Journal of Biomaterials and Nanobiotechnology, 2010, 1, 17-30
doi:10.4236/jbnb.2010.11003 Published Online October 2010 (
Copyright © 2010 SciRes. JBNB
Water—A Key Substance to Comprehension of
Stimuli-Responsive Hydrated Reticular Systems*
Miloslav Milichovsky
Department of Wood, Pulp and Paper, University of Pardubice, Faculty of Chemical Technology, Pardubice, Czech Republic.
Received August 3rd, 2010; revised August 24th, 2010; accepted September 20th, 2010.
Thermo-responsive hydrated macro-, micro- and submicro-reticular systems (TRHRS), particularly polymers forming
hydrogels or similar networks, have attracted extensive interest because comprise biomaterials, smart or intelligent
materials. Phase transition temperature (LCST or UCST, i.e. low or upper critical solution temperature, respectively)
at about the TRHRS exhibiting a unique hydration-dehydration change is a typical characteristic. The characterization
and division of the TRHRS are described followed by explanation of their behaviour. The presented original explana-
tion is based on merely combination of basic thermodynamical state of individual useful macromolecule chains
(long-chain or coil) with inter- and intra-mutual action of attractive and repulsive intramolecular hydration forces
among them being strongly dependent upon temperature. Acquainted with this piece of knowledge, a theoretical con-
cept of really biological systems movement, e.g. muscle tissues or artificial muscle etc., can be formulated.
Keywords: Thermally Responsive Materials, Hydrogels, Hydration Forces, Volume Phase Transition
1. Introduction
Stimuli-responsive polymers – so-called smart polymers
– have attracted great interest in academic and applied
science recently. Most commonly, approaches take ad-
vantage of thermally induced, reversible phase transitions.
In this context, polymers forming hydrated reticular sys-
tems found great interest. Hydrated reticular systems, i.e.
networks in water environment, feature all of bio-objects
and the products of their existence. We can identify these
structures in nano- (submicro-), micro- and macro-scale
as submicro-, micro- and macro-reticular hydrated sys-
tems, respectively. Supramolecular and hypermolecular
structures are typical, e.g. the hydrogels on peptide basis
and fibre-networks on cellulosic basis.
Hydrogels consist of elastic networks that can uptake
as much as 90–99% w/w of water in their interstitial
space. Hydrogels have high water content and a soft and
rubbery consistency. Such systems have been especially
focused in the biomedical area as they provide adequate
semiwet three-dimensional environment for cells and
tissue interaction and they can be combined with bio-
logical or therapeutic molecules. They can be also chemi-
cally controlled and designed to tailor their mechanical
and functional properties [1-3]. Therefore hydrogels have
been proposed for a series of biomedical and biological
applications, including tissue engineering [3-4], drug
release systems [5-9], biological sensors [12-14], tem-
perature and light-responsive films [15] or tuneable hy-
drogel photonic crystals as optical sensors [16]. The most
common hydrogels are the ones obtained by chemical
crosslinking of hydrophilic macromolecules. Such link-
ages prevent the dissolution of the material but water can
penetrate within the structure, causing the swelling of the
structure without disrupting the mechanical and geomet-
rical integrity of the structure. If the macromolecules
composing the network react with some external variable,
e.g. temperature, switching between a stretched to a
squeezed states then the corresponding hydrogel could
reversible swell and deswell in response to this stimulus.
Such smart hydrogels have been proposed for a series of
biomedical applications [17-18], not only in the delivery
of therapeutic agents [5-9], but also in tissue engineering
[10-11], intelligent microfluidic switching [19-21], sen-
sors/diagnostic devices [22-23] and actuators [24-25].
For these purposes predominantly the recent new sur-
face techniques are utilised. These smart designs are
mostly based on stimuli-responsive materials forming
self-assembled monolayers and polymer films. Methods
such as spin coating, chemical vapour deposition, laser
*This work was supported by the Ministry of Education, Youth and Sports
of the Czech Republic under the Research Project MSM0021627501.
Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems
ablation, plasma deposition and chemical or electro-
chemical reactions have been widely applied to the fab-
rication of thin polymer films [26]. Further utilisation of
the effect of external temperature stimuli was already
demonstrated in several applications of nanometre-thick
poly(N-isopropylacrylamide) (PNIPAAm)-grafted sur-
faces for separation processes [27-38] including gel per-
meation chromatography [33], size exclusion chroma-
tography [36] and aqueous chromatography [27-30,37]
inclusive high performance liquid chromatography
(HPLC) [31,33-36]. In most of these applications, the
packing material is modified with PNIPAAm to change
the property of the stationary of the column in response
to alteration of temperature.
