Ординатура / Офтальмология / Английские материалы / Biomaterials and regenerative medicine in ophthalmology_Chirila_2010

surgery ophthalmic in repair wound for sealants Hydrogel

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16.1 Continued

ophthalmology in medicine regenerative and Biomaterials 414

16.3Hydrogel sealants

16.3.1 Amniotic membrane transplantation (AMT)

One of the exciting uses for a hydrogel sealant is to secure an amniotic membrane (AM) after its placement on the ocular surface. Amniotic membranes are used in ophthalmic surgeries/procedures when conventional treatments fail to restore the integrity of the tissue (i.e. epithelium or sclera). Specifically, the AM is used as a scaffold or temporary graft on which the tissue can regrow. It is the innermost layer of the fetal membrane, which consists of an endodermal layer of epithelia cells. This membrane is an avascular stromal matrix which: (a) inhibits blood vessel growth in bordering tissues (Koizumi et al., 2000); (b) promotes epithelialization (Tseng et al., 1997); (c) exhibits anti-inflammatory properties (Kim et al., 2000; Solomon et al., 2001); and

(d) assists in cellular migration to the treated areas (Tseng et al., 1997); therefore, this technique minimizes scar tissue, making it very appealing for ophthalmic applications. Unfortunately, to secure this graft, the surgeon usually uses 10-0 nylon sutures, which inflict new trauma and unwanted scarring (Azuara-Blanco et al., 1999). In order to overcome this problem,

Takaoka and coworkers evaluated a sutureless AMT using either fibrin glue

(Sekiyama et al., 2007) or, even more recently, a Schiff base hydrogel sealant (Takaoka et al., 2008). Fibrin glues are a two-component adhesive made from fibrinogen and thrombin delivered by a two-barrel syringe bearing a ‘needle’ mixing chamber. The thrombin converts the fibrinogen to fibrin by enzymatic reaction which can be controlled by the amount of thrombin added. Fibrin glues have been used for more than 20 years in Europe and have been approved since 1998 in the United States for cardiac surgeries but not for ophthalmic use, so these applications are considered off-label use. Even though good results were achieved using fibrin glue to seal the transplant, this technique has three main drawbacks. First, the preparation of the fibrin glue is difficult and time consuming. Second, since the fibrin glue has two main components, fibrinogen and thrombin directly extracted from blood plasma, some viruses, such as parvovirus B19 (HPV B19) which is particularly difficult to remove or inactivate, can lead to infection (Hino et al., 2000; Kawamura et al., 2002). Third, fibrin glue contains fibrinogen, which is well known to have a fundamental role in the process of inflammation that slows down the healing process (Tang, 1998; Forsyth et al., 2001; Hu et al., 2001; Rubel et al., 2001). In order to overcome these drawbacks, the same group also studied a Schiff base hydrogel made from ε-poly(l-lysine) and aldehyde-derivatized polysaccharide to seal the AMT to sclera tissue as shown in Fig. 16.2 (Takaoka et al., 2008). For ease of application, they used a double-barrel syringe with a mixing chamber needle for this study. One barrel contained a 14 wt% solution of aldehyde Dextran (0.43 aldehyde per sugar unit, molecular weight (MW) 75 kDa) and the other barrel contained

418 Biomaterials and regenerative medicine in ophthalmology

and generation 1 and 2 of PEGand lysine-based dendrimers, was used. By screening these different formulations of dendrimers, they found the lysine first-generation (G1) dendrimer to be the best candidate in terms of rheological and swelling properties to seal a sclera incision. This hydrogel sealant was transparent, easy and quick to set up; furthermore, it was strong and elastic, and had only moderate swelling. Using 5–10 μL of this formulation, they were able to seal a 1.4 mm full-thickness sclerotomy wound to an IOP of up to 250 mmHg, while the control groups – those left untreated and those using sutures – were only able to hold to 6 and 140 mmHg, respectively. This research also demonstrated the ability of the synthetic hydrogel sealant to be tuned in order to match the clinical requirement for a scleral sealant.

