method of forming a dry adhesive which includes
forming an electrospun non-woven of a spinnable polymer, wherein the polymer fiber
forming the non-woven is aligned. A dry adhesive is provided that comprises aligned
20 polymeric nanofibers. The polymeric nanofibers may be formed from a mixture of
highly spinnable material is combined with an adhesive component to further enhance
the adhesion onto substrates. The non-woven can further be processed by plastic
deformation to create microprotrusions.
However, this technology is based on forming nano-fibers by electro-spinning method,
25 which suffers with the disadvantage that it is difficult to generate such fibrous surface
over a large area. Further, this technology is able to generate high aspect ratio
nanofibers but not nanopillars. So, there is a requirement of a highly scalable
technology which could generate nano-pillars over a large area.
4
WO 2009053714 provides adhesive microstructure shaving significantly improved
adhesion strengths and process of its fabrication. WO ‘714 provides a
fabricated adhesive microstructure comprising a deformable material which, in use,
deforms to provide an adhesion strength at a substantially smooth glass surface of at
least 120kPa in air at one atmosphere pressure and at least 10kPa less 5 (preferably at
least 20kPa less, or more preferably at least 50kPa less) adhesion strength in vacuum
than that at one atmosphere pressure.
The method of fabricating adhesive microstructure of WO ‘714 comprising the steps of:
10 (i) providing a mould structure; (ii) introducing a curable liquid polymer into the
mould structure;(iii) curing the polymer in the structure; and thereafter (iv) separating
the polymer from the mould structure to form the microstructure.
However, the method described in patent number WO 2009053714 A1, involves
generation of mushroom shaped dry adhesive pad via top-down approach of etching and
15 lithography on silicon wafer to form templates. Lithographic methods are in general
expensive and are not suitable for large area patterning. It is difficult also to generate
pillars with nano-scopic dimension by these methods, as a result the pillars with their
flaps, as described in this patent, are microscopic and non-hierarchical. There is still a
need of such technology which could generate nano-pillar based structures.
20 US 20110300339 provides high aspect ratio adhesive structure and a method of forming
the same. The method comprises forming a high aspect ratio adhesive structure, by
fabricating a porous template comprising at least a first tier and a second tier; followed
by introducing a softened polymer into the template; and separating the polymer from
the template.
25 However, this technology involves the fabrication of nano-porous anodic aluminum
substrates which are used as template for generation of high aspect ratio pillars on
adhesive surface. Also, since it is a single anodization process, pores remain uniform
with essentially one single length scale and also due to the use of nano-imprinting
5
employed in the above pattern, there is always a high risk of accurately transfer of
pattern.
WO 2013069013 discloses a microstructure made of flexible material and including a
plurality of projections extending from a common substrate, a distal end of each
projection including a resilient flap, wherein all lateral dimensions 5 of each of the
projections increase monotonically with increased distance from a surface defined by
distal ends of the flaps when the flaps are unbent, and wherein each of the projections
extends from the common substrate in a general direction that is locally substantially
perpendicular to the surface defined by the ends of the flaps, such that when a shear
10 force is applied to each of the flaps by relative motion between that flap and a
cooperating surface in contact with that flap, that flap bends so as to increase the area of
contact of that flap with the cooperating surface and to affect adhesion between the
microstructure and the cooperating surface. However, this technology suffers with the
issue of large area patterning.
15
3719/DEL/2011 discloses reusable, pressure-sensitive adhesive material that includes a
visco-elastic layer and an elastic skin layer or includes a sandwich viscoelastic layer
between two elastic layers. It further discloses that the visco-elastic layer includes a
weakly or partially cross-linked elastomer of at least one of silicone, rubber elastomer,
20 acrylate copolymer, mono-acrylate oligomer, multi acrylate oligomer, styrene -
butadiene-styrene, styrene-ethylene/butylene-styrene, styrene-ethylene/propylene,
styrene-isoprene-styrene, or vinyl ether. It also discloses that such pressure sensitive
composite adhesives show adhesive strengths similar to their initial adhesive strengths,
even after being used. These composite PSA's is such that a second and/or subsequent
25 use of the composite PSA still provides an adhesive strength of at least about 100-3000
mJ/m2
. The composite PSA’s of 3719 is such that a second and/or subsequent use of the
composite PSA still provides an adhesive strength of at least about 100 mJ/m2 , 800,
1000, 1200, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2400, 2600, 2800,
3000, or 4000, or 30,000 mJ/m2; the adhesive surface has an adhesive strength
30 following 15 cycles of adhesion and debonding, and the adhesive strength is at least
6
about 400 mJ/m2 , 800, 1000, 1200, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100,
2200, 2300, 2400, 2600, 2800, 3000, or 4000 mJ/m2. However, there still remains a
need in the art to develop adhesives that are simple in making, washable and retain their
adhesive strengths on repeated use against different adherents.
5
Further reference is made to a review article by Carlo Menon et al.: Recent advances in
nanostructured biomimetic dry adhesives; Front Bioeng Biotechnol., 2013; 1: 22
summarizes the recently published literatures on bio-inspired, nanostructured dry
adhesives presented with an emphasis being placed on fabrication techniques. This
10 article discusses about overall nanostructure fabrication process which encompasses
mold preparation, mold pre-treatment, material infusion, demolding, and post-molding
treatments of the demolded adhesive. Several different molding techniques that have
been used to create nanostructures including melting, UV-cross-linking, polymerization,
capillary force nano-imprinting, and two-photo-polymerization have been discussed in
15 this review article. However, there still remains a need in the art to develop a
biocompatible adhesive which is simple in making involving self-organization process
and can be washed and reused against different adherents on rough surfaces.
Lot of research is being carried out in the domain of finding an effective adhesive and
20 its process of fabrication. Particularly, there is always a high need for a cost effective
patterning processes using principles of self organization for the fabrication of
adhesives and wherein the size and shape of the pores in adhesive could be under
control. Also, there is a need of such adhesive based on bio-compatible material, which
remains usable even after its multiple applications and have superior ability to adhere to
25 smooth and rough surfaces.