The most peculiar property of these systems, however,
is probably their stimuli-responsive behaviour. The thermo-
responsive behaviour is typical but only for hydrogels
because fibre-networks are composed of high consis-
tency hydrogel fibres distributed in macro-space of water
environment. Thermo-responsive hydrogels undergo a
phase transition in response to temperature changes. Up
to now, almost all of the thermo-responsive hydrogels
have been featured with negatively thermo- responsive
volume phase transition, i.e. with the existence of a
lower-critical solution temperature, LCST. Below LCST,
the un-crosslinked polymer chains are soluble in water
whereas above LCST the polymer chains form submicro-
and micro-aggregates, which separate from solution.
Thermo-responsive hydrogels composed of cross-linked
polymer chains undergo fast [38-39], reversible structural
changes from a swollen to a collapsed state by expulsing
water. However, also another kind of thermo-responsive
hydrogels exist which is opposite to that LCST-hydrogels,
i.e. the hydrogels with an upper-critical solution tem-
perature, UCST. These hydrogels shrink at lower tem-
perature and swell at higher temperature.
Obviously, due to short history of this family and the
fact that these materials are not commercially available, a
great deal of fundamental knowledge regarding their
properties is still lacking. Polymer interactions are very
complex and no complete molecular-level understanding
exists to date. Mostly, the absence of water molecule
interaction is typical for theoretical interpretation of this
specifically behavior.
2. Classification of TRHRS
According to behavior of thermo-responsive hydrated
reticular systems (TRHRS) during dilution we can divide
them onto water dilute-able and non dilute-able, the
crosslinked 3D networks (see Figure 1) or crosslinked
2D networks – films. Additionally, it is possible to divide
the dilute-able TRHRS onto fully dilute-able polymer
solutions at T< LCST (or T > UCST) and coacervated
[51-53] submicro- or micro-TRHRS or flocculated
The dilute-able TRHRS coacervate or flocculate in
water environment due to weak bonds among of polymer
chains, micro-particles and hydrogel particles or fibers
and micro-fibers, respectively. It is typical of the submi-
cro-, micro- and macro-networks that are disrupting dur-
ing dilution process, i.e. the quasi-hydrogels are coacer-
vating and the fiber networks are flocculating, respec-
tively. As a temperature changes, the sol-gel reversible
hydrogels transition occurs due to non-chemical cross-
links being formed among grafted and branched elements
of copolymers.
The crosslinked structures created by strong particu-
larly chemical bonds among polymer chains like micro-
and macro-sponges have been then swelled or shrunk in
response to the temperature change over the LCST.
2.1. LCST Hydrogels
PNIPAAm, has been the most used macromolecule in
thermo-responsive hydrogels. The changing in properties
with temperature in PNIPAAm is based on a phenome-
non that is thermodynamically similar to that causing
temperature-induced protein folding [17]. Above the
LCTS a reversible structural transition occurs from ex-
panded coil (soluble chains) to compact globule (insolu-
ble state), at around 32°C in pure water [3,13-14,40-42].
Below the LCST, the hydrogel is swollen and absorbs a
significant amount of water, while above LCST, the hy-
drogel dramatically releases free water and begins to
shrink. Mostly opinion is prevailing that the solubility is
affected because the amphiphilic PNIPAAm chains hide
the hydrophilic amide groups and expose the hydropho-
bic isopropyl groups in the compact globule structure.
The most common LCST hydrogels are the ones ob-
tained by chemical crosslinking of hydrophilic macro-
molecules. Such linkages prevent the dissolution of the
material but water can penetrate within the micro-re-
ticular structure, causing the swelling of the structure
without disrupting the mechanical and geometrical integ-
rity of the structure.
During a volume transition the hydrogel anti-bonding
system formed between water molecules and the poly-
meric chains is disturbed, being this thermodynamically
favourable increasing in entropy the main driving force
for the occurrence of the transition. For the case of
crosslinking systems this transition can be seen through
an abrupt shrinkage of the hydrogel above the LCST as-
sociated with the change in the swelling capability of the
hydrogel. In addition, DSC analysis indicates that this
process is accompanied especially for cross-linked hy-
drogel with heat consumption, i.e. an endothermic proc-
ess [1-2,39]. The LCST, the characteristic temperature
Copyright © 2010 SciRes. JBNB
Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems
Copyright © 2010 SciRes. JBNB
for solution–to-gel transition, is also defined as the tem-
perature at which the elastic modulus crosses over the
viscous modulus [10]. From this reason, the rheological
behaviour of hydrogels as the function of temperature is
oft measured [1,11].
N-isopropylacrylamide may be also copolymerized in
order to include linear, end functionalized and cross-
linked binary and ternary copolymers [4-5], graft and
block copolymers [1,17-18]. However, PNIPAAm is
non-biodegradable and it is not readily cleared from the
body at physiological temperature. That is why bioad-
sorbable thermo-responsive polymer systems have been
achieved by incorporation of biodegradable segments
such as hyaluronic acid [46], gelatine [12], peptides [47]
and collagen [1,32] into PNIPAAm-based polymers.