16.3.3 Corneal wounds

Corneal laceration

In the United States, approximately 50 000 eye laceration surgeries are performed each year. If the corneal wound is left untreated, it could result in permanent damage of the visual acuity over time caused by a decrease of the IOP that changes the curvature of the lens. Current repair strategies are insufficient for inducing normal tissue regeneration and complete visual recovery. The current clinical standard of care is suture fixation, which can cause trauma to the wound site, astigmatism, and infection. Cyanoacrylate glues have also been used off-label, but they are opaque and cytotoxic, as well as abrasive. In order to overcome these drawbacks, the use of a hydrogel adhesive is very appealing, since it can be made to be soft, non-toxic, transparent, and elastic enough to prevent astigmatism. The first report of a hydrogel sealant for cornea wound repair was published by the Grinstaff group (Miki et al., 2002). Specifically, the authors developed and evaluated a new hydrogel formed from modified hyaluronic acid (HA). Their idea was to use this biological macromolecule present in the eye, as it may likely afford a biocompatible hydrogel material. They modified HA with methacrylate groups that can be photopolymerized by exposure to the radiation emitted by an argon laser (514 nm, 200 mW) to afford a crosslinked HA hydrogel. The new hydrogel sealant was tested in vivo on a rabbit model by performing either a 3 mm linear laceration or a 3 mm stellate laceration at the center of the cornea, and the results were compared with those treated with standard sutures. In a follow-up examination of the rabbit at 28 days, the authors used a slit-lamp (Fig. 16.4) to detect inflammation or infection, a Seidel test to check for wound leakage, and a Schiotz tonometer to measure the IOP at different time intervals (6 hours, and 1, 4, 7, 14, 21, and 28 days). At day 28, they euthanized the animal and performed histology on the corneas to check for inflammation and the quality of epithelial, stromal, and endothelial healing.

16.4 Clinical appearance over time of a stellate laceration with epithelial removal in the eye of a representative rabbit that was treated with the laser-activated HA– methyl acrylate (MA) polymer.

(a) Slit-lamp photograph with retroillumination 1 day after the experimental laceration was sealed. The anterior chamber had reformed and the sealant filled the laceration which was still a gaping wound. (b) Seidel test showing that the sealant was still present, the laceration was closed, and no leakage was detected. Pictures extracted from Miki et al. (2002) and displayed with the publisher’s authorization.

16.5 IOP from day 1 to day 28 following application and photopolymerization of HA–MA on experimental linear and stellate corneal lacerations in rabbit eyes. Linear lacerations, N = 8, black bar. Stellate lacerations, N = 14, white bar. Data for eyes with and without epithelial removal were combined since the results were not significantly different. Data extracted from Miki et al. (2002) and displayed with the publisher’s authorization.

Almost all (37/38) of the rabbits had their IOP restored by day 7 (Fig. 16.5) and 97% (37/38) of the rabbit corneas healed with no signs of inflammation or infection. Additionally, the integrity of the cornea was re-established by stroma cell proliferation, which created new connective tissue.

5 min). Finally, since it is elastic, the sealant should lead to a reduced risk of astigmatism.

Fibrin glue in conjunction with an AMT procedure has been used to treat three patients with a 2 mm central corneal laceration resulting from an ulcer (Duchesne et al., 2001). The lacerations were secured with the fibrin glue and an AMT was placed on top of them to prevent failure that might result from the mechanical rubbing on the plug owing to the movements of the eyelids. Using this combination of treatment, they were able to seal these lacerations successfully but unfortunately they had to use a bandage contact lens for 3 weeks after surgery, which is not the ideal outcome for hydrogel sealant.

In 2003, Kalayci and coworkers reported the use of a modified, commercially available hydrogel (Confluent Surgical Inc.) to seal corneal wounds in an ex vivo rabbit model (Kalayci et al., 2003). Central corneal lacerations from 1 to 5 mm wide were used. This hydrogel was made from two different PEG solutions, one containing a PEG bearing succinimidyl ester groups and the other amine functions. The hydrogel was applied to the wound using a spray system with only relative success, since the reproducibility of the coating was very difficult to control. Too much spraying led to a thick and bumpy layer, whereas too little spraying led to a thin coating unable to hold high

IOPs. In spite of the difficulty of spraying the hydrogel, they were still able to get better results on all laceration sizes than those of the control group, which used one 10-0 nylon suture. This result was very promising, for they were able to secure (well above normal IOP) lacerations as wide as 5 mm with a non-toxic, soft, transparent, and elastic adhesive sealant that auto-set quickly without heat generation.

A BSA-based adhesive hydrogel was recently reported (Khadem et al., 2004), which can be activated using the radiation emitted by a diode laser (665 nm, 300 mW, 2 min) instead of that from an argon ion laser. Two different photochemical initiators, chlorine e6 and Janus Green, were used to covalently bond the collagen surfaces together. The first was a BSA bearing chlorine e6 (BSA-ce6) and the second was a BSA bearing Janus Green (BSA-JG). Both hydrogel sealants were tested (2 wt%) in vivo on a 6 mm linear central corneal laceration in rats. The animals were assessed up to 14 days, and before they were killed the IOP was measured and samples were sent for histology. The hydrogel was completely gone at day 14 (and was only occasionally present at day 7), and satisfactory healing results

(from the histology) with only minimal inflammation and hyperemia were obtained. The sealed lacerations were able to withstand IOP well above normal. The advantageous properties of hydrogel sealants combined with the promising ex vivo and in vivo results in corneal wound repair will lead to the development of a hydrogel sealant for ophthalmic application in the clinic.