Review Article Adhesive and sealant interfaces for general surgery applications Francesca Scognamiglio, 1 Andrea Travan, 1 Isabella Rustighi, 1 Paola Tarchi, 2 Silvia Palmisano, 2 Eleonora Marsich, 2 Massimiliano Borgogna, 1 Ivan Donati, 1 Nicol o de Manzini, 2 Sergio Paoletti 1 1 Department of Life Sciences, University of Trieste, Italy 2 Department of Medical, Surgical and Health Sciences, Internal Medicine Clinic, University of Trieste, Italy Received 25 October 2014; revised 15 January 2015; accepted 26 February 2015 Published online 00 Month 2015 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.33409 Abstract: The main functions of biological adhesives and sealants are to repair injured tissues, reinforce surgical wounds, or even replace common suturing techniques. In general surgery, adhesives must match several requirements taking into account clinical needs, biological effects, and material features; these requirements can be fulfilled by spe- cific polymers. Natural or synthetic polymeric materials can be employed to generate three-dimensional networks that physically or chemically bind to the target tissues and act as hemostats, sealants, or adhesives. Among them, fibrin, gela- tin, dextran, chitosan, cyanoacrylates, polyethylene glycol, and polyurethanes are the most important components of these interfaces; various aspects regarding their adhesion mechanisms, mechanical performance, and resistance to body fluids should be taken into account to choose the most suitable formulation for the target application. This review aims to describe the main adhesives and sealant materials for general surgery applications developed in the past deca- des and to highlight the most important aspects for the development of future formulations. V C 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 00B: 000–000, 2015. Key Words: adhesion, biomimetic, interface(s), polymer, tissue adhesion How to cite this article: Scognamiglio F, Travan A, Rustighi I, Tarchi P, Palmisano S, Marsich E, Borgogna M, Donati I, de Manzini N, Paoletti S. 2015. Adhesive and sealant interfaces for general surgery applications. J Biomed Mater Res Part B 2015:00B:000–000. INTRODUCTION Despite sutures are considered a mainstay for several treat- ments and procedures in general surgery, they also have some drawbacks mainly associated with high infection rate, extensive handling, risk of blood-borne disease transmission and tissue reactivity. 1,2 Moreover, the presence of sutures or staple materials in surgical wounds is considered to increase the risk of infections, which may retard wound healing, cause wound chronicity, and also threaten the patient’s life. 3,4 For these reasons, a general trend toward simpler, quicker, and minimally invasive surgical procedures has encouraged the development of sutureless techniques like the use of adhesive and sealant interfaces to restore soft tissue integrity and functionality. These interfaces can be successfully employed in the treatment of emergency hemostasis, 2,5 in sealing leaks of gas or fluids, 6 and in the reinforcement of sutures. 7 Hemostats work by causing blood to clot and are indicated to stop nonsuturable or noncauter- izable bleeding particularly in anticoagulated or coagulo- pathic patients; several surgical operations require a perfect hemostasis, so that the principal aim is the reduction of post-operative bleeding and leakage, especially when paren- chymal resections or vascular anastomoses are performed. The use of sealants has been widely described in liver sur- gery to reduce postoperative blood loss and bile leak, impacting both short and long-term prognosis as they are the most detrimental complications in liver surgery. 8 Spleen traumas represent another field for the application of seal- ant interfaces. Laparoscopic spleen-preserving procedures have been used for patients with hemodinamically stable splenic injuries; in these patients the topical application of sealants like fibrin glues has shown to enable good bleeding control, even in patients lacking clotting factors or platelets or taking anticlotting medications. 9 Sealants can also be used to prevent the leakage of organic fluids, including lymph cerebrospinal fluid and gastrointestinal contents. Anastomotic leakage can occur at all levels of gastrointesti- nal surgery; recent studies have shown that this risk appears to be reduced by the use of sealants. 10–12 Tissue approximation of wounds with no tension represents another field in which adhesives can be very useful 13 ; in these cases the adhesives need to be strong, water resistant Correspondence to: Marsich E; e-mail: [email protected]V C 2015 WILEY PERIODICALS, INC. 1
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Review Article
Adhesive and sealant interfaces for general surgery applications
Francesca Scognamiglio,1 Andrea Travan,1 Isabella Rustighi,1 Paola Tarchi,2 Silvia Palmisano,2
Eleonora Marsich,2 Massimiliano Borgogna,1 Ivan Donati,1 Nicol�o de Manzini,2 Sergio Paoletti1
1Department of Life Sciences, University of Trieste, Italy2Department of Medical, Surgical and Health Sciences, Internal Medicine Clinic, University of Trieste, Italy
Received 25 October 2014; revised 15 January 2015; accepted 26 February 2015
Published online 00 Month 2015 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.33409
Abstract: The main functions of biological adhesives and
sealants are to repair injured tissues, reinforce surgical
wounds, or even replace common suturing techniques. In
general surgery, adhesives must match several requirements
taking into account clinical needs, biological effects, and
material features; these requirements can be fulfilled by spe-
cific polymers. Natural or synthetic polymeric materials can
be employed to generate three-dimensional networks that
physically or chemically bind to the target tissues and act as
hemostats, sealants, or adhesives. Among them, fibrin, gela-
How to cite this article: Scognamiglio F, Travan A, Rustighi I, Tarchi P, Palmisano S, Marsich E, Borgogna M, Donati I, deManzini N, Paoletti S. 2015. Adhesive and sealant interfaces for general surgery applications. J Biomed Mater Res Part B2015:00B:000–000.