N,N´-methylenebisacrylamide (MBA) is oft utilized as
crosslinker [39,43]. Such macromolecular design widens
the applicability of such systems in a variety of biomedi-
cal applications inclusive biomineralization on biode-
gradable substrates [44]. Such modifications are particu-
larly important to tailor the LCST of PNIPAAm-based
systems. For example, random copolymers containing
hydrophilic units (e.g. acrylic acid) or Ba2+ ions exhibit
higher values of the LCST [4-5], and an opposite trend is
observed in copolymers containing hydrophobic groups
(e.g. n-butyl methacrylate) [34] or Cs+ ions [5]. It was
also observed (laminin-1-functionalized methylcellu-
lose by periodate oxidation) [10] that the LCST is de-
pendent on polymer concentration, as decreasing poly-
mer concentration increased the LCST. This observation
is in contradiction with UCST increase vs. temperature –
see Figure 1.
Figure 1. Schematic illustration of classification of thermo-responsive hydrated reticular systems.
Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems
Instead of PNIPAAm-based polymers also other
polymer bases exist, e.g. poly(N,N-diethylacrylamide)
(pDEAAm) [45], poly(N-cyclopropylacrylamide) [15], a
“dual” nanocomposite based on poly(vinyl acetate)
(PVAc) and cellulose whiskers [12], poly-D-lysine-
functionalised chitosan [11], the hydrogel photoresist,
which was formulated by mixing poly(HEMA-co-MMA)
synthesized by radical copolymerization of 2-hy-
droxyethyl methacrylate (HEMA) and methyl methacry-
late (MMA) with a crosslinker, tetramethoxymethyl gly-
coluril [16]. Most promising family of protein polymers
is elastine-like polypeptides (ELP) [1,32]. ELPs have
shown an outstanding biocompatibility. In addition, the
ELPs have an acute “smart” nature.
Below the transition temperature, the uncrosslinked
polymer chains are soluble in water but above LCST, the
polymer starts a complex self-assembling process that
leads to an aggregation of polymer chains, initially
forming nano- and micro-particles, which segregates
from the solution [32]. The copolymers with branched
i.e. prepared by copolymerization of NIPAAm, acrylic
acid (AAc) and biodegradable monomer hydroxyethyl
are accompanied above LCST with sol-gel transition
occurred immediately when the clear solution is im-
mersed into the water bath. After incubation, a highly
flexible gum-like material is then formed and de-swelling
is further observed during continued warming in water
bath [4].
For the case of crosslinking three-dimensional systems
this transition can be seen through an abrupt shrinkage of
the hydrogel above the LCST associated with the change
in the swelling capability of the hydro-reticular material.
The volume phase transition process upon heating con-
nected with water molecules expulsion from hydro-re-
ticular spaces - known as hydrogel syneresis [50] - is
reversible with thermal and stimuli responsivity [13,39,
48-49] but with different response dynamic and volume
changes. It was proved that the reversible transition dur-
ing the heat cycle is due to the elasticity of crosslinked
hydrogels [16]. Since the rapid response dynamic and
large volume changes due to temperature variation is the
essential function for intelligent hydrogels applications,
the thermo-responsive hydrogels with improved response
rate and large volume changes to an external temperature
stimulus are preferred [31,39]. The improved response
dynamic of the hydrogels are obtained through incorpo-
rating siloxane linkage [31], cold polymerization and the
pore-forming agent, etc. [39].
Though a lot of information exists describing the be-
haviour of TRHRS some of the mechanisms involved in
the volume transition critical solution temperature are
still not well understood. The smart surface designs
mostly based on stimuli-responsive materials forming
self-assembled monolayers (SAMs) and surface-tethered
polymers, known as polymer brushes, suggested that the
polymer chains of PNIPAAm and its copolymers have
two structures in aqueous solution [26]. Below its LCST,
PNIPAAm polymer is in an extended, solvent-swelled
structure, but when heated up above LCST, the polymer
undergoes a phase transition to yield a collapsed mor-
phology that excludes water [17,44]. For example, these
widespread structural changes enable the multiresponsive
surfaces reversible change on silicon substrate to be real-
ised between superhydrophilicity and superhydrophobic-
ity [13]. It is important that the silicon surface roughness
becomes the main factor in intensifying this behaviour.
In contrast to magnitude of the contact angel changes on
flat film, a remarkably large change in this one was in-
duced on rough substrate. Logically, for a rough surface
with a high surface free energy, the film is more hydro-
philic or more hydrophobic. Nevertheless, PNIPAAm
surfaces cannot be described only in terms of surface
wettability, because above the LCST the surfaces are
only partially dehydrated [33].
