REVERSIBLE THERMOSENSITIVE GLUEFOR RETINAL IMPLANTS
MURAT TUNC, MD,*† XUANHONG CHENG, MD,‡ BUDDY D. RATNER, MD,§ELLIS MENG, PHD,¶ MARK HUMAYUN, MD*Purpose: To determine in vitro effects of a plasma polymerized N-isopropyl acrylamide(pNIPAM) coating for thermally controllable adhesion to retinal tissue.
Methods: Polyimide (50 ␮m), parylene C [poly(monochloro-p-xylylene)] (20 ␮m), andpoly(dimethyl siloxane) (PDMS) (200 ␮m) coated with pNIPAM were used as implantmaterials to test retinal adhesion in enucleated pig eyes. Following preparation of theimplant materials (n ⫽ 5) and retina, the authors held the implants over the retinal tissue at22°C and gradually increased the temperature of the water bath within 15 minutes. While
increasing the temperature the authors monitored the adhesion with the retina and pNIPAM coated implant. The authors measured the adhesive force by a traction test using asuture attached to the implant and a strain gauge. Then the authors checked the reversibility of the adhesion by lowering the temperature of the water bath.
Results: There was no retinal adhesion at room temperature (22°C). The adhesion
developed strongly within 60 seconds after reaching the critical temperature (ⱖ32°C). Thisadhesion was persistent when the authors applied tractional forces of 98 mN and 148 mN
between 32 and 38°C. When the authors lowered the temperature back to 22°C by
irrigation with cold BSS, the implants detached from the retinal surface without using any
tractional force.
Conclusion: pNIPAM provides effective in vitro retinal adhesion between 32 and 38°C
and this adhesion is completely reversible by lowering the temperature of the physiologicmedium.
RETINA 27:938 –942, 2007

B

uilding reliable interfaces between biologic andengineered systems is one of the great challenges
in biomimetic applications and for drug delivery purposes. A safe and effective adhesive can be very
useful to implant a biomimetic microelectronic device
inside the eye. Several adhesives such as hydrogels,fibrin sealants, and photocurable glues have beentested in previous studies for this purpose.1–3 These

reports showed limitations in previously tested adhesives such as inflammation, toxicity, insufficient adhesive strength, irreversibility, and deformation of theocular tissue.1– 6
Polymeric systems that may modify their adhesive
properties in response to changes in the physical andchemical characteristics of the physiologic medium
are promising candidates to achieve reversible tissueadhesion. Several groups have explored the use of
dynamic stimulus-responsive surface chemistries forcell patterning.1,7–9 Thermoactive,7 electrical-active,8and photoactive1,9 chemistries have been defined forcellular adhesion. In general, all of these chemistries
operate under the same principle: these substances canbe switched from a state that prevents cellular attachment to a state that promotes it.

From *Doheny Eye Institute, ¶Biomedical Engineering, University of Southern California, Los Angeles; †Ophthalmology, Duzce
University Medical School, Turkey; and ‡Bioengineering Department and §Department of Bioengineering and Department ofChemistry, University of Washington Engineered Biomaterials,Seattle.
Supported by National Science Foundation grants EEC-9529161
and EEC-0310723. (Invention disclosure application was submitted.)
Reprint requests: Murat Tunc, MD, Inkilap Sokak 19-1, Kizilay,Ankara 06420, Turkey; e-mail: mtunc@isbank.net.tr

938

REVERSIBLE THERMOSENSITIVE GLUE ● TUNC ET AL

939

Fig. 1. Schematic diagram
demonstrating the experimentaldesign. (F ⫽ tractional force.)

In this study, we planned to demonstrate in vitro
retinal tissue adhesion properties of a thermoresponsive adhesive: polymerized N-isopropyl acrylamide(pNIPAM).
Methods
We used three types of base materials: polyimide,
parylene C [poly(monochloro-p-xylylene)], and poly(dimethyl siloxane) (PDMS) for retinal implantation.
These are inert materials that are commonly used forinsulation of the epiretinal electrode for epiretinal
stimulation. Polyimide was prepared as 50 ␮m thickfilms, parylene as 20 ␮m thick films, and PDMS wasprepared at 200 ␮m in thickness at the BiomedicalEngineering Department of the University of Southern
California.
Coating of the base materials with a polymerized
form of NIPAM was performed by plasma deposition
as described elsewhere.10 N-isopropyl acrylamide (NIPAM) (97%) was purchased from Aldrich (Milwaukee, WI) and following plasma deposition, the exposed surfaces of the implant materials (polyimide,parylene, and PDMS) were coated by pNIPAM. The
pNIPAM-grafted surfaces were rinsed three times
with cold deionized water to remove uncrosslinked
molecules before use.
Enucleated pig eyes were transferred to the laboratory in cooled oxygenated 0.01 moles/L phosphatebuffered saline (PBS, pH 7.4), and the cornea, lens,and vitreous were totally removed. Vertical relaxing
incisions were performed through the eyecup from theperiphery towards the optic disk, leaving the retina
intact at the posterior pole. The retina was stabilized
over a soft plastic sheet by pinning with four 25Gauge needles, to keep the retina flat and facing