INTRODUCTION
Despite sutures are considered a mainstay for several treat-ments and procedures in general surgery, they also havesome drawbacks mainly associated with high infection rate,extensive handling, risk of blood-borne disease transmissionand tissue reactivity.1,2 Moreover, the presence of sutures orstaple materials in surgical wounds is considered toincrease the risk of infections, which may retard woundhealing, cause wound chronicity, and also threaten thepatient’s life.3,4 For these reasons, a general trend towardsimpler, quicker, and minimally invasive surgical procedureshas encouraged the development of sutureless techniqueslike the use of adhesive and sealant interfaces to restoresoft tissue integrity and functionality. These interfaces canbe successfully employed in the treatment of emergencyhemostasis,2,5 in sealing leaks of gas or fluids,6 and in thereinforcement of sutures.7 Hemostats work by causing bloodto clot and are indicated to stop nonsuturable or noncauter-izable bleeding particularly in anticoagulated or coagulo-pathic patients; several surgical operations require a perfecthemostasis, so that the principal aim is the reduction of
post-operative bleeding and leakage, especially when paren-chymal resections or vascular anastomoses are performed.The use of sealants has been widely described in liver sur-gery to reduce postoperative blood loss and bile leak,impacting both short and long-term prognosis as they arethe most detrimental complications in liver surgery.8 Spleentraumas represent another field for the application of seal-ant interfaces. Laparoscopic spleen-preserving procedureshave been used for patients with hemodinamically stablesplenic injuries; in these patients the topical application ofsealants like fibrin glues has shown to enable good bleedingcontrol, even in patients lacking clotting factors or plateletsor taking anticlotting medications.9 Sealants can also beused to prevent the leakage of organic fluids, includinglymph cerebrospinal fluid and gastrointestinal contents.Anastomotic leakage can occur at all levels of gastrointesti-nal surgery; recent studies have shown that this riskappears to be reduced by the use of sealants.10–12 Tissueapproximation of wounds with no tension representsanother field in which adhesives can be very useful13; inthese cases the adhesives need to be strong, water resistant
and able to function as antibacterial barrier. Another com-mon procedure in general surgery is the use of implantablebiomaterials that should be maintained in situ in close con-tact with the target tissue; for instance, implanted deviceslike meshes, gauzes, webs or catheters need to be kept inplace to properly fulfill their functions. Also in these cases,sutureless techniques offer considerable advantages.14,15
Overall, general surgery requires an increasing use ofadhesive and sealant interfaces for a wide range of opera-tions and treatments. All these strategies are based on theconcept of bioadhesion, defined as the process whereby syn-thetic and/or natural macromolecules adhere to a biologicaltissue for an extended period of time in the body.16 To cre-ate stable and safe interfaces, an ideal bioadhesive shouldpossess several properties. Provided the biocompatibility ofthe formulation, which must not be locally irritating, inflam-matory, toxic or antigenic, the adhesive should be easilyapplied or injected in a form of liquid or hydrogel on thetarget surface. Then, the reticulation process should takeplace in the presence of body fluids in a conveniently shorttime, according to the requirements of the specific opera-tion. After reticulation, the adhesive should be as pliable asthe tissue, in order to follow its physiologic expansion/con-traction, while at the same time ensuring strong bindingefficacy; for this reason adequate mechanical properties arerequired for a proper elasticity/compliance of the interface.In some cases, the adhesive should progressively undergobiodegradation after having exerted its function. Finally, oneof the main challenges of bioadhesion is bonding in a wetphysiological environment.16,17