Usually, this behaviour is based on confusing explana-
tion that a hydrogen bond network between the amide
groups and water molecules are formed at lower tem-
perature, whereas at higher temperatures the stabilizing
H-bonds break up and the hydrophobic interactions be-
come predominant [26]. Thus the hydrophobic interac-
tions among the hydrophobic groups become stronger
which subsequently induce the freeing of the entrapped
water molecules from the hydrogel network [13,26,39].
However, indication confirming an important role of
water molecules in behaviour of TRHRS was observed.
Temperature- and light-responsive polyacrylamide co-
polymers featuring salicylideneanilin as a photochromic
group is reported which structure by irradiation and
turning off the UV light is changed, but the respective
LCSTs values remained higher than before irradiation.
The LCST shift after irradiation can be explained by an
intramolecular stabilization of the exited keto form in
high polar media such as water because after evaporation
of the samples solutions and redissolving in water, the
values for the LCST were the same as before irradiation
A similar mechanism of structural transition from ex-
panded coil to squeezed proper thermodynamically ad-
vantageous structure is possible to expect at crosslinked
TRHRS as the temperature is raised above the LCST,
because macroporous hydrogels, i.e. hydrated macro-
reticular systems, are consisted from walls which are
formed by micro- and submicro-porous sections of the
hydrogel character as well as. These facts ensue from the
Copyright © 2010 SciRes. JBNB
Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems21
observations of non-expectable low wall density of ma-
croporous thermo-sensitive hydrogels from recombinant
elastin-like polymers (ELP) [1]. For example, according
to myself recalculation of the results presented in liter.
[1], the wall densities of ELP hydrogels in clear water
were 0,4405 g/cm3 at T = 4oC and 0,6081 g/cm3 at T =
37oC but had been lowered dramatically with increasing
salt/polymer ratio at T = 4oC to 0,0677 – 0,0850 g/cm3
although the densities at T = 37oC were approximately
unchanged (0,436 - 0,617 g/cm3). By parallel action of
all submicro- and micro-sections composing the walls of
macro-reticular system, the hydrated macroporous sys-
tem is then swelled or de-swelled in dependence on the
temperature changes crossing a value of the LCST.
Summarizing up of the all above mentioned facts, we
can conclude that at usually conditions, i.e. at room tem-
perature and inert environment, the PNIPAAm polymer
has preferred thermodynamically advantageous a coil
conformation because the hydrophobic interactions
among isopropyl pendant groups. However, due to pecu-
liar water activity, the coil conformation is stretched at
temperature below the LCST in contradiction with
squeezed the original coil conformation above the LCST
as the repulsive domain activity is weakened – see sche-
matic illustration in Figure 2. The peculiar water activity
is accompanied below the LCST by origination of repul-
sive water action among polymer chains, its segments,
submicro- and micro-colloidal particles etc. arising from
equally water molecules orientations at interacting inter-
face micro-domains due to hetero- followed by homo-
H-bonds among them, i.e. a hydration anti-bonding sys-
tem. Obviously, a width of vicinal immobilised water
within interacting polymer interfaces decreases with in-
crease of polymer concentration because improving dis-
ruption action of the hydration repulsive forces being
weakened dramatically with a temperature increase. As
result, the LCST decreases with polymer concentration
increase [10].
Figure 2. Schematic illustration of temperature and concentration influence on behaviour of hydrated micro- and
nano-reticular systems distinguished by LCST.
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Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems
2.2. UCST Hydrated Reticular Systems
Until very recently [17,43], little works has been reported
on positively thermo-responsive microgel particles with
UCST, i.e. hydrogels that shrink at lower temperature
and swell at higher temperatures, although they should be
preferred to negatively thermo-responsive microgels in
certain applications. The UCST hydrogels are mainly
composed of an interpenetrating polymer network (IPN)
of polyacrylamide (PAAm) and poly(acrylic acid) (PAAc)
or poly(acrylamide-co-butyl methacrylate) crosslinked
with MBA [43]. The formation of helices (double or tri-
ple in polysaccharides such as agarose, amylase, cellu-
lose derivatives and carrageenans or in gelatine, respec-
tively) and corresponding aggregation upon cooling,
forming physical junctions, are on the base of hydrogel
formation [17]. Dilute-able but coacervating quasi-hy-
drogel with UCST are represented by urea-formaldehyde
(UF) pre-condensates [51-53].
Thermo-responsive volume phase transition behaviour
of these TRHRS is opposite to that of PNIPAAm-based
polymers. Again, the description of this behaviour is
based on confusing explanation [43] that the IPN hy-
drogels form intermolecular complexes via hydrogen
bonding at temperatures lower than the UCST while dis-
sociate at temperatures higher than the UCST. According
to this explanation, driven by the hydrogen bonding, the
PAAm/PAAc based IPN hydrogels shrink at lower tem-
peratures and swell at higher temperatures revealing
positively thermo-responsive volume transition behav-
iour. Aside from the facts that mostly parts of IPN are
penetrated with water molecules, it is interesting to look
in closer way at the areas in the LCST and UCST hy-
drated reticular systems.