There is a need for a process which could provide non-uniform pores but of hierarchical
length scale and at the same time providing accurate transfer of pattern.
OBJECTS OF THE INVENTION:
7
An object of the present invention is to overcome the drawback / disadvantages of the
prior art.
Yet another object of the present invention is to provide adhesive surfaces with
hierarchical nano-microscopic pillars generated by self organization route thus allowing
large 5 area patterning.
Yet another object of the present invention is to provide adhesive surfaces which remain
usable even after repeated applications and allow easy removal of particulate
contaminants.
10 Yet another object of the present invention is to provide adhesive surfaces having
superior ability to adhere to smooth and rough surfaces.
Yet another object of the present invention is to provide a cost effective, easy and highly
scalable efficient process of fabrication of adhesive surfaces.
15
Yet another object of the present invention is to provide patterning processes using
principles of self organization.
Yet another object of the present invention is to provide a patterning processes wherein
20 nano-pillars over a large area could be fabricated.
Yet another object of the present invention is to provide such a process for the
fabrication of adhesive surface wherein the porosity and pore size of the template could
be controlled.
25
Yet another object of the present invention is to provide such a process for the
fabrication of adhesive surface wherein the pore size and shape of the template could be
controlled by the crystals of the precipitant which are formed during the gel forming
reaction.
30
8
Yet another object of the present invention is to provide such a process for the
fabrication of adhesive surface wherein the pore size and shape of the template could be
controlled by altering the initial water content in the gel forming solution.
Yet another object of the present invention is to provide a process 5 which involves
development of non-uniform but of hierarchical lengthscale pores on the template in a
single stage process.
Yet another object of the present invention is to provide an inexpensive route of
preparing a suitable template by self organization process against which the
10 crosslinkable elastomer is crosslinked generating pillars with hierarchical length scales.
Yet another object of the present invention is to provide a process which could facilitate
transfer of patterns from nano-microscopic structures.
Yet another object of the present invention is to provide a fabrication process in which
15 the porous template is generated by simultaneous phase separation and crosslinking of a
hydrogel material.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a fabrication process
20 for a reusable adhesive surface with hierarchical nano-microscopic pillars, such process
comprising the steps of:
i) preparation of a porous hydrogel template by crosslinking a pre-polymer
solution in water as the monomer with a cross-linker, a promoter, a redox initiator and a
salt as precipitant for the non-aqueous phase.
25 ii) removal of varied amounts of water from the pores of the said hydrogel
template of step (i) by drying under vacuum or atmospheric pressure, and/or squeezing
the gel layer by applying negative pressure of a vacuum pump, and/or squeezing the gel
in a centrifugal field to regulate the height and pore diameters of the said template;
9
iii) pouring a cured crosslinkable elastomer liquid mixed with a crosslinking
agent on the said gel template formed by steps (i) and (ii) to fill the pores of the said gel
template partially or completely;
iv) crosslinking of the said elastomer mixture on the said gel template first at 25-
270C for 4 hours, second at 500C for 4 hours and at finally 800 5 C for 4-5 hours.
v) separation of the crosslinked elastomeric layer from surface of the said gel
template.
According to another aspect of present invention there is provided a reusable pressuresensitive
adhesive consisting of non-uniform, hierarchical nano-pillars of diameters
10 varying in between 50 to 900 nm and heights 5 nm to 450nm patterned over a large
surface area.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1: illustrates flow chart depicts the method of preparation of pHEMA gel layer.
15 Figure 2: illustrates Optical micrographs (a) and (b) represent smooth and wrinkled
HEMA gel surfaces respectively.
Figure 3: illustrates the removal of water from the porous pHEMA gel by three
different methods: (a) drying by blowing hot air over the gel layer, (b) by squeezing the
gel layer under centrifugal field and (c) by vacuum squeezing the water out of the gel
20 inside a vacuum chamber.
Figure 4: (a) illustrates Flow chart depicts the method of preparation of the PDMS
adhesive using template prepared as in figure 1. The process within the dotted box could
be carried out in three different ways as depicted in this figure and that described in
figures 3(b and c); 4(b) shows flow chart depicts the method protocol for dilution of
25 PDMS in presence of toluene; 4(c) shows flow chart depicts the protocol for vibration
of porous template.
Figure 5: illustrates flow chart depicts the method of preparation of the Ecoflex
adhesive using pHEMA gel template prepared as in figure 1.
10
Figure 6: illustrates effect of water removal on the pore morphology of pHEMA
Figure 7: illustrates optical and AFM image of adhesive film made of PDMS.
Figure 8: illustrates shear adhesion test, in which the adhesive layer was strongly
attached to a vertical wall; a scotch tape was used as an additional enforcement for
preventing the slippage of the adhesive 5 on this surface.
Figure 9: illustrates effect of removal of water from the pHEMA gel on the shear
adhesion strength of the adhesive prepared using Sylgard 184 as the adhesive material
by the method described in figure 3.
Figure 10: illustrates effect on shear adhesion strength, of the final temperature at
10 which PDMS (Sylgard 184 elastomer) layer prepared by the method described in figure
3 was heated.
Figure 11: illustrates the effects of longitudinal vibration of the template during
crosslinking of the elastomer against the template and the presence of toluene in the
crosslinkable liquid, on the final shear adhesion strength of the adhesive
15 Figure 12: illustrates extent of removal of water by pHEMA gel by the method
described in figure 2 affects the shear adhesion strength of Ecoflex (silicone based
polymer).
Figure 13: illustrates the adhesion strength of adhesive layers made of Ecoflex cured
against a pHEMA gel template.
20 Figure 14: illustrates the suitability of the adhesive for applications which demand
repeated use. Figure 14 demonstrates that adhesive is washable and has been reused
over more than 100 cycle without any significant decrease in adhesion strength.