This wide range of functions is pursued by employingpolymers capable of generating a three-dimensional networkthat binds to the target tissue. Current surgical adhesive andsealants are either based on natural compounds or on syn-thetic materials; the former are generally well accepted bytissues but often exhibit low adhesive strength while the lat-ter typically display higher strength but lower biocompatibil-ity. Depending on the nature of the polymers, the mainclasses of adhesives for general surgery include fibrin,18–20
gelatin,21 and formulations based on proteins and polysac-charides,3,22 cyanoacrylates,23,24 polyurethanes,25,26 and poly-ethylene glycol (PEG).27,28 Beside polymers, novel adhesivestrategies considering the topography of gecko-foot29 as wellas the use of silica nanoparticle solutions for gluing gels andtissues are being investigated.30 The reticulation step can fol-low different routes: it can be triggered by the chemical reac-tivity of the adhesive compounds or by the interaction withbiological molecules. Chemical approaches include polymer-ization by contact with physiological fluids (for example, cya-noacrylates) and reticulation triggered by crosslinkers (forexample, glutaraldehyde or carbodiimide) or by reactive sub-stituents on the polymer backbone. Biological approaches toinitiate network formation include the enzymatic crosslinkingas in the case of fibrin-based adhesives in which occur theexploitation of transglutaminase-catalyzed reactions thatoccur during blood coagulation.21,31 In general, biochemicalcrosslinking approaches are preferred, because they providea more biocompatible adhesion strategy.
The effectiveness of a given formulation stems from acompromise between cohesive and adhesive forces,32 theformer being due to molecular forces within the interface(bulk-bulk bonding), the latter being due to attractive forcesbetween the adhesive and the target surface. Cohesive inter-actions are required only to a certain extent since too muchcohesion may result in a hardened material without signifi-cant affinity for a surface. However, adhesive interactionswith the target tissue are a fundamental aspect that mustbe considered for each specific organ of the body.
This review is aimed at considering the most importantadhesive and sealant materials for general surgery applica-tions, thus highlighting the scientific progress over recentyears and suggesting the importance of continuous researchin this field.
ADHESIVES BASED ON NATURAL PRODUCTS
Adhesives based on natural products refer to a class of sub-stances formulated from bio-based raw materials, which areemployed as adhesives in man-made technology33; some ofthese bioadhesives work in wet environment,21,34 which is oneof the most important properties for a surgical adhesive, andthey typically show good biocompatibility.35 However, batch-to-batch variation may be a serious concern and sometimes itis difficult to establish reliable large scale production proc-esses. Most bioadhesives proposed for general surgery arebased on a variety of substances like proteins (for example,collagen, fibrin, gelatin, and albumin)36,37 and polysaccharides(for example, chitosan, starch, and dextran).38,39 Protein-basedmaterials are more commonly used, although polysaccharide-based systems are gaining increasing attention.
Protein-based adhesivesProteins such as gelatin, fibrin, and albumin have been usedin general surgery for many years; the main advantage ofprotein based formulations is related to their haemostaticproperties that can assist the coagulation process.40,41 Theycan also be combined with traditional wound closure meth-ods such as stitches, grafts or sutures.42,43 The main disad-vantages of employing proteins as adhesives are theirsource, the enzymatic degradability, and the high sensibilityto fluids, as well as the relatively high price. The major com-ponents of these bioadhesives are directly extracted fromhuman biological sources, such as blood, or are based onproteins isolated from animals, such as porcine gelatin orbovine albumin. Fibrin-based haemostatic adhesives carry arisk of disease transmission due to the presence of plasmaderived components.44 These adhesives can be used withoutthe involvement of any other chemical reagents or in combi-nation with active agents (chemical, enzymatic, or photo-chemical crosslinkers) that trigger crosslinking reactions ofthe glue while simultaneously forming covalent bonds withthe tissue surface.2 The main protein-based adhesives andsealants are described hereafter.