Really, the mechanism of the reversible behaviour is
similar to that of TRHRS with the LCST but in opposite
manner - see schematic illustration in Figure 3. At usu-
ally conditions, i.e. at room temperature and inert envi-
ronment, the UCST hydrated crosslinked and un-cross-
linked polymers have preferred thermodynamically ad-
vantageous a long-chain structure which is squeezed in
water environment to compressed coil conformation due
to origination of weak hydration bonding system. The
hydration bonding system [52-58] among polymer chains,
its segments, submicro- and micro-colloidal particles etc.
Figure 3. Schematic comparison of both the TRHRS with LCST and UCST. The weak bonding system is represented in this
case by coacervating of quasi hydrogel system with UCST.
Copyright © 2010 SciRes. JBNB
Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems23
arise from opposite water molecules orientations at in-
teracting interface micro-domains because hetero- H-bonds
to proton-acceptor or proton-donor groups of polymer
chains and homo-H-bonds among water molecules. As
the temperature increases the hydration bonding system
is weakened because the increase of water molecules
kinetic energy. Above the UCST, the hydration bonding
system is weaker than inner opposite stress of compressed
polymer chains of TRHRS and the polymer chains have
been expanded. As result, the crosslinked TRHRS are
swelled and the polymer chains in non-crosslinked
TRHRS are dissolved. Under the UCST, the hydration
bonding system is stronger than inner opposite tension of
compressed flexible polymer chains and the polymer
long chains have been compressed. The process is re-
sulted in de-swelling and coacervating of crosslinked
hydrogel and dissolved polymers, respectively.
With increase of polymer concentration in contradic-
tion with LCST systems a width of vicinal immobilised
water also increases within interacting polymer interface
micro-domains, because improving stabilization action of
the hydration attractive forces although they are dis-
turbed as well with a temperature increase. As result, the
UCST increases with polymer concentration increase [51]
– see Figure 4.
3. TRHRS with Weak Bonding System
As already said, the UCST hydrated polymers with pre-
ferred the long-chain flexible structure are squeezed in
water environment to compressed coil conformation due
to origination of weak intra- and inter-hydration bonding
system. Obviously, both the intra-hydration bonds squee-
ze the long-chain un-crosslinked polymer structure to
compressed coil conformation and the inter-hydration
bonds squeeze the long-chain but crosslinked structures
to de-swelled form in temperatures below UCST. How-
ever, other characteristic behaviour is observed if a hy-
drated reticular system is composed of relative rigid rod
like particles as short polymer chains or fibres. The short
polymer chains or fibres in hydrated submicro- or macro-
reticular systems, respectively, are formed through in-
ter-hydration bonds and the inter-hydration repulsive
domains, i.e. mutually functioning hydration bonding and
de-bonding sites. As typical, due to increased fluctuation
at the bonding and de-bonding activities of interacting
micro-sites during dilution the submicro-reticular sys-
tems are coacervating and the macro-reticular systems
are flocculating.
Figure 4. Schematic illustration of temperature and concentration influence on behaviour of hydrated micro- and submi-
cro-reticular systems distinguished by UCST.
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Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems
The submicro weak bonding hydro-reticular system
accompanied by coacervation during a dilution is well
demonstrated by use of UF pre-condensate [51-53]. The
UF pre-condensates form in concentrated state optically
homogeneous systems with water, but when gradually
diluted the quasi-hydrogel system having reached so-
called critical degree of dilution (CDD) gets turbid, i.e.
the quasi-hydrogel – coacervate transition [51] is taken
place. The behaviour of such systems can be simply
explained by concept of hydrations forces. Individual
oligomeric molecules or short relative rigid polymer
chains are evenly distributed in water environment at
concentrated state at room temperature, when the hydra-
tion forces are functioning to sufficient extent. Minimum
internal energy can be reached if all water molecules are
contained only in mutually hydration spheres of diffusing
character around the hydrated molecules or short poly-
mer chains. The whole system is isotropic with regard to
the sizes of its structural units and more viscous owing to
the attraction of hydration forces. The lower is CDD, the
higher is viscosity of this system. In the water system,
hydration forces that repulse alternate regularly with hy-
dration forces that attract. On gradual diluting, hydrated
structural units separate from each other, becoming more
free and mobile owing to the fluctuation effect of the
attractive and repulsive forces and as well a heat. They
are less and less limited in their motion, so that they take
a more preferable orientation in their collisions. Owing to
non-isometric form of the structural units, some of these
ones take a new better arranged state at suitable moment
after breaking a 3D submicro-network, becoming ori-
ented to each other in a certain order similar to the con-
centrated state. The whole system of structural units be-
haves during dilution like a stretching network which is
gradually ruptured after reaching CDD. As a rule, the
quasi-hydrogel – coacervate transition is accompanied
logically by exothermic heat effect followed by increase
of density, viscosity and surface tension of sedimented
coacervate phase [51]. A lot of additional components
influence the attractive forces connecting the individual
weak links of network of UF pre-condensates and also
the properties of the coacervates. This influence is either
a positive one, i.e. an increase of the attraction, or nega-
tive one, i.e. a decrease of attractive hydration forces. As
we can expect the UCST is increased due to increasing
attractive hydration forces, i.e. with improving hydration
bonding ability of the TRHRS.