Figure 15: illustrates bar chart representing the shear adhesion strength of an adhesive
(one corresponding to symbol in figure 12, for 50% water removal) against adherents
25 of different roughness; the glass slide (smooth surface), R0, R1 and R2 represent
adherent surfaces having root mean square roughness of 1.6 nm, 50 nm, 280 nm and
820 nm respectively; the data show that within the range of adherent roughness
examined, the strength of adhesion increases with roughness.
30 DETAILED DESCRIPTION OF THE INVENTION:
11
The following description is intended to describe working of the present invention, and
not intended to limit the scope of the invention. Other embodiments may also be
covered under the scope of the invention.
The present invention essentially provides an adhesive and process of its production.
The process of preparing the adhesive of present invention comprises 5 moulding a
crosslinkable material on a template. Template according to the invention is a porous
hydrogel selected from poly(hydroxyethyl methacrylate) (pHEMA) gel,
n-isopropylacrylamide (NIPA) gel, poly-acrylamide gel and agarose gel. The gel
samples can be either used as template without any further processing, or it can be dried
10 to desired extent to make its pores available for pattern transfer. When a crosslinkable
elastomeric material is cured against such templates, it results in surface patterns of
different length scales, varying from few microns to tens of nano-meters. Prior art
methods implement variety of top-down processes like molding against a
lithographically prepared templates of various kinds which result in patterns of uniform
15 dimension, and the ones which consist of two-three different well-defined length scales.
These “hierarchical” dimensions are achieved in multiple step process and the resultant
length scales vary from 20 nm to 3 μm. In contrast, present hierarchically patterned
adhesive is prepared in single step process by molding a cross linkable liquid against a
porous and wet, hydrogel template containing heterogeneous pores of different size,
20 shape and dimension and the pore walls moistened to different extent. The length scales
thus achieved vary from nanoscopic e.g. 10 nm to macroscopic, e.g. 5 mm, i.e. over a
range much larger than any other process described in literature.
Importantly, although AFM imaging of the surface of the porous gel template shows
features having minimum length scales ~1 μm, the minimum feature size achieved on
25 our adhesive surface, which is prepared by molding against this template, is found to be
much smaller ~10nm, thus suggesting occurrence of interfacial instability induced by
variety of mechanisms, e.g. dewetting, development of interfacial stresses and gradient
in interfacial tension. The present method comprises coupled effect of top-down
approach of molding against a heterogeneous template and the self-organized processes
30 of interfacial instability.
12
Unlike several processes described in the review article, present method does not
employ any electrochemical process or any other etching processes for generating the
template or any imprinting method.
Since, the porous hydrogel template in present method is prepared by phase separation
mediated by a precipitant like NaCl, it allows to vary the shape and size 5 of the pores by
varying the type of precipitant, its concentration; the water content of the prepolymer
solution too affects the above parameters.
The following definitions are provided to better define the present invention and to
guide those of ordinary skill in the art in the practice of the present invention. Unless
10 otherwise noted, terms are to be understood according to conventional usage by those of
ordinary skill in the relevant art.
The term “Self organization” as used in the present invention broadly refers to the
ability of a material to spontaneously evolve under the influence of a set of physical
15 forces which act either on the surface or at the bulk of it.
The term “Hierarchical pattern” as used in the present invention broadly refers to the
pattern that consists of features of dimension which varies over several orders of
magnitude, such as micropillars having diameter @10 μm, dividing into smaller pillars
20 of @1 μm, which further divides into nanoscopic pillars of diameter @100 nm and so
on.
The term “Nano- microscopic pillars/nano pillars” as used in the present invention
broadly refers to the diameter of the pillars, e.g. pillars having diameter in the range of
10nm to ~1 μm.
25 The term “Aspect ratio” as used in the present invention broadly refers to the ratio of the
length/diameter of pillars
The term “cycle” or “repeated cycles” as used in the present invention broadly refer to
repeated use of the present adhesive against different adherents, wherein the adhesive
surface gets contaminated with particulate matters of different kind, e.g. dust particles,
30 oil droplets, aerosols; the adhesive is then washed by rinsing with ethanol/surfactant
13
solution and dried, which in turn is capable of being reused against adherents. In figure
14, the data of adhesion correspond to such “cycles”. This definition of “cycle” is in
contrast to many references in the prior art such as 3719/DEL/2011 in which “cycle”
simply means repeated attachment and detachment from a smooth adherent.
The term “Superior” or “superior ability” as used in the present invention 5 broadly imply
to several aspects of the adhesive all considered together, such as its ability to adhere to
smooth as well rough surfaces, its reusability, wash ability and number of cycles that the
adhesive can be subjected to adhesion to different adherents and washing of the soiled
adhesive surface. Taking into account all these characteristics, the present adhesive is
10 superior to others known in the art.
The term “easy” as used in the present invention broadly refers to various things in
different contexts as mentioned below;-
The term “easy” as used in the present invention in context to the term “removal
of contaminants” implies to the dirt or dust particles adhered to the adhesive surface that
15 can be removed by simply immersing the adhesive in surfactant solution and by gentle
scrubbing by hand.
The term “easy” as used in the present invention in context to the term
“detachment of crosslinked material”, implies that the crosslinked material is removed
from the template by gentle pull.
20 The term “easy” as used in the present invention in context to the term “scale
up” implies that the process of fabrication of the adhesive in lab scale production can be
adopted for making in larger scale without increasing the number of steps.
The term “highly scalable” as used in the present invention broadly refers to various
things in different contexts as mentioned below;-
25 The term “highly scalable” as used in the present invention in context to the term
“removal of contaminants” implies to the dirt or dust particles adhered to the adhesive
surface that can be removed by simply immersing the adhesive in surfactant solution
and by gentle scrubbing by hand.
The term “highly scalable” as used in the present invention in context to the term
30 “detachment of crosslinked material”, implies that the crosslinked material is removed
from the template by gentle pull.