Fibrin glue. Fibrin-based formulations are currently one ofthe main biological sealant systems in general surgery appli-cations; they are designed to mimic the last stage of blood
2 SCOGNAMIGLIO ET AL. ADHESIVE AND SEALANT INTERFACES FOR GENERAL SURGERY APPLICATIONS
clotting, during which fibrinogen is converted into fibrinclot through a complex coagulation cascade (Figure 1).45
The process requires the catalysis by thrombin and factorXIIIa, enzymes belonging to the family of transglutaminases.Factor XIIIa catalyzes the formation of covalent bondsbetween the side chains of different fibrin molecules, con-tributing to stable crosslinking and resistance to dissolution.Crosslinking occur through the formation of amide bondsbetween glutamine (Gln) and lysine (Lys) residues inproteins. The transglutaminases have, for this reason, beenclassified as natural biological adhesives.46
Commercial formulations of fibrin glue are typically sup-plied as a two-component system, in which thrombin (incombination with a calcium chloride solution) and a concen-trated solution of human-derived fibrinogen (together withfactor XIII) are placed in separate syringe tubes; the twocomponents are mixed together prior to the application onthe wounded tissue. In some preparations of fibrin glue, anantifibrinolytic agent is included, in order to prevent prema-ture lysis of the clot and to control gelling kinetics.44 TheVivostatVR system (Vivolution A/S Alleroed, Denmark) is anautomated medical device that allows the preparation of anautologous fibrin sealant starting from patient’s blood.47
This approach enables to eliminate the risk of transmittingblood-borne diseases, which is one of the major concernsrelated to the clinical use of fibrin glues. However, the timerequired to produce fibrin this way is approximately twodays; for this reason, the use of autologous fibrinogen is notcompatible with trauma and emergency surgery. All com-mercial fibrin glues are biodegradable and bioresorbableand the degradation of the fibrin clots occurs throughthrombolysis in a time that ranges from few days to weeks.The properties of commercial fibrin glues can be modulatedby varying their composition48: in a typical formulation thefibrinogen concentration is higher than the one in humanplasma, which positively contributes to the strength of thefibrin glue, while thrombin concentration determines thecuring time to achieve maximum adhesive strength.49 Theuse of fibrin glue hastened when the ability of producinghighly concentrated fibrinogen was developed, accountingfor stronger adhesion properties.50 The main physical andchemical processes used for obtaining the fibrinogen neces-sary to prepare fibrin glues and sealants are based oncryoprecipitation and precipitation with ammonium sulfate,
ethanol or PEG. Cryoprecipitation involves several cycles offreezing/thawing and although being a time consuming pro-cess, it presents the advantage of avoiding the addition ofexogenous chemicals51,52; however, this method enables toobtain low concentrations of fibrinogen, which reduces theeffectiveness of the adhesive formulation. Conversely, chemi-cal precipitation is considered a fast and efficient method toobtain high fibrinogen concentrations, but with insufficientpurity.53 Fibrin-based adhesives are clinically used in gen-eral surgery mainly as hemostats, primary wound closureagents, and as adjuncts to sutures and staples. The majorityof reported applications are in surgical procedures, to con-trol bleeding and leaking during and after surgery.18–20
Fibrin glue is also used as hemostatic agent and sealant invascular surgeries, particularly to prevent bleeding fromsuture line and graft area, which is a common issue in thistype of operations.54–56 The application of fibrin glues inthe treatment of gastrointestinal diseases, such as inpatients suffering from bleeding peptic ulcers, has also beeninvestigated with the aim of replacing surgical proceduresby noninvasive endoscopic injections.57 The main disadvan-tages of these adhesives are the poor mechanical strengthand adhesion in wet environment and the concerns relatedto the safety of the products. The main concern regardsviral transmission (such as HIV, parvovirus B19, hepatitis B,or hepatitis C) from formulations prepared using pooledblood.2 Only in 1998, the FDA approved the productTisseelVR (Baxter Healthcare, Deerfield, IL), the first genera-tion of commercial fibrin glues. However, some adverseresponses can be associated to the use of fibrin-based adhe-sives. For instance, allergic skin responses58 or anaphylacticreactions59 were reported in patients who have beenexposed to the bovine aprotinin contained in fibrin sealant.Recently, the efficacy of fibrin sealants was tested in a clini-cal trial: patients undergoing laparoscopic Roux-en-Y gastricbypass were treated with human fibrin sealant and a reduc-tion of the postoperative bleeding was observed.37
Gelatin based adhesives. Gelatin is an irreversibly hydro-lyzed form of collagen with many industrial, pharmaceutical,and biomedical applications. Gelatin has been used for cen-turies as an adhesive for technical applications and wasamong the first polymeric components to be adapted formedical adhesives. Gelatins are cheap, biocompatible and
FIGURE 1. Formation of clot based on fibrin crosslinking.