upward over the scleral surface. This preparation wasirrigated by phosphate buffered isotonic ringer lactate
solution in a water bath.
We used a thermostatic heater-controlled water bathto adjust the temperature of the retinal tissue. We
checked the tissue temperature continuously during
the experiment. pNIPAM coated materials, polyimide,parylene, and PDMS were cut into 3 ⫻ 2 mm pieces.
Five pieces were prepared for each material. A 7/0
suture was passed through one corner of the polyimide
and parylene implants to apply tractional force (peeltest) controlled by a strain gauge mechanism to measure the adhesive force (Figure 1). As PDMS was too
fragile to apply traction using the suture method, weonly used vitreoretinal microforceps to check the retinal adhesion.
Five pre-prepared pNIPAM-coated implants foreach material and five controls without pNIPAM coating were used in our experiments. Before retinal implantation, the water bath temperature was set at 22°C.
We then gradually increased the temperature of thewater bath to 38°C within 15 minutes. During theheating period we held the implant over the retina withvitreoretinal forceps. We checked the retinal adhesion
between the pNIPAM coated materials and controls
continuously until we reached body temperature(37°C). Where adhesion was observed, we measured
the adhesive force by a strain gauge (Somfy-TecFrance) attached to the suture. Then we lowered thetemperature back to 22°C to test whether the adhesion
is reversible.
Results
At room temperature (22°C) there was no retinaladhesion in any of the pNIPAM-coated materials940

RETINA, THE JOURNAL OF RETINAL AND VITREOUS DISEASES ● 2007 ● VOLUME 27 ● NUMBER 7

Fig. 2. Temperature-related changes in adhesive properties of polymerized N-isopropyl acrylamide (pNIPAM) coated implants.

In PDMS, we observed strong adhesion between 32and 38°C and we were not able to detach the implantfrom the retinal surface with vitreoretinal forceps
without detaching the retina from the scleral bed.
In the remaining three samples we lowered thetemperature back to 22°C by irrigation with cold BSS.
When we reached temperatures below 31°C all attached implants started to detach from the retinal
surface spontaneously without using tractional force
and they detached from the retinal surface completely
within 2 minutes when we reached 22°C. This showedthat the retinal adhesion of pNIPAM coated implants
was reversible. There was no retinal adhesion in anyof the controls without pNIPAM coating between
22°C and 38°C.
Discussion(polyimide, parylene, and PDMS) and controls. Retinal adhesion developed in all pNIPAM coated test
materials (polyimide, parylene, and PDMS) when thetemperature reached 32°C. The adhesion developed
strongly within 60 seconds after reaching the criticaltemperature (ⱖ32°C). This adhesion was persistent
between 32 and 38°C (Figure 2). We performed a pulltest by a suture attached to one corner of the polyimide
and parylene implants. Stable forces of 98 mN and
148 mN were applied for 5 seconds to pull these
implants away from the retinal surface (Figure 3). The
adhesion was stable during this test for both materials.
We increased the tractional force up to 250 mN in twocases for each tested material and the retina tore ordetached following increased peeling force.

Fig. 3. Adhesion between the retina and pNIPAM coated implant
during a peel test when we increased the temperature to body temperature. Retinal adhesion continues when we increase the pulling force.