14
The term “highly scalable” as used in the present invention in context to the
term “scale up” implies that the process of fabrication of the adhesive in lab scale
production can be adopted for making in larger scale without increasing the number of
steps.
The term “Cost effective” as used in the present invention refers 5 to the sticky pads
which are imported from abroad that costs @Rs. 6000/- per m2 area, whereas, the
approximate making cost of the present adhesive (including material and processing
cost) is less than Rs. 1000/- per m2.
10 In the present invention, the inventors have utilized one single elastomeric layer with
hierarchical pattern surface to form the adhesive surfaces. The inventors have found that
these adhesive surfaces after being contaminated with particulate matters of different
kind, e.g. dust particles, oil droplets, aerosols, can be washed by rinsing with
ethanol/surfactant solution and dried which in turn is capable of being reused against the
15 adherents on rough surfaces without any significant decrease in their adhesive strengths.
The present invention provides hierarchically patterned adhesive in a single step process
by molding a cross linkable liquid against a porous and wet, hydrogel template
containing heterogeneous pores of different size, shape and dimension and the pore
walls moistened to different extent. The present invention comprises of coupled effect
20 of top-down approach of molding against a heterogeneous template and the selforganized
processes of interfacial instability.
The present invention also demonstrates fabrication of the adhesive over a large area,
wherein by employing different techniques such as by altering the moisture content of
the gel different adhesion strengths can be achieved. Such adhesive surfaces of the
25 present invention are capable of being reused against different adherents on rough
surfaces without any significant decrease in their adhesive strengths.
In an embodiment of present invention, the process for fabricating a surface patterned
with nano to microscopic features is performed by moulding against a template having
30 microscopic structures. The process for generating such a surface comprises the
following steps:
15
• Preparation of a layer of porous hydrogel of different molecules, e.g. 2-
hydroxyethylmethacrylate (HEMA), acrylamide but not excluding others.
• As an example, the hydrogel of 2-hydroxyethyl methacrylate (HEMA),
acrylamide is prepared by crosslinking a pre-polymer solution in water of 2-
hydroxyethylmethacrylate (HEMA) as 5 the monomer with
ethyleneglycoldimethacrylate (EGDMA) as cross-linker, TEMED as a promoter,
ammonium per sulphate (APS) as a redox initiator and a suitable salt, e.g.
sodium chloride or calcium chloride as precipitant for the non-aqueous phase.
The gel layer thus prepared consists of water filled pores, the dimension of
10 which can be varied by using gel monomer and water at different weight ratio.
For different samples the pore diameter was found to vary from ~1 μm to 20
μm.
• Removal of water from the pores by a suitable process, e.g. drying under
vacuum or atmospheric pressure, and/or squeezing the gel layer by applying
15 negative pressure of a vacuum pump, and/or squeezing the gel in a centrifugal
field, so as to remove desired quantity of water.
• Pouring a cross linkable elastomer, e.g. poly(dimethylsiloxane) mixed with the
crosslinking agent at a desired w/w ratio on the above layer of hydrogel, so that
the elastomeric phase can fill the pores of the gel partially or completely.
Crosslinking of the elastomer first at 25-270C for 4 hours, at 50020 C for 4 hours and at
800C for 4-5 hours.
The process of manufacturing nano-patterned elastomeric materials involves following
steps:
Step a) Preparation of template:
25 The template was prepared by crosslinking a pre-polymer solution in water of 2-
hydroxyethyl methacrylate (HEMA) as the monomer with Ethylene glycol
dimethacrylate (EGDMA) as cross-linker (1% by weight of monomer), TEMED as a
promoter (1% by weight of monomer), ammonium per sulphate (APS) as a redox
initiator (0.05% by weight of monomer) and sodium chloride (7.81% by weight of
30 water) as a monovalent precipitant or calcium chloride (7.81% by weight of water) as a
divalent precipitant for the non-aqueous phase. Thus in all the experiments, the
16
crosslinker, promoter, and the initiator, all were used in respective specific weight
fractions of the monomer. From sample to sample, the monomer-to-water weight ratio
was varied such that the final product contained 80%–95% by weight of water. The
above weight fractions could also be varied in order to achieve desired property of the
5 gel.
Step b -Filling the pores of the porous template by crosslinkable liquid:
The elastomeric layer was made of commercially available reagents, e.g. Dow Corning
product Sylgard 184 elastomer, Ecoflex (silicone based elastomer) from Smooth-On and
so on. In each case, the crosslinkable liquid was prepared by mixing the reagents as
10 suggested by their respective suppliers:
(a) For example, in order to prepare Sylgard 184 pre-polymer liquid, the oligomer and
the curing agent was mixed in 10:1 weight ratio.
(b) For preparing adhesive layers made of Ecoflex-30, the procedure suggested by the
supplier: Smooth One, was followed.
15
In essence, the precursor liquids were thoroughly mixed in 1:1 volume ratio, the mixed
liquid was degassed inside a vacuum chamber at 0.05 mbar pressure. Desired quantity
of the above liquid was then poured over the template and was uniformly spread over it
using a doctor’s blade and spacer of known thickness. The liquid fills the pores of the
20 gel template, particularly at the vicinity of the surface. In order to facilitate this process
of pore filing, several methods were explored (illustrated in Figure 4):
(a) The above template with the elastomer precursor liquid spread on it was placed
inside a vacuum chamber for 10 min at 0.05 mbar pressure (illustrated in Figure 4a).
(b) The above template with the elastomer precursor liquid spread on it was subjected to
25 transverse vibration at a frequency of 70 Hz for 1 hours. Vibration minimizes the effect
of pinning of the pre-polymeric liquid on the substrate and thereby enhancing the extent
of filling of pores (illustrated in Figure 4b).
(c) In order to reduce the viscosity of the elastomer precursor liquid small quantity of a
solvent e.g. toluene was added to it, which was then spread on the template for filling
30 the pores (illustrated in Figure 4c).