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bioresorbable materials that can form strong, transparent,and flexible gels and films, granting them suitable proper-ties for internal surgery. Gelatin-based tissue adhesives havebeen recently proposed in clinical field for the treatment ofaortic dissection: the amino groups of gelatin were modifiedwith cholesteryl residues conferring improved properties tothe adhesive in terms of bonding strength and tissue pene-tration.60 Because gelatin hydrogels are relatively unstablein aqueous solutions (they swell and typically dissolveabove 35 �C), various chemical crosslinking methods havebeen used to confer stability under biological conditions tomeet bioadhesive properties. The primary purpose of thechemical modification of gelatin with a crosslinker is toincrease its adhesion strength and control its degradationrate; crosslinking can be achieved through chemical, photo-chemical, and enzymatic approaches, as described in thenext sections.
Chemically crosslinked gelatin (gelatin-resorcinol-formaldehyde, GRF glue). In these formulations, gelatinchains are crosslinked by aldehydes through a polyconden-sation reaction. Simultaneously, gelatin amine groups reactwith amine groups of tissue proteins to form a covalentbond with it (Figure 2); in addition, resorcinol moleculesare reticulated by means of formaldehyde to yield a three-dimensional network.2
The curing profile of GRF adhesives can be altered byadjusting the ratio of the components; these adhesives arecapable to bind to wet tissues and form covalent linkageswith functional groups on the tissue surface. Bondingstrength is ensured by the penetration of the componentsinto the tissue. Nevertheless, its performance is limited bythe cytotoxicity associated with formaldehyde.61 Resorcinol isless toxic than other phenols because it is less oxidized andproduces lower levels of oxygen radicals.62 Some researchers
FIGURE 2. Crosslinking of gelatin with aldehydes and resorcinol (Reproduced from Ref. 2).
4 SCOGNAMIGLIO ET AL. ADHESIVE AND SEALANT INTERFACES FOR GENERAL SURGERY APPLICATIONS
argued that the GRF glue may become innocuous if anoptimal composition of the components can accomplish apolymerization with no residual formaldehyde.61 There issubstantial evidence that GRF glue has beneficial effects onperioperative bleeding and on the incidence of reopera-tion.63,64 Surgical application of GRF glues is recommendedin cases in which tissue integrity is poor, hemostasis is chal-lenging, and high bonding strength is absolutely imperative.
Enzymatically crosslinked gelatin (gelatin-mTGadhesive). Gelatin can be used as a sealant in combinationwith a microbial transglutaminase (mTG),34 which is capa-ble to catalyze its crosslinking.21,65,66 mTG catalyzes the for-mation of a covalent bond between a free amine group of apeptide-bound Lys and the acyl group at the end of the sidechain of a peptide-bound Gln, with the production of a mol-ecule of ammonia (Figure 3).
The safety of mTG for medical applications has not beenextensively tested, but it is worthwhile to note that thisenzyme is approved for food uses. Both gelatin and mTGare commercial products obtained from sources that raiseless concern than blood. The mTG-catalyzed crosslinking ofgelatin does not require low MW compounds (that is, mono-mers, initiators, and crosslinkers) prior functionalization ofthe polymer backbone, nor photopolymerization. The cur-rent in vitro evidence indicates that the gelatin–mTG adhe-sive is effective under wet conditions21,34 and that thisadhesive confers strengths comparable to other soft-tissueadhesives like fibrin based sealants.34 The resulting cross-linked network resorbs as a result of normal proteolyticprocesses. Viscosity and elasticity of the glue (but not itsadhesive strength) depend on gelatin type and concentra-tion.67 One limitation of the gelatin-mTG adhesive is thatthe protein forms a physical gel at room temperature, and itneeds therefore to be warmed to 37 �C prior to use, whichcould be inconvenient for surgical techniques. Additionallong-term studies are required to ensure the biocompatibil-ity and biodegradability of this adhesive and to assess thepotential of the gelatin-mTG adhesives to promote woundhealing process.
