Environmentally responsive hydrogels with dynamically tunable properties as extracellular matrix mimetic

DOI 10.1515/revce-2012-0022 Rev Chem Eng 2013; aop

Shani Eliyahu-Gross and Ronit Bitton *

Environmentally responsive hydrogels with dynamically tunable properties as extracellular matrix mimetic Abstract: Hydrogels are hydrophilic polymers with three-

dimensional cross-linked structure that swell in aqueous

solutions without dissolving in them. Environmentally

responsive hydrogels have the ability to change their con-

formation when a specific stimulus such as temperature,

pH, light or enzymes is applied. Such hydrogels have been

interesting for biomedical uses as they can deform in situ

under physiological conditions and provide the advan-

tage of convenient administration. Over the past decade,

hydrogels have been studied as materials for the develop-

ment of artificial extracellular matrices (ECMs). Recently

the ability to utilize external stimuli to mimic the dynamic

character of natural ECMs has been explored. The scope of

this paper is to review the recent developments in stimuli-

responsive hydrogels whose properties can be tuned on

the time and length scales of cell development.

Keywords: ECM; hydrogel; stimuli-responsive.

*Corresponding author: Ronit Bitton, Department of Chemical

Engineering, Ben-Gurion University of the Negev, P.O. Box 653,

Beer Sheva 84105, Israel, e-mail: [email protected]

Shani Eliyahu-Gross: Department of Chemical Engineering,

Ben-Gurion University of the Negev, P.O. Box 653, Beer Sheva

84105, Israel

Introduction The extracellular matrix (ECM) is a heterogeneous compo-

sition of proteoglycans, proteins and signaling molecules

that was originally known for its role in providing struc-

tural support to cells and as a milieu for cell migration

(Figure 1 ). Recent investigations of the ECM have clarified

its role beyond an inert background to an active com-

ponent in cell signaling (Cukierman et al. 2001 , Berrier

and Yamada 2007 ). The ECM influences cell differentia-

tion, proliferation, survival and migration through both

biochemical interactions (cell adhesion, presentation of

growth factors) (Cukierman et al. 2001 , Lutolf and Hubbell

2005 , Berrier and Yamada 2007 ) and mechanical cues

(stiffness, deformability) (Gieni and Hendzel 2008 ).

Among various tissue engineering scaffolds that have

been investigated, hydrogel-hydrophilic polymers swollen

by water that are insoluble owing to physical or chemical

cross-links are particularly interesting. Their structural

similarity to the natural ECM, inherent biocompatibility,

high water content and high permeability for oxygen and

essential nutrients makes them appealing candidates for

the formation of scaffolds (Peppas 1997 , Lee and Mooney

2001 , Jia and Kiick 2009 ). A variety of hydrogel materi-

als have been utilized for tissue engineering applications

(Anderson et al. 2004 , Ladet et al. 2008) including recon-

stituted ECM components or natural proteins and car-

bohydrates (Wallace and Rosenblatt 2003 , Jia and Kiick

2009 ), self-assembling peptides (Hartgerink et al. 2001 ),

and synthetic hydrogels (Rinaudo 2008 , Hoffmann et al.

2009 , Van Vlierberghe et al. 2011 ). Current effort in the

engineering of synthetic ECM has focused on installing

molecular features (peptides, proteins and biointerac-

tive polymers) within insoluble scaffolds, either by self-

assembly or through covalent modifications of polymer or

biopolymer networks (Kopecek 2007, 2009 , Jia and Kiick

2009 , Place et al. 2009 , Collier et al. 2010 ).

Materials with properties that are static in time are

often insufficient for mimicking natural cellular environ-

ments in which temporal variations are believed to be

important in a wide variety of milieus including cell devel-

opment, differentiation and morphogenesis, the progres-

sion of diseases and even the maintenance of homeostasis

(Kim and Hayward 2012 ).

Responsive materials, defined here as materials that

exhibit an acute change in properties upon a change in

environmental conditions (Figure 2 ), have attracted much

attention over the past two decades (Ravichandran et al.

2012 ) in a variety of applications such as nanodevices,

active membranes, engineered tissues and vehicles for

drug delivery (Mano 2008 , Mohammed and Murphy 2009 ,

Stuart et al. 2010 , Tomatsu et al. 2011 ).