17
Step c-Curing of the precursor/crosslinkable liquid:
The crosslinking reaction was then carried out as suggested by the supplier:
(a) For example, Sylgard 184 was cured at three different temperature
(b) Ecoflex was cured at 313K for 4-5 hours and then temperature was increased to
353K at which PDMS is crosslinked (illustrated in Figure 5). Initially, 5 sample was kept
for 4-5 hours at room temperature. An elastomeric layer of thickness 0.1-15 mm was
thus formed.
Step d-Separation of the crosslinked elastomeric layer from gel surface:
10 After crosslinking reaction is complete, the pHEMA gel, along with the crosslinked
elastomer was placed inside a pool of DI water which swells the hydrogel. The
elastomeric layer could then be easily removed. Elastomeric layer was then first
sonicated inside a pool of ethanol and water for at about 30 minutes in order to remove
any gel flake from its surface and was then dried in a hot air oven at 800C temperature.
15
In an embodiment of the invention, the porous gel template can be made of
poly(hexaethyldimethylacrylamide), poly(acrylamide) gel, agarose gel but not
excluding others.
In another embodiment of the invention, the specific shape of the pores of the gel
20 template can follow the specific shape and size of the salt crystal which is formed
during formation of gel.
In another embodiment of the invention, the elastomer can be poly(dimethylsiloxane),
ecoflex, poly(urethane), but not excluding others.
25 In another embodiment of the invention, different monomer to water weight ratio is
used for varying the porosity of the hydrogel and the pore sizes towards development of
templates of different length scales.
In another embodiment of the invention, the pore shape of the adhesive is controlled by
using crystal of different salts.
30
18
In another embodiment of the invention, water is removed to different extents from the
pores of the hydrogel by different squeezing techniques towards development of
templates of different length scales.
In another embodiment of the invention, the interfacial instability driven 5 nano-patterns
on the surface of the crosslinked elastomer is generated during fabrication of adhesive.
In another embodiment of the invention, the surface of the crosslinked layer of
elastomer consists of nano-pillars of diameter varying from 50 to 900 nm and height 5
10 nm to 450nm. The nano-pillars may be spatially separated with a characteristic
separation distance or may be randomly organized. The nano-pillars remain vertically
aligned or aligned at an angle. The height of the nano-pillars can be enhanced by using
vibration of the substrate with frequency 10 to 100 Hz. The height of the nano-pillars
can be enhanced by decreasing the viscosity of the crosslinkable elastomer, e.g. by
15 adding to it a suitable solvent like Toluene, so that the crosslinkable elastomer can fill in
the pores to larger extent.
In another embodiment of the invention, the surface of elastomer consisting of nanomicroscopic
structures can also be used as pressure sensitive adhesive for applications
which demand reusability.
20
In another embodiment of the invention, the adhesion strength of the adhesive can be
varied from 0.2-1.3 N/cm2 by altering the extent to which water can be removed from
the pores of the gel. The adhesion strength of the adhesive can be varied from 0.2-1.3
N/cm2 by varying the aspect ratio of the pillars, e.g. by altering the extent to which the
25 crosslinkable elastomer liquid can be allowed to fill in the pores of the gel. The pattern
on the elastomer surface can be used as template for further transfer to other kind of
materials e.g. PVA, RF gel, but not excluding others. The patterns on these surfaces can
be used for several other applications, e.g. as electrode material. If the initial water
content in pre-polymer solution is less than 80%, the adhesion strength of the adhesive
is same as that of a smooth adhesive, i.e. 0.2 N/cm2 30 . The data shown in Figure 15
19
shows that the strength of adhesion increases with roughness within the range of
adherent roughness examined,
The present process involves self-organization in at least two different levels i.e. first in
generation of a template and further during moulding of a polymeric material against
that template. During preparation of the template, precipitation 5 of the non-aqueous
phase results in a porous structure in a self-organization process. Such a surface is found
to be useful in many applications, e.g. in strong and reversible adhesion.
The process of the present invention is useful for generating nano-microscopic patterns
10 on surfaces of different materials using porous hydrogel as templates. The method of
preparing the template, in order to make it suitable for the above process, has also been
developed and optimized. The patterns have been transferred on silicone elastomers and
Resorcinol Formaldehyde gel. AFM and SEM imaging of the patterned surfaces reveal
that their dimensions are at least an order of magnitude smaller than the size of those on
15 the template. The parameters which can be tuned in order to control the dimension of
the patterns have also been identified. The applications of these patterns have been
demonstrated by designing reusable adhesives.
In an aspect of the invention, there is provided a process for generating patterns of
20 nanoscopic length scales on the surface of a soft elastomer by using templates which
consist of features having microscopic length scales. The templates consist of layers of
porous hydrogel, the scanning electron micrograph (SEM) imaging of which show that
the pore sizes vary from 1.2-20 μm depending on the initial water content in the gel prepolymer
solution. Therefore, when a cross linkable elastomer is cured on such a surface,
25 the crosslinked material is too expected to have topographical features of similar
dimensions. Contrary to this expectation, the crosslinked surfaces get decorated with
long pillar like structures of diameter: 50-900 nm and height 5-440 nm whole through
their surface. This observation shows that patterns get generated not only via moulding
against template but also via additional interfacial interactions which lead to instability
30 driven morphological self-organization. Thus the invention relates to interfacial
20
instability at a confined space of a material going from oligomeric liquid phase to
crosslinked network phase. Since this process allows patterning over a large surface
area, it holds the promise of easy scale up and therefore large variety of applications.
In another aspect of the invention, the elastomers decorated with nano-pillars can be
used as reusable pressure sensitive adhesives, the adhesion strength 5 of which depend
upon several physical factors: their flexibility allow them to follow the roughness of a
surface and secondly multiple crack arrest and initiation, that occur at the interface
when they are detached from an adherent, similar to natural adhesives present at the feet
of many insects and wall climbing animals. Thus this invention is also related to bio10
mimicking of natural materials.