Photocrosslinked gelatin. The synthesis of a tissue sealantbased on a photocrosslinkable gelatin was recently reportedand the formulation showed high elasticity while retainingexcellent adhesive strength.68 In this case, self-associatingproteins, for example, resilin and fibrinogen, can be cova-lently crosslinked via di-tyrosine bonds within secondsusing visible light.68 Elvin et al. proved that naturally self-associating proteins that contain surface accessible Tyr resi-
dues can be crosslinked into polymers using the ruthenium-based photochemistry.69 The main drawback of the photo-polymerized gelatin is its high swelling ratio (over 240%within 24 h); in an attempt to reduce this swelling, gelatinwas derivatized with phenolic residues to increase itsamount of tyrosine residues.68
The potential of photocurable gelatin in tissue sealing wastested in a sheep surgical model68: the photopolymerized gel-atin sealed a wound in lung from leakage of blood and air,with excellent post-surgery outcomes. In another study, a pho-tochemically crosslinked gelatin sealant was used in rabbitand canine gastrointestinal models with good mechanical andbiological outcomes; the sealant demonstrated high elasticityand adhesive strength and good tissue integration.70
The effectiveness of a gelatin-based adhesive was eval-uated in an experimental study on rat’s liver: the resultspointed out its efficacy in the establishment of a good tissueadhesion and hemostasis.71 Sato et al. reported a case wherethe use of gelatin-resorcinol-formal glue was effective in thetreatment of postoperative fistula following a low anteriorresection in colorectal surgery.72 Despite all the advantagesof this material, potential contamination with animal infectiveagents is still the major concern on the use of gelatin.
Albumin-based glues. Albumin–glutaraldehyde adhesivesare able to establish covalent bonds with functional groupson the tissue surface, thus creating an elastic seal. Theseglues also adhere to synthetic graft materials throughmechanical bonding within the interstices of the graftmatrix. The reticulation of bovine serum albumin (BSA)/glutaraldehyde tissue adhesives occurs by a condensationreaction between amino groups of Lys residues in the BSAprotein and glutaraldehyde. Albumin–glutaraldehyde gluestend to degrade slowly and they can persist at the repairsite for up to 2 years after application.73 The commercialformulation BioGlueVR (Cryolife, Kennesaw, GA) is a tissueadhesive composed of BSA mixed with glutaraldehyde andis able to adhere to tissues and to synthetic graft materials.It is currently being used as an adjunct for securing hemo-stasis at vascular anastomoses.74,75 The effectiveness of Bio-GlueVR in preventing air leakage in pulmonary surgery wasdemonstrated on rats.76 In a pilot clinical study, the useful-ness of BioGlueVR for the treatment of high transsphincteranal fistulas was reported.36 An improper use of albumin–glutaraldehyde glues was reported to cause negative out-comes in case of excessive application.77 An in vivo study onrabbits reported that the release of glutaraldehyde uponpolymerization could cause a certain extent of cytotoxicitywhen applied on lung and liver tissue.74
FIGURE 3. Crosslinking of Gln and Lys residues of gelatin by mTG. (Reproduced from Ref. 21).
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Polysaccharide-based adhesivesIn nature, polysaccharides and proteins (or a combination ofthe two) are natural mediators of adhesion and have foundmany industrial and pharmaceutical applications over thepast decades. They represent a very attractive class of biomo-lecules for various biomedical fields, including general sur-gery. In this field, two polysaccharides from marine source,alginate and chitosan, are particularly attractive owing totheir biocompatibility, hydrophilicity, adhesiveness, and hemo-static activities.78,79 These two polysaccharides have beenused for the preparation of adhesive nanosheets.80 As topolysaccharide adhesivity, it should also be mentioned thatsome microorganisms use acidic or neutral exopolysacchar-ides (that is, dextran, heparan sulfate, levan) to adhere to avariety of substrates.81 Certain polysaccharides are able toform hydrogels that exhibit high swelling ratios; althoughthis is a desirable feature when polysaccharides