The macro-reticular systems with weak bonding sys-
tem are represented by papermaking pulp slurries com-
posed of fibres of cellulosic or ligno-cellulosic character.
It is typical for components with marked papermaking
properties that forms fibre network which is only com-
pressed during sedimentation, i.e. process behaviour
called as rheosedimentation [59-60]. Basic condition of
rheosedimentation is an ability of pulp fibre to form a
network with special behaviour, i.e., due to weak bond-
ing system a fibre network is compressed by gravity
[60-61]. The homogeneous pulp fibre network is formed
at concentration higher than1 kg/m3 of suspension. Fol-
lowing dilution of the suspension under the concentration
of 1 g/l is then accompanied by flocculation and rheo-
sedimentation. However, the rheosedimented fibre net-
work is not in fully homogeny state because lack of shear
forces (agitation) disturbing rheosedimenting floccules.
The temperature influences of both the macro-reticular
fibre system and the hydrogels structure of fibres with
complicated morphology. Predominantly, the tempera-
ture-responsive activity of hydrated microstructure of
fibres is important from practical point of view. We have
been observed during wet pulp beating that characteristic
decreasing of pulp drainage ability with increasing input
beating energy is abruptly increased if the temperature of
beating pulp slurry is higher then 40oC – see Figure 5.
This fact indicates some LCST behaviour of hydrogels
forming the beated fibres. As the temperature is raised
above the LCST the fibre hydrogels deswell by contrar-
ies with the swelled state below the LCST, i.e. at tem-
perature above the LCST of fibres the pulp slurry is bet-
ter drained and vice versa.
3.1. SCHL Theory and Hydration Bonding
The SCHL (structural changes in hydration layers) theory
[62] has been designed to deal with the interaction
Notice: SR – degree of pulp beating according to Schopper-Riegler (ČSN
EN ISO 5267-1) - the drainage ability of pulp slurry decreases with in-
creasing of SR; Effective beating energy consumption (kWh/kg of oven dried
pulp fibre). Beating conditions: - Laboratory ring beater; - Non-bleached
hemp pulp prepared by alkaline cooking method; - Pulp beated at 3% con-
sistency during 49 minutes at approximately constant operating beater edge
Figure 5. Temperature of pulp slurry influence upon
drainage ability of beated hemp pulp.
Copyright © 2010 SciRes. JBNB
Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems25
mechanism in hydrated hydrophilic systems. The hydra-
tion bonding concept, i.e. a formation of hydration weak
bonds or anti-bonds, follows up of the SCHL theory. The
idea of the origins and effect of hydration forces is based
on typical dipole character of water molecules and on
their two possible basic orientations in hydration spheres
(called as immobilised or vicinal water) around the hy-
drophilic sub-micro domains [52-57] depending upon
their nature. The possible orientations of water molecules
with regard to the hydrophilic domains forming hydro-
philic phase interface vary essentially between the fol-
lowing extreme positions (see Figure 6):
orientation with the H-atoms of water molecules to
the submicro domain with proton acceptor activities,
orientation with the O-atoms of water molecules to
the submicro domain with proton donor activities.
Owing to this orientations of water molecules, an in-
termolecular field of force produced by hydrogen bonds
formed among them will then spread by means of the
other molecules through the hydration sphere under the
influence of this orientation of water molecules, becom-
ing more and more diffused until it equals the zero value
in bulk of water. This effect serves as origin of the force
action between interacting sub-micro domains of phase
interfaces, i.e. the hydration forces. If the orientation of
water molecules is equal to each of the interacting do-
mains, the two sub-micro domains will affect each other
with repulsive hydration forces, i.e. the hydration de-
bonding system prevails. In the opposite case, when the
orientation of water molecules to each of the sub-micro
domains is different, the interacting surface domains will
affect each other with attractive forces, i.e. creation the
hydration bonding system. According to this theory, the
groups forming hydrogen bonds with water followed by
hydration bond formation can be divided into three types:
1) H-donor groups and molecules: such as primary al-
coholic OH-groups, secondary amino groups and primary
amino groups.
2) Amphoteric groups and molecules: such as H2O,
secondary alcoholic OH-groups in polysaccharides, and
partially primary amino groups, amido groups etc.