In another aspect of the invention, the shape and size of pores can be controlled. Since
monovalent and bivalent salts like NaCl and CaCl2 are used as precipitant for the nonaqueous
phase, and the gel monomer crosslinks around the crystal of such salt as the
template, both the shape and size of the pores in the gel template attains that of the
15 crystal. Thus while in conventional processes, only the size of the pores is controlled,
here the shape of the pores too can be controlled.
In another embodiment the present invention provides a reusable, pressure sensitive
adhesive surface comprising non-uniform, hierarchical nano-pillar structures patterned
20 over a large surface area. The pressure sensitive adhesive surface of present invention
comprises crosslinked poly(dimethylsiloxane). Oligomers of this material is crosslinked
using a curing agent which is mixed with it at a defined weight fraction.
The adhesives are having superior ability to adhere to smooth and rough surfaces. These
25 adhesives adheres reversibly and can be easily cleaned and reused several times without
any decrease in adhesion property.
Advantages of the present invention:
21
• By using different squeezing techniques, water inside pores can be removed to
desired extent, so that, both height and diameter of pores, i.e. the dimension of
the template can be tuned in a one step process.
• The feature size on the elastomeric layer can be controlled by varying the
dimensions 5 of the template.
• In addition to controlling pore dimensions, the feature size on elastomeric layers
can also be controlled by altering the viscosity of the pre-polymeric elastomeric
liquid, i.e. by varying the extent to which the pores at the vicinity of the gel
surface get filled in by this liquid.
• 10 The template can be reused multiple times.
• The process makes use of environmentally benign material for preparing
template as well as the final product.
• Coupled effect of top-down approach for moulding against a micro-patterned
template and bottom-up effect of interfacial interaction driven instability of a
15 soft surface leads to generation of dense nano-pillars over a large area.
• Since hydrogel can be swelled by water, the process allows easy removal of the
crosslinked material from the template surface, which, in turn renders the
template reusable.
• Since, the process is essentially driven by physical effects, chemical character of
20 the materials is of less importance; as a result, elastomers of different kind can
be used for preparing the textured surface.
• The process is amenable for easy scale-up.
• The process allows variety of auxiliary techniques to be implemented for
controlling the dimension of the structures, e.g. transverse and longitudinal
25 vibration of the substrate and reduction of viscosity of the elastomer prepolymer
liquid by addition of a suitable solvent, both of which increases the
adhesion strength.
• Since the pillars are of irregular shape and they remain randomly dispersed on
the adhesive surface, they are inherently hierarchical in structure.
22
• Hierarchical structure of pillars prevents their self adhesion, an effect known to
be responsible for irreversibly diminishing the adhesion strength of many
reusable adhesives.
• The adhesive surface gets contaminated by particulate matters which diminish
its adhesion but can be easily cleaned by immersing the adhesive 5 layer under
common surfactant solution which removes the dirt rendering the adhesive as
fresh as its initial state with completely recovered adhesion strength.
Applications of the present invention:
• 10 The invention will be useful for all kinds of applications requiring re-usable
adhesion, e.g. sticky mat in laboratories, hospitals and cleanrooms; keyboard
guard for laptop and desktop, sticky pad for wall hanging purposes etc.
• Adhesive pads for locomotion of wall climbing robots.
• Gloves and pads useful for handling heavy equipments.
• 15 Sticky feet and supports for heavy duty ladders.
• Sticky handkerchief for dry cleaning of hand.
The invention is further described by way of non-limiting examples:
EXAMPLE 1:
Preparation of template:
20 For preparing 20 gm of the gel, two kinds of stock solution were prepared:
16gm of well mixed solution 1 in distilled water containing desired quantity of
monomer, crosslinker, promoter and the additive and 4gm of solution 2 containing
required quantity of APS. The solution 2 was added to solution 1 while continuously
25 stirring; followed by it, the mixed solution was allowed to crosslink for about three
hours. In most of the application of the present adhesive surface, a thin layer of gel of
thickness 1-5 mm is prepared by carrying out the crosslinking reaction on a solid
substrate, e.g. a polystyrene or glass petridish, microscope glass slide. The surface of the
petridish can be coated with a SAM of silane molecules, e.g. octadecyltrichlorosilane, in
23
order to prevent adhesion or bonding of the reagent molecules on its surface. The top
surface of the gel remains smooth and devoid of any macroscopic topographical features
when the crosslinking reaction is carried out on such a substrate (illustrated in Figure
2a). However, for one without such surface modification, e.g. hydrophilic glass slide or
polystyrene petridish, the gel surface gets decorated with wrinkle patterns 5 (illustrated in
Figure 2b). In either case, the gel remains porous, the water from which is removed by
any of the following methods (illustrated in Figure 3). Figure 3a illustrates Drying by
blowing air where pHEMA gel layer is placed horizontally inside a hot air oven
maintained at 30-350C temperature. Water evaporates out of the gel with concomitant
10 shrinkage of the gel and the pores. Figure 3b shows Centrifugal Mode wherein the
petridish containing the pHEMA gel layer in it is attached vertically to the inner wall of
a rotating drum. Under centrifugal field the water gets squeezed out of the gel layer.
This non-contact rapid method of water removal ensures that pores remain intact so that
crosslinkable liquid PDMS can get into the pores. While figure 3c shows Vacuum
15 Drying where the pHEMA gel layer is placed on a filter paper or a porous substrate
inside a vacuum chamber at 0.2-0.5 mbar for 15-30 minutes. 40% to 70% of water
present initially in the gel is removed such that the pores at the surface of the gel do not
get damaged or blocked. The gel surface attains porous sponge like structure, the
porosity of which varies from 1.2 μm to 20 μm. The template thus formed is used for
20 forming the elstomeric layer against it.