Hydrogels are perhaps the most common class of

responsive biomaterials used today as tissue engineer-

ing scaffolds (Mohammed and Murphy 2009 ). The most

explored stimuli-responsive hydrogel scaffolds are ones

that have the ability to turn from solution to gel when a

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2 S. Eliyahu-Gross and R. Bitton: Hydrogels with dynamic properties as ECMs

specific stimulus is applied due to their potential as drug

delivery vehicles (Traitel et al. 2008 , Fleige et al. 2012 ) and

injectable scaffolds (Mano 2008 , Motornov et al. 2010 ).

Recently, the idea of utilizing stimuli responsiveness as

a tool for elucidating the dynamics between materials ’

properties and cellular function has gained much inter-

est. In this review we will provide an overview of recent

developments of stimuli-responsive hydrogels, whose

responsiveness is designed to tune their mechanical and

chemical properties on the time and length scales of cell

development.

Stimuli-responsive materials Light, pH or temperature may act as external variable that

cause the system to undergo dramatic conformational

changes. These types of changes happen under narrow

variation of the stimuli around a tailored critical point.

Figure 1 Schematics of the ECM.

FIgure 2 Schematic representation of stimuli-responsive

hydrogels.

Although less explored, enzymes have also been utilized

as stimuli for the development of stimuli-responsive

hydrogels.

Each of these external stimuli has its own advantages

in ECM design. Temperature is probably the most intuitive

stimuli of all, as the physiological temperature of cellular

matrix is usually higher than the external ambient tem-

perature of the materials. Light is a noninvasive stimulus

applied on a system. It may affect the system components ’

mutual interactions, leading to formation changes of the

whole system. The pH may change in diverse biological

areas and may cause a system response under a specific

pH of the target point. In all biological and metabolic

processes, enzymes are present and have a leading role

at the molecular level in the chemistry of living organi-

sms. Therefore, there is a major effort to utilize them for

responsive ECM systems because they are highly selective

and work under mild conditions.

Polymer hydrogels

Photoresponsive hydrogels

Light-responsive hydrogels are very attractive, as light pro-

vides a noninvasive tool for spatial and temporal control

of specific reactions. Most photoresponsive polymers

contain light-sensitive chromophores such as azoben-

zene (Ichimura et al. 2000 ), spiropyran and nitrobenzyl.

The variety of photoresponsive groups used in hydro-

gels for biomedical applications was recently described

by Tomatsu et al. ( 2011 ). Photoresponsive hydrogels

are typically designed with a photoreactive moiety in

a polymeric network. The photochromic molecules are

sensitive to the optical signal. The chromophores in the

photoreceptor convert the photoirradiation to a chemi-

cal signal through a photoreaction. The signal is trans-

ferred to the functional part of the hydrogel and controls

its properties. In the context of dynamic hydrogels, light

responsiveness has been utilized to induce network deg-

radation or to display, hide or release biomolecules such

as the cell-adhesive RGD peptides. Luo and Shoichet

(2004a,b) used o -nitrobenzyl moieties to modify agarose

hydrogels in order to create photolabile hydrogel materi-

als for convenient photoimmobilization of biomolecules.

GRGDS peptide was immobilized in selected volumes in

the 3D hydrogel and was shown to be cell-adhesive and

to promote neurite outgrowth from primary dorsal root

ganglion neurons. Goubko et al. (2010) used 2-nitrobenzyl

as a selectively removable photocage for RGDS peptides


S. Eliyahu-Gross and R. Bitton: Hydrogels with dynamic properties as ECMs 3

immobilized on a hyaluronic acid (HA) hydrogel. The

cell nonadhesive surface was switched to compose cell

adhesive regions upon near-UV light exposure through a

pattern. The Anseth lab (Kloxin et al. 2009 ) has developed

a polyethylene glycol (PEG)-based photolytically degra-

dable hydrogel. They used o -nitrobenzylethers to form a

photodegradable cross-linker or a photoreleasable tether

to a peptide. They used the photoresponsiveness to tune

the gels ’ cross-linking density and thus their mechanical

properties in situ . Photocrosslinked gels based on multi-

vinyl macromers of PEG and polylactic acid with PEG

macromer concentrations varying from 10% to 20% were

found to exhibit moduli in the range of 60 to 500 kPa. The

photoresponsiveness was also utilized to create temporal

changes in epitope presentation, which has been shown

to promote chondrogenic differentiation of human mes-

enchymal stem cells, as well as to tune their cross-link-

ing density and thus their mechanical properties in situ

(Kloxin and Anseth 2009 , Kloxin et al. 2009, 2010a,b ).