A thermosensitive reversible adhesive could have
many applications in ophthalmology such as in posterior segment surgery, implantation of biomimetic
microelectronic devices, and ocular drug delivery. Inaddition, many other sites in the body could benefit
from a reversible bioadhesive strategy for localized
drug delivery, surgical repair, or the attachment of
prosthetic devices. Previous studies showed that pNIPAM had a lower critical solution temperature of31°C in an aqueous environment.11,12 This means thatthe thermo-reversible hydrogel exhibits decreased solubility or swelling in water as the temperature isincreased, due to a phase transformation at the lower
critical solution temperature.11,12 In this manner, pNIPAM can be switched from a state that prevents cellular attachment to a state that promotes the cellularattachment, merely by changing the temperature of thesurface.11 As this pNIPAM coating thermal transition
is between room temperature and body temperature,
the sharp property change and the ability to immobilize the polymer onto a solid support should becategorized as an enabling technology that can facilitate applications that were previously difficult orimpossible.
Parylene, polyimide, and PDMS are inert materials
that are commonly used for insulation of an epiretinal
electrode for retinal stimulation. In our experiments
pNIPAM coated implants showed strong adhesion
with the retinal tissue in vitro above the transition
temperature. There is no standardized method to testin vitro adhesive power of a material with the retina
but when compared with previously tested adhesives,
the adhesive strength of pNIPAM with the retinal
tissue seems to be superior to cyanoacrylate,2 fibrin
sealants,1 and Cell-Tak1 and similar to hydrogels (succinimidyl succinate polyethylene glycol [SS-PEG]

REVERSIBLE THERMOSENSITIVE GLUE ● TUNC ET AL

and succinimidyl propionate polyethylene glycol[SPA-PEG]).1 Cell adhesion onto a material surface iscontrolled by complex combination of physicochemical interactions including hydrophobic, coulombic,and van der Waals forces between the cell membrane
and the material surface and molecular interpenetrations of macromolecules. SPA-PEG hydrogels canmake covalent links with the retina.1 pNIPAM undergoes its phase transition by changes in hydrophobicinteraction and the breaking of H-bonds. Also, spectroscopic data showed a change of the polymer backbone conformation.13,14 Thus, the adhesive force between retina and pNIPAM may be associated with thesoluble, freely coiling chains that have slightly interpenetrated into the outer molecular structure of theretina and then undergone thermally induced hydrophobic collapse (the change in backbone conformation) locking themselves within the outer zone of theretina.
The ideal adhesive for intraocular use should be
nontoxic and biocompatible. Previous reports showedthat hydrogels such as SS-PEG and styryl-polyethylene glycol (ST-PEG) were effective but short-lasting
and SS-PEG was toxic to the retina.1 Although Nisopropylacrylamide monomer is toxic to neural tissue, following plasma polymerization, the pNIPAM
molecule is no longer toxic to neural tissue and commonly used in cell cultures and tissue cultures for its
reversible cell adhesion properties.11,15,16 Previous reports showed cells attached and detached from pNIPAM coated culture dishes did not exhibit morphologic changes.11,15 pNIPAM has also been used inretinal pigment epithelial (RPE) cell cultures toprovide RPE sheets for transplantation and RPE
cells preserved their morphology with no signs oftoxicity.15 Interestingly, pNIPAM has also been used
to stop bleeding in experimental liver injuries and notoxicity has been reported.17 To our knowledge there
is no previous report on in vivo effects of pNIPAM
inside the eye. Our in vivo study is underway toexamine the long-term in vivo effects of pNIPAM onretina.
In vitro studies in porcine cadaver eyes showed thatmost of the measured force during retinal surgicalmanipulations was below 67 mN in magnitude.18 In
our study, pNIPAM coated test materials (polyimideand parylene) were resistant to tractional forces of 98
mN and 148 mN. Additionally, the adhesive force
between the retina and pNIPAM-coated PDMS wasstrong enough to resist the pulling force of a vitreoretinal forceps. In regards to an intraocular implant,
the force from any device on the retina depends on theweight of the device in air and then adjusted for thebuoyancy effects of the intraocular environment.

941

Our study showed that all pNIPAM-coated implants
can be attached and detached from the retinal surface,
simply by changing the temperature of the retinal
tissue. This reversibility theoretically may provide theability to remove an epiretinal stimulating electrodewhenever necessary simply by using a cooled BSS ininfusion, facilitating exchange with a newer design inthe future. In addition to its reversibility, pNIPAM
coated implants may also provide better apposition ofthe electrode with the retinal surface for effective
stimulation.19
Our in vitro results suggest that with patterning,
especially in conjunction with microfabrication, thermoreversible hydrogels may be helpful in implantation of nanotechnological systems inside the eye. This
thermoresponsive smart polymer may also have otherpotential applications in ophthalmic surgery and drugdelivery. Our in vivo studies will provide detailed
information for possible clinical applications of pNIPAM as a thermosensitive reversible adhesive.
Key words: biomimetics, pNIPAM, retina, thermosensitive glue.
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