Example 2:
The figure 14 demonstrates the suitability of the adhesive for applications which
demand repeated use. Here a pHEMA gel template was first prepared by the method in
figure 1. The gel pre-polymer solution was prepared using 2-hydroxyehtylmethacrylate
25 monomer and water in 15:85 w/w ratio in pre-polymer solution; monovalent salt NaCl
was used as the precipitant (7.81% by weight of the water). Following polymerization,
43% of the initial water content was removed by the method depicted in figure 2. The
Sylgard 184 elastomer was crosslinked against this template as in figure 3. Adhesive
thus prepared was subjected to shear adhesion test as in figure 8 and the test was
30 repeated n -times. Between two successive tests, the adhesive was cleaned by first
ultrasonication for 5 minutes inside a pool of ethanol, followed by rinsing it with DI
24
water and then drying by blowing nitrogen gas. The symbols representing this data
show that even after 15 test cycles, the adhesion strength does not diminish any
significantly.
5 Example 3:
The adhesion strength of adhesive layers made of Ecoflex cured against a pHEMA gel
template which was prepared with bivalent salt CaCl2 as the precipitant is shown in
Figure 13. The template was prepared by using the 2-hydroxyehtyl methacrylate
monomer and water in 20:80 w/w ratio in the pre-polymer solution; bivalent salt CaCl2
10 (7.81% by weight of the water) was added to it as the precipitant. Following
polymerization, the water was removed to different extent e.g. 15 – 70% of the initial
water content. The Ecoflex oligomer was crosslinked against this template as in figure
4. The adhesive layer thus prepared was subjected to shear adhesion test as in figure 8.
15 Example 4:
Figure 12 shows the extent of removal of water by pHEMA gel by the method described
in figure 2 affects the shear adhesion strength of Ecoflex. pHEMA gel templates were
prepared by using the 2-hydroxyehtyl methacrylate monomer and water in 20:80, 15:85
and 10:90 w/w ratio in the pre-polymer solution; monovalent salt NaCl (7.81% by
20 weight of the water) was added as the precipitant. The symbols represent
these different templates from which water was removed to different extent e.g. 15 –
70% of the initial water content 30% of this water was removed. The ecoflex oligomer
was crosslinked against these gel samples as the template by the process described in
figure 3. The shear adhesion strength of the adhesive thus prepared was measured by the
25 method described in figure 8.
Example 5:
Figure 11 shows the effects of longitudinal vibration of the template during crosslinking
of the elastomer against the template and the presence of toluene in the crosslinkable
30 liquid, on the final shear adhesion strength of the adhesive. A pHEMA gel template was
prepared by using the 2-hydroxyehtyl methacrylate monomer and water in 20:80 w/w
25
ratio in the pre-polymer solution; monovalent salt NaCl (7.81% by weight of the water)
was added as the precipitant. Following polymerization, 30% be weight of the water
was removed by the method described in figure 2. 90% w/w PDMS (Sylgard 184
elastomer), mixed with 10%w/w curing agent, was crosslinked against this template in
three temperature steps: at 25-270C for 4 hours, at 500C for 4 hours and at 800 5 C for 4-5
hours. The template was vibrated in transverse direction at a frequency 70Hz and 50-
500 μm amplitude during the first 1 hour of this curing process. In few experiments, the
crosslinkable liquid was prepared by mixing 83% w/w PDMS (Sylgard 184 elastomer),
8.33% w/w curing agent and 8.33% w/w Toluene, which was used as diluent for
10 diminishing the viscosity of the crosslinkable liquid.
The vibration would affect de-pinning of the crosslinkable liquid on the template and
diluent like Toluene would diminish the force required for driving the liquid into the
confined space of the pores both the process finally leading to longer and more dense
15 nano-structures on the adhesive surface and enhanced strength of adhesion. The data in
the bar chart show that adhesion strength indeed increased by combined effect of
substrate vibration and presence of diluent in the crosslinkable liquid.
Example 6:
Effect on shear adhesion strength, of the final temperature at which PDMS (Sylgard 184
20 elastomer) layer prepared by the method described in figure 3 was heated, is shown in
Figure 10.
A pHEMA gel template was prepared by using the 2-hydroxyehtyl methacrylate
monomer and water in 15:85 w/w ratio in the pre-polymer solution; monovalent salt
NaCl (7.81% by weight of the water) was added as the precipitant. Following
25 polymerization, 43% by weight of this water was removed by the method described in
figure 2. PDMS (Sylgard 184 elastomer) layer was crosslinked against this template in
three temperature steps: at 25-270C for 4 hours, at 500C for 4 hours and at 800C for 4-5
hours. Following this process, the crosslinked solid was separated from the template and
was then heated for one hour at an elevated temperature, Tf: 80, 95 and 1300C
30 respectively. These adhesive samples were subjected to shear adhesion test as in figure
26
8. The data in the bar chart show that adhesion strength of the adhesive decreases with
increase in Tf because possibly of irreversible processes, e.g. self adhesion of the nanostructures
at the interface.
5 Example 7:
Figure 9 shows the effect of removal of water from the pHEMA gel on the shear
adhesion strength of the adhesive prepared using Sylgard 184 as the adhesive material
by the method described in figure 3. The symbols represent respectively
the 2-hydroxyehtyl methacrylate monomer and water w/w ratios of 20:80, 15:85 and
10 10:90 in the pre-polymer solutions; monovalent salt NaCl (7.81% by weight of the
water) was added as the precipitant. Following polymerization, the water present in
these gel samples were removed to different extent, e.g. 15–70% of the initial water
content by the methods described in figure 2. The poly(dimethylsiloxane) oligomer was
crosslinked against these gel samples as the template and the resultant shear failure load
15 was measured. The data show that adhesion strength increases with the initial water
content in pHEMA gel and the extent of removal of water from it.
Example 8:
Figure 8 depicts the shear adhesion test, in which the adhesive layer was strongly
20 attached to a vertical wall; a scotch tape was used as an additional enforcement for
preventing the slippage of the adhesive on this surface. A glass plate was used as a
model adherent that was contacted with the nano-patterned surface of this adhesive.