pH-responsive hydrogels

pH-responsive hydrogels typically contain pendant

acidic and basic groups that accept or release protons

in response to change in pH (Podual et al. 2000 ). Nat-

ural-based polymers such as chitosan as well as syn-

thetic weak polyelectrolytes such as poly(acrylic acid)

(PAA), poly(metacrylic acid) (PMAA) and poly(diethyla

minoethylmethacrylate) (PDEAEMA) have been exten-

sively studied in the development of pH-responsive

gels (Mano 2008 ). Weak polyelectrolytes, however,

can only swell or shrink, limiting responsiveness to

these two scenarios. Another strategy for incorporat-

ing pH responsiveness into hydrogels is to exploit

hydrolyzable bonds. Several functional groups exist

that remain stable at physiological pH yet hydrolyze

quickly at pH 5, including hydrazones, orthoesters and

acetals. Hydrazones are the most commonly employed

functional group of this class (Kalia and Raines 2008 ).

Because pH differences exist or may occur in the body

[i.e., tumor tissue is extracellularly acidic (Vaupel et al.

1989 , Rofstad et al. 2006) ], the main application ion of

pH-responsive hydrogels has been drug delivery. In the

field of scaffolds, pH responsiveness has been mainly

used for incorporation and delivery of bioactive agents

(Prabaharan and Mano 2006 , Mano 2008 ). Gau et al.

used a chitosan scaffold for the delivery of plasmid

DNA encoding a growth factor; a better biological per-

formance could be observed compared with the corre-

sponding matrix without the plasmid.

pH responsiveness has also been utilized to dynami-

cally control the mechanical properties of ECM mimetics.

Yoshikawa et al. studied thin hydrogel films composed of

ABA triblock copolymer, where A is pH-sensitive poly(2-

(diisopropylamino)ethylmethacrylate) (PDPA) and B

is biocompatible poly(2-(methacryloyloxy)-ethyl phos-

phorylcholine) (PMPC)]. The modulus of these hydrogel

films was shown to be reversibly modulated by 30-fold

via only a modest change in pH from 7.0 to 8.0 (Yoshikawa

et al. 2011 ).

Thermoresponsive hydrogels

Hydrogels containing polymers, such as poly( N -isopro-

pylacrylamide) (PNIPAAm) (Tanaka 1978 , Hirotsu et al.

1987 , Beltran et al. 1991 ) methyl cellulose (Takahashi

et al. 2001 , Li et al. 2002 ), pluronics (Alexandridis and

Hatton 1995 ), Tetronics (Cho et al. 2012 ), and N -vinyl cap-

rolactam (Makhaeva et al. 1998 ), are characterized by

the temperature-dependent transition from a solution

to a gel, which is commonly referred to as sol-gel transi-

tion (Peppas et al. 2000 , Qiu and Park 2001 , Klouda and

Mikos 2008 ). Some hydrogels exhibit a separation from

solution and solidification above a certain temperature.

This threshold is defined as the lower critical solution

temperature (LCST) (Schild 1992 , Aoki et al. 1994 , Shibay-

ama et al. 1996 , Mikheeva et al. 1997 , Makhaeva et al. 1998 ,

Idziak et al. 1999 , Van Durme et al. 2004 ). Below the LCST,

the polymers are soluble. Above the LCST, they become

increasingly hydrophobic and insoluble, leading to gel

formation. Thermoresponsive phase transition has been

utilized for potential tissue regeneration because gelation

can be realized simply as the temperature increases above

the LCST, which is designed to be below body tempera-

ture. In contrast, hydrogels that are formed upon cooling

of a polymer solution have an upper critical solution tem-

perature (UCST) (Lee et al. 2001 , Schmaljohann 2006 ).

Many studies of these materials as potential ECMs

have been performed. In recent studies, materials with

such typical transition are used as key elements in

complex multicomponent systems for ECM utilization.