3) H-acceptor groups: such as hemiacetal oxygen in
saccharides, carbonyl groups, and tertiary amino groups.
Interestingly, under the same conditions, the repulsive
forces are effective over a greater distance and the effect
of attractive forces prevails on short distances (approxi-
mately smaller than 4 nm) but at the shortest distances
the attractive forces are stronger than repulsive forces.
This difference appears in the interactions of hetero-
geneous mosaic surfaces [62] containing sub-micro do-
mains in which repulsive and attractive hydration forces
act simultaneously as a kind of equilibrium established in
which the two interacting surfaces reach a definite
optimum distance from each other – see Figure 7. In the
interaction, mutual diffusion of their hydration spheres
takes place, connected with a change of their structure.
The effects of hydration forces decrease with a tempera-
ture increase and practically disappear at boiling point of
water. The structural changes take place on the molecular
level, being accompanied by appropriate heat effects
[51-53,62]. Theoretically [62] it has been shown and
confirmed experimentally [51] that the action of attractive
forces is an exothermic process connected with decreasing
of entropy while the action of repulsive hydration forces
(i.e. under influence of external forces) has an endother-
mic character connected with increasing of entropy.
|(O)||(O)|FF o
180|(O) ||(O) |FF
 o
Figure 6. Conception [62] of the orientation of water mole-
cules around various types of sub-micro domains at phase
interface. o
α - average axis angle of water molecules re-
lated to the phase interface normal at its close vicinity; F(0)
– potential energy of water molecule in d (distance) = 0; A –
proton acceptor group, D – proton donor group.
Figure 7. The course and dependence [51] of isopotentials
on the distance between interacting domains of heteroge-
neous surfaces – the creation of hydration bond system. >; <
- depiction of the prevailing orientation of water molecules
in the hydration layers; d – distance from the phase bound-
ary; Φo (relative potential of water molecule in d = 0) = F1(0)
/ F2(0), F2(0) > F1(0).
Copyright © 2010 SciRes. JBNB
Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems
Moreover, the SCHL theory predicts and experiments
support the fact that density of immobilised water is
higher than the density of bulk water in dependence on
its vicinity to the hydrophilic domain interface. With
increasing distance from the domain interface the density
of water decreases. The difference [63] between vicinal
and bulk water is changed in the range of 2 to 40 %. Be-
side of this, an important implication is possible to derive
that density of water, comparatively to the bulk of water,
located between interacting hydrophilic surfaces is
higher in the case of attractive hydration forces and lower
in the case of repulsive ones.
3.2. Biomimetic Systems Especially Artificial
Recently, discovery of cellulose as a smart material was
described that can be used for biomimetic sensor/actuator
devices and micro-electromechanical systems [25]. This
smart cellulose is termed electroactive paper (EAPap)
because it can produce a large bending displacement with
low actuation voltage and low power consumption. The
authors in [25] are proposed that electroactive paper is
advantageous for many applications such as micro-insect
robots, micro-flying objects, micro-electromechanical
systems, biosensors, and flexible electrical displays. By
use of this phenomenon it is possible also to explain and
simulate muscles movement.
EAPap is made with a cellulose film (cellophane) on
which gold electrodes are deposited on both sides. An
EAPap actuator was supported vertically in environment
chamber that can be controlled the humidity and tem-
perature. By excitation of voltage application to the ac-
tuator a bending deformation is evoked. The authors [25]
believe that the actuation is due to a combination of two
mechanisms: ion migration (diffusion of sodium ions to
anode?) and dipolar orientation. Again, in spite of their
confusing and irrational explanation of the EAPap
movement the received results have high inspiring poten-
tial and challenge. The tip displacement of the EAPap
actuator is dependent on applied electric field, its fre-
quency, EAPap sample thickness and temperature but
predominantly on humidity. The humidity affects the
displacement, where a high relative humidity leads to a
large displacement. It is no problem to explain this
behaviour by use of SCHL theory.
An orientation of water molecules in immobilized lay-
ers around cellulose macromolecules in stratified struc-
ture of EAPap actuator is determined by presence of
proton donor groups or proton acceptor groups at their
interacted surfaces. The overall film structure and its
shape are formed among structural cellulosic units due to
both the hydrogen-bonding bridging in dry state and the
hydration-bonding bridging in wet state. Extent and in-
tensity of this bonding system is determined by size,
concentration and distribution of nano-domains either
with the attractive or the repulsive force action, i.e.
among interacting opposite nano-surfaces with reversal
or identical basic orientation of water molecules, respec-
tively. The basic orientation of water molecule is given
by presence of surface proton donor groups or proton
acceptor groups of cellulose. Whilst hemiacetal and gly-
cosidic oxygen in cellulose is typical proton-acceptor
groups the hydroxyl groups can behave as proton-donor
and proton-acceptor groups. Nevertheless, one is sup-
posed that mostly behaviour of hydroxyl groups in cellu-
losic materials has more a proton-donor character.