Shear load were exerted at the interface between the adhesive and the adherent by
hanging different dead weights onto the glass slide with the help of a clamp. The loads
25 were increased in small steps in order to find out the specific value at which failure
occurred. The experiment was repeated 3-4 times in order to have a representative
value.
Example 9:
27
Figure 7 shows AFM image of adhesive film made of PDMS prepared against pHEMA
gel template, which was prepared by using 2-hydroxyehtylmethacrylate monomer and
water at 10:90 w/w ratio in the pre-polymer solution; monovalent salt NaCl was used as
the precipitant (7.81% by weight of the water). Following polymerization, 31% by
weight of the initial water content in the gel was removed by the method 5 described in
figure 2. PDMS (Sylgard 184 elastomer) layer was cross-linked against this template as
in figure 3. The AFM image depicts that the adhesive surface consists of topographical
structures having micro to nanoscopic dimensions.
10 Example 10:
Effect of Water Removal on the Pore Morphology of Phema
SEM micrograph of pHEMA gel which was prepared by using 2-hydroxyehtyl
methacrylate monomer and water mixed in 10:90 w/w ratio in the pre-polymer
solutions; monovalent salt NaCl (7.81% by weight of the water) was added as the
15 precipitant. Following polymerization (a) 40% and (b) 52% by weight of the initial
water present in the gel was removed by following the process depicted in figure 2. The
dashed rectangles depict the specific shape of the pores which match with the cubic
crystal structure of NaCl.

WE CLAIM:
1. A fabrication process for a reusable adhesive surface with non-uniform,
hierarchical nano-microscopic pillars, comprising 5 the steps of:
i) preparing a porous hydrogel template by crosslinking a pre-polymer
solution in water with a cross-linker, a promoter, a redox initiator and a
salt as precipitant for the non-aqueous phase;
ii) removing varied amounts of water from the pores of said hydrogel
10 template obtained instep (i) by drying under vacuum or atmospheric
pressure, and/or squeezing the gel layer by applying negative pressure of
a vacuum pump, and/or squeezing the gel in a centrifugal field to
regulate the height and pore diameters of the said template;
iii) pouring a cured crosslinkable elastomer liquid mixed with a crosslinking
15 agent on said gel template obtained in steps (i) and (ii) to fill the pores of
the said gel template partially or completely;
iv) crosslinking of said elastomer mixture on said gel template first at 25-
270C for 4 hours, second at 500C for 4 hours and at finally 800C for 4-5
hours; and
20 v) separating crosslinked elastomeric layer from surface of the said gel
template.
2. The process as claimed in claim 1, wherein the porous hydrogel pre-polymer
solution is selected from group consisting of polyhydroxyethyl methacrylate
(pHEMA) gel, n-isopropylacrylamide (NIPA) gel, poly-acrylamide gel.
25 3. The process as claimed in claim 1, wherein the cross-linker is
ethyleneglycoldimethacrylate (EGDMA).
29
4. The process as claimed in claim 1, wherein the promoter is
tetramethylethylenediamine (TEMED) as a promoter.
5. The process as claimed in claim 1, wherein the redox initiator is ammonium per
sulphate (APS).
6. The process as claimed in claim 1, wherein said salt as precipitant 5 for the nonaqueous
phase is selected from sodium chloride or calcium chloride.
7. The process as claimed in claims 1 and 3, wherein said cross-linker is used in
1% weight fraction of monomer.
8. The process as claimed in claims 1 and 4, wherein said promoter is used in 1%
10 weight fraction of monomer.
9. The process as claimed in claims 1 and 5, wherein said redox initiator is used in
0.05% weight fraction of monomer.
10. The process as claimed in claim 1, wherein said salt is present in an amount of
7.81% by weight of water as a monovalent precipitant.
15 11. The process as claimed in claim 1, wherein the monomer-to-water weight ratio
is ranges from 80%–95% by weight of water.
12. The process as claimed in claim1, wherein pore diameter ranges from 1 μm to
20 μm.
13. The process as claimed in any of the preceding claims, wherein the template is
20 made of environmentally benign material.
14. The process as claimed in claim 1, wherein the crosslinkable elastomer is
selected from polydimethylsiloxane and polyurethane.
15. The process as claimed in claim 1, wherein the cured elastomer liquid layer is of
25 thickness 0.1-15 mm.
30
16. The process as claimed in claims 1, wherein said cured elastomer liquid on the
gel template is degassed inside a vacuum chamber at 0.05 mbar pressure for 10
min.
17. The process as claimed in claim 1, wherein said cured elastomer 5 liquid system
the gel template is subjected to transverse vibration at a frequency range of 10-
100 Hz.
18. The process as claimed in claim 1, wherein the viscosity of the said elastomer
10 liquid is reduced by adding solvent, preferably toluene.
19. The process as claimed in claim 1, wherein said separated elastomeric layer is
sonicated inside a pool of ethanol and water for 30 minutes.
15 20. The process as claimed any of the preceding claims, wherein the crosslinked
layer of elastomer consists of nano-pillars.
21. The process as claimed in claim 20, wherein diameter of said nano-pillars ranges
from 50 to 900 nm and heights ranges from 5 nm to 450nm.
20
22. A reusable, pressure sensitive adhesive surface comprising non-uniform,
hierarchical nano-pillar structures patterned over a large surface area.
23. The adhesive as claimed in claim 22, wherein the nano-pillar structures are
25 spatially separated or randomly organized.
24. The adhesive as claimed in claim 22, wherein the nano-pillar structures are
aligned vertically or at an angle.
31
25. The adhesive as claimed in claim 22, wherein the nano-pillars with hierarchical
length scales provide superior adhesion strength to the adhesives on both smooth
and rough surfaces.
26. The adhesive as claimed in claim 23, wherein the diameter of 5 said nano-pillar
ranges from 50 to 900 nm and heights ranges from 5 nm to 450nm.
27. The adhesive as claimed in claim 23, wherein said adhesive is having a tensile
strength of 1.3 N/cm2.
10
28. The adhesive as claimed in claim 22 comprises crosslinked