HA hydrogels are widely pursued as tissue regenera-

tive and drug delivery materials because of their excellent

biocompatibility and biodegradability. Lee et al. (2010)

developed a thermoresponsive injectable and tissue-

adhesive material based on HA/Pluronic F127 composite

hydrogels. HA conjugated with dopamine (HA-DN) was

mixed with thiol end-capped Pluronic F127 copolymer

(Plu-SH) to produce a lightly cross-linked HA/Pluronic

composite gel structure (Figure 3 ). The sol-gel transition



temperatures of HA/Pluronic hydrogels were shown to

decrease with increasing concentration of Plu-SH. More-

over, it was shown that HA/Pluronic composite hydrogels

with different gel strengths could be readily fabricated by

controlling the extent of cross-linking between HA and

Pluronic F127 via catechol-thiol reaction. As the gelation

kinetic rates above the LCST were very fast, these compo-

sites have a suitable profile for injectable materials. When

the HA/Pluronic hydrogel was heated from 5 ° C to 37 ° C,

the elastic modulus value instantaneously increased, sur-

passing the viscous modulus value within 5 s.

Fu et al. (2012) report on tricomponent biomimetic

hydrogel composite for bone regeneration. The mate-

rial was composed of triblock poly(ethylene glycol)-poly

( ε -caprolactone)-poly(ethylene glycol) copolymer (PECE),

collagen and nano-hydroxyapatite (n-HA), and its inject-

able properties and thermoresponsibility were investi-

gated (Fu et al. 2012 ). The inner microstructure exhibits

an interconnected porous structure with sizes of several

micrometers to ca. 40 mm. It was found that the three-

component composite had characteristic thermoproper-

ties similar to that of the PECE hydrogel. At temperatures

lower than 30 ° C, the material has viscoelastic proper-

ties with detectable G ′ and G ′ ″ . Yet, it lost its flowability

at 37 ° C, indicating a sol-gel transition. The combination

of PECE hydrogel with hydroxyapatite and collagen were

chosen to mimic the composition of natural bone. Each

component increases the ability of the material to serve

as ECM. As such, this tricomponent system can gather the

osteoconductivity of HA, bioabsorbability of collagen and

thermal sensitivity of PECE copolymer together. Evalua-

tion of in vivo bone regeneration performance was carried

out in calvarial defects of New Zealand White rabbits,

which showed good capacity for guided bone regenera-

tion compared to a self-repair process. This might be due

Plu-SH+

HA-DN

RT

Flow (Injectable)

37°C

No flow(Hydrogel)

Figure 3 Schematic representation of the bulding blocks and

structural changes of temperature response of HA/Pluronic

composite (Berrier and Yamada 2007 ).

to the support of the collagen scaffold for new bone tissue

ingrowth and osteoconduction of HA.

Another tricomponent system was described in the

work of Chen and Cheng (Chen and Cheng 2009 ). Poly( N -

isopropylacrylamide) end-capped with a carboxyl group

(PNIPAM-COOH) was grafted to chitosan for synthesiz-

ing thermoreversible chitosan-g-poly( N -isopropylacryla-

mide) (CPN), which was further grafted with HA to form

HA-g-CPN (HA-CPN). CPN is a comb-like copolymer with

a PNIPAM side chain extending from the chitosan back-

bone. The double-grafted HA-CPN was synthesized by

grafting CPN onto the HA molecules. The LCST was not

significantly affected by polymer concentration and was

found to be between 28 ° C and 30 ° C. The addition of HA

to CPN increased LCST during gel forming and decreased

LCST during gel melting due to its hydrophilic moie-

ties such as carboxylic acid (-COOH) and hydroxyl (-OH)

groups. Moreover, the water content of hydrogel equili-

brated at -37 ° C was dependent on the concentration

and composition of the hydrogels. Owing to the effect of

volume repulsion between polymer molecules and water

molecules, more concentrated polymer solution resulted

in hydrogels with lower water contents. Owing to the con-

tribution of chitosan and HA components, known for high

water sorption and water retention, in the polymer hydro-

gels the CPN and HA-CPN hydrogels contain significantly

more water than the PNIPAM-COOH hydrogel. The addi-

tional grafting of HA to CPN led to reduced mechanical

strength but provides a polymer solution with better flow

property as an injectable cell carrier and alleviates limita-

tion on cell proliferation.

Lee et al. (2009) formed spherically symmetric aggre-

gates of mesenchymal stem cells (MC) with a relatively

uniform size in the MC hydrogel system. An aqueous MC

blended with phosphate-buffered saline (PBS) was used

to prepare a thermoresponsive hydrogel system. PBS, a

buffer solution, has many uses in biochemistry because

it is isotonic and nontoxic to cells. This aqueous MC

underwent a sol-gel reversible transition upon heating

or cooling at approximately 32 ° C. Blending of salts in

an aqueous MC lowers its gelation temperature. The

salts blended usually have a greater affinity for water

molecules than polymers, resulting in the removal of

water of hydration from polymers and thus dehydrat-

ing or “ salting out ” the polymeric molecules. Thus, the

surface of the MC hydrogel in the hydrated state is con-

siderably hydrophilic than in the dried state, making it

nonadherent to cells. As a result, cells self-assembled

to uniform spherical symmetric aggregates, which are

crucial for a better control of cell delivery via intramu-

scular injection.



Enzyme-responsive hydrogels

Enzyme-responsive materials are designed to go through

macroscopic transition due to catalytic activities of selec-

tive enzymes. These systems usually have enzyme-reac-

tive components either along the polymeric backbone

or as a side group. Therefore, two main classes can be

defined for enzyme-responsive polymeric hydrogels: (i)

cross-linked hydrogels, which are composed (synthe-

sized) or decomposed (hydrolyzed) by enzymes causing

a sol-gel or gel-sol transition, respectively; (ii) hydrogels

with side groups that are enzyme sensitive. Thus, specific

enzymatic reactions have been used for network forma-

tion, network degradation and the release of tethered

biomolecules.

Transglutaminases, tyrosinase and horseradish per-

oxidases are enzymes known for cross-linking catalyza-

tion of a diverse range of materials such as elastin, PEG

and gelatin (Hu et al. 2012 ). They are the most-studied

enzyme systems involved in network formation for tissue

engineering; their cross-linking mechanisms and exam-

ples of current applications were recently reviewed by

Teixeira et al. (2012) .

Recently, a hydrogel constructed from two classes

of recombinant protein polymers that can be enzymati-

cally cross-linked was developed. By evenly spacing the

enzymes at specific sites and incorporation of elastin

sequencing along the protein backbone, a modular hydro-

gel with tunable characteristics was obtained (Davis et al.

2010 ). A similar synthetic system has not been reported

yet.

In the case of network degradation, matrix metal-

loproteinases (MMPs) are probably the most studied in

synthetic polymer hydrogels. By incorporating two pep-

tides, one that is cleaved by a specific enzyme and one

that induces cell adhesion (i.e., RGD), into the hydrogel

backbone, West and Hubbell created a hydrogel that can

be infiltrated and remodeled by migrating cells (West and

Hubbell 1999 ). Lutolf et al. showed that incorporation of

peptides that are cleavable by MMPs in a PEG network

can result in local degradation by MMPs secreted by cells

(Lutolf et al. 2003 ). This concept has been implemented

in other polymeric systems that have been described in

several review papers (Ulijn 2006 , Cabane et al. 2008 ,

Mano 2008 , Hu et al. 2012 , Ravichandran et al. 2012 ).

Another approach is to use polysaccharides (e.g., dextran,

chitosan, heparin, alginate and HA), which are biodegrad-

able, as building blocks for enzyme-degradable hydrogels

(Kumashiro et al. 2004 ).

In an attempt to mimic the remodeling capability

of the biological ECM, hydrogels in which cross-linking

is achieved by one type of stimulus and degradation by

another have been explored. An example for such a system

was synthesized by Kim and Healy; hydrogels copolymer-

ized from PNIPAM, AA and collagenase-cleavable peptide

cross-linkers, thus creating a temperature-responsive gel

formation and enzyme-responsive gel degradation (Kim

and Healy 2003 ).

The combination of light and enzyme responsiveness

was used in an attempt to mimic the ECM spatial control

of chemical cues. Deforest et al. synthesized four-arm PEG

hydrogels with enzymatically cleavable peptide sequences

and photoresponsive groups for the conjunction of bio-

molecules at specific locations (DeForest et al. 2009 ).

Peptide-based hydrogels Apart from conventional polymer hydrogels, over the past

decade the use of low molecular hydrogels as scaffolds

for cell growth has been studied. The modular nature of

amino acids as building blocks and their versatile inter-

molecular interactions open the path to the design of

external stimulus-responsive systems. Peptides are driven

to self-assemble by noncovalent interactions such as

hydrogen bonding, hydrophobic interactions, π - π stack-

ing and Van der Waals forces. As a result, secondary struc-

tures such as α -helices, β -sheets and β -turns may form in

the nanoscale as controllable 3D networks. Peptides offer

the widest variety of functionality and cell signaling capa-

city with rapid and facile synthesis of complex molecules

having inherent biocompatibility and biodegradability

properties, which give an advantage to such systems (Cui

et al. 2010 ). A comprehensive description of the structural

changes stimuli-responsive peptides undergo upon expo-

sure to stimulus is beyond the scope of this manuscript

and can be found in several review papers (Mart et al.

2006 , Lowik et al. 2010 ).

As in the case of photoresponsive polymers, light-sen-

sitive synthetic peptides contain a strategically positioned

chromophore that can change their conformation upon

absorption of light. In pH-responsive peptides, the confor-

mation change is often a consequence of protonation or

deprotonation of basic and acidic amino acids.

Thermoresponsive peptide hydrogels, however, are

typically composed of elastin-based peptides (EBPs).

Elastin is one of the most abundant ECM proteins and is

responsible for the elasticity in skin, blood vessels and

the lungs, among other organs. Its structure is largely

composed of Xaa-Pro-Gly-Yaa-Gly pentapeptide repeats,

where Xaa and Yaa represent a variety of amino acids



in natural elastin (Foster et al. 1973 ). EBPs exhibit LCST

behavior (Reiersen et al. 1998 , Pechar et al. 2007 ). Enzyme-

responsive peptides are typically defined as peptides

whose structure changes upon exposure to enzymes. For

example, incorporation of a phosphorylated serine within

the sequence of a peptide amphiphile (PA) was shown

to control its self-assembly (Webber et al. 2011 ). Here we

focus on peptides that contain an enzyme-site-specific

sequence as well as self-assembly motifs.

Self-assembling peptides have been extensively

studied as bioactive gels for cell cultures (Trabbic - Carlson

et al. 2003 , Fernandez -Trillo et al. 2007 , Conrad and

Grubbs 2009 , Hauser and Zhang 2010 , Nagarkar et al.

2010 , Matson et al. 2011 , Kim et al. 2012 ).

Due to the nature of these supermolecular scaffolds,

external stimulus is often used to trigger the self-assem-

bly process or partially disassemble them (Lowik et al.

2010 ). Thus, the dynamic behavior of such gels is limited

to degradation and release of proteins or small molecules

(Cardin and Weintraub 1989 , Rajangam et al. 2006 , Stend-

ahl et al. 2008 , Branco et al. 2010 , Gelain et al. 2010 , Chow

et al. 2011 ).

As in the synthetic polymer hydrogels, enzymatic

degradation and enzymatic cross-linking are utilized to

mimic the remodeling capability of the ECM. One of the

challenges in doing so is to not alter the self-assembly of

the peptide.

Giano et al. (2011) incorporated degrading peptides

with MMP-13-specific cleavage sites into a β -hairpin

peptide. This self-assembling peptide forms a gel under

physiological conditions. In the presence of enzymes the

gels demonstrated tunable degradation profiles depend-

ing on the primary sequence of the β -hairpin peptide,

thus creating a peptide hydrogel with dynamic mechani-

cal properties.

The Hartgerink lab reported the incorporation of an

enzyme-cleavable site in the head group sequence of a

PA molecule with tunable properties. A sequence spe-

cific to MMP-2 was incorporated into an RGD containing

PA, creating a cell-adhesive, enzyme-cleavable hydro-

gel made from self-assembling peptide (Jun et al. 2005 ,

2008 , Galler et al. 2010 , 2012 ). They also reported the

enzymatic cross-linking of a multidomain PA; the lysine

residues were cross-linked by lysyl oxidase, leading to

an increase of the gel ’ s storage modulus over a period

of 2 weeks without noticeable changes in the hydrogel

nanostructure (Bakota et al. 2011 ).