In consequence of this preposition, the domains of pre-
vailing hydration-bonding bridging are regularly distrib-
uted within cellulosic material with flat formation. By
any disturbing this distribution, the paper strip curling is
evoked because the inner tension equilibrium is disturbed.
As schematically presented in Figure 8, by application of
oriented electric field on cellulosic material in wet state
the water molecules in bonding nano-domains contained
nearest the electrodes are reoriented. However, reorienta-
tion at cathode is different of the reorientation at anode –
at anode are reoriented only all the water molecules hav-
ing been oriented to this pole with hydrogen atoms and at
cathode only these ones having been oriented to this pole
with oxygen atoms at basic origin state. Moreover, the
distribution of attractive forces formed around both the A
and D and the D and A nano-centres is not the same – it
is supposed a prevailing A - D structure orientation in
bonding domains. At this situation, an application of dc
electric field is evoked a weaker bond system in layers
laying near anode and vice-versa a stronger bond system
in layers near cathode. Due to this effect the paper strip
gets to bend to anode. Logically, the effect is strongly
dependent upon relative humidity, the reorientation of
water molecules is independent on diffusion process and
it is relatively quickly.
Obviously, by similar effect, but in microscale, a mus-
cles movement is possible to explain. The main preposi-
tion – the non-symmetrical distribution of attractive
forces formed around both the A and D and the D and A
The enhancement of the protein folding owing to the
physical properties and microstructure of the host organic-
inorganic nanoporous silica matrix induced by the nature
of the functional groups and the siloxane network is
probably a further similar effect of the hydration forces
system activity [64].
4. Conclusions
During last decade a lot of information was collected
Copyright © 2010 SciRes. JBNB
Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems
Copyright © 2010 SciRes. JBNB
describing the behaviour of hydrated reticular systems (e.g.
hydrogels, quasi-hydrogels, pulp fibre network) with a
temperature response, i.e. the hydrogels with LCST and
the hydrogels with UCST. Particularly, the TRHRS with
LCST were studied because their biomedical importance.
It seems that the behaviour is given by specific thermo-
dynamically base conformation state of a main polymer
forming the TRHRS at room temperature, i.e.:
- squeezed state of polymer coil conformation above
LCST being stretched by an influence of inner repulsive
intermediary forces under LCST and,
- more stretched state of polymer coil conformation
above UCST being contrary compressed by an influence
of the intermediary attractive forces under UCST.
Figure 8. Schematically representation of water molecules reorientation in vicinity of electric input field inside nano-localities
of cellulosic materials.
Water—A Key Substance to Comprehension of Stimuli-Responsive Hydrated Reticular Systems
It was shown that both sorts of the intermediary forces
are the repulsive and attractive intramolecular hydration
forces theoretically explained by SCHL theory.
However, more challenging attention is evoking by
swelling or de-swelling activity of elastic crosslinked
polymers forming hydrogels with rapid thermoresponsive
dynamics, because their similarity to biomimetic dy-
namic of muscle tissues of animals. At present, the aim
to better understanding to movement of biological sys-
tems prompting the TRHRS to offer the crosslinked hy-
drogels serve as artificial objects of experimental studies.
Acceptable moveable hydrogel system, e.g. with strati-
fied structure composed of useful crosslinked both the
LCST and the UCST hydrogels, is possible to create and
to evoke its movement by changing temperature around
LCST and UCST but only in a connection with mutually
water transport. The recently described biomimetic sys-
tems not behave as a really artificial muscle but only as
the hydrogel with rapid syneresis. Obviously, a muscle
movement represented by mutual movement of different
muscle tissue like myosin and actins take place on an-
other principle than that one connected with water expul-
sion and retention. Moreover, the dynamic of real muscle
response is incomparable higher because it is not con-
nected with water transport.
As a matter of fact, actually how is it possible to real-
ise this basic bio-property?
As theoretically follows of the hydration bonding - de-
bonding concept being applied upon micro- and submit-
cro-reticular systems, the stratified muscle tissue is
squeezed by prevailing of hydrated bond formation and
stretched by prevailing of hydrated anti-bonds. The all of
changes are evoked only by merely overturning the ori-
entation of interacting water molecules in hydration lay-
ers around interacting hydrophilic interface micro-do-
mains. This overturning is relative very quickly and it is
connected with heat evolving - the formation of attrac-
tive hydration forces is prevailing - or heat consumption -
the formation of repulsive hydration forces is prevailing.
A real mechanism evoking this water molecule orienta-
tion changing by inner or outer stimuli in muscle tissue is
still not known. Probably, the evocation of amphoteric
hydroxyl or amino interface groups is responsible for this
alternating orientation.
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