The possibility of gaining a dynamic behavior while

maintaining the structural integrity of a peptide hydrogel

was recently explored by the Stupp laboratory. Matson

and Stupp (2011) and Matson et al. (2012) reported

incorporation of hydrozones (pH-responsive molecules)

in PAs as a linker from small molecules. Incorporation of

the hydrozones was shown not to affect the self-assembly

of the PAs (Matson and Stupp 2011 , Matson et al. 2012 ),

while releasing the small molecule in a controlled manner

(Webber et al. 2012 ). Taking this concept a step further

toward making dynamic materials as synthetic ECMs,

Sur et al. (2012) included a photocleavable nitrobenzyl

ester group on a PA that self-assembles into cylindri-

cal nanofibers. The PA was designed in such a way that

upon exposure to light the RGD cell adhesion epitope is

removed. An arrest in fibroblast spreading was observed

when the culture was exposed to light, demonstrating a

dynamic shift in cell response.

Outlook In this focused review we have described the use of

stimuli-responsive materials for the creation of new bio-

materials that will better mimic the dynamic nature of the

natural ECM. In particular, we highlighted recent devel-

opments of light-, pH- and temperature-sensitive hydro-

gels. These responsive materials capable of acute changes

in properties upon exposure to the designed stimuli have

shown great promise as tools for studying dynamic cell-

material interactions. However, the road from the current

state of the field to achieving the goal of creating an arti-

ficial ECM that will truly mimic the natural one is a long

and challenging one.

A few of the obstacles that need to be addressed are

as follows: (i) Whereas the existing synthetic materi-

als allow a controlled change of only one parameter, in

natural systems several parameters can be tuned simul-

taneously but independently. (ii) The systems described

above generally require a change to a large portion of the

polymer to effect a macroscopic change in the hydrogel,

unlike natural responsive materials (e.g., muscles) where

small structural changes are multiplied over several

length scales to generate large macroscopic changes. (iii)

Hydrogels resemble the 3D network of the ECM; however,

they lack the hierarchical organization of tissues such as

cartilage or bone.

Exploring new stimulus that can be applied to bioma-

terials, such as electric or magnetic fields and incorpora-

tion of several different stimuli-responsive groups in one

hydrogel, are two approaches that are being implemented

in the development of hydrogels whose mechanical and

chemical properties can be tuned on the time and length

scales of cell development.



Although the number of papers on such materials is

still relatively small, we believe the hydrogels with tem-

poral control of chemical and mechanical properties are

the next generation of scaffolds for tissue engineering and

regenerative medicine.

Acknowledgements: R.B. gratefully acknowledges the

support of The Joseph and May Winston Foundation

Career Development Chair in Chemical Engineering.

Received December 12, 2012; accepted February 27, 2013

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Shani Eliyahu-Gross received her MSc in Chemistry from the

Weizmann Institute of Science at 2006, focusing on nanotechnol-

ogy research. She received her PhD from Ben-Gurion University at

2011 under the guidance of Rachel Yerushalmi-Rozen, specializing

in ultrathin polymeric-nanoparticle systems at soft interfaces. Shani

is currently a postdoctoral scholar in the group of Ronit Bitton at

Ben-Gurion University. Her research interests include hydrogels,

drug delivery, dynamic self-assembling, and the development of

new materials for regenerative medicine.

Ronit Bitton received her BSc in Chemical Engineering in 1997 from

the Technion-Israel Institute of Technology. She obtained her MSc

(2003) and her PhD (2007) in Biotechnology also from the Technion.

After postdoctoral work in the group of Sam Stupp at Northwestern

University she joined the department of Chemical Engineering at

the Ben-Gurion University of the Negev. She is a member of the Ilse

Katz Institute for Nanoscale Science and Technology. Her research

focuses on experimental investigation of hierarchical (nano-,

micro-) structure and properties of complex materials of interest in

regenerative medicine. Materials of interest include hydrogels from

polysaccharides and dynamic self-assembling peptides.


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Environmentally responsive hydrogels with dynamically tunable properties as extracellular matrix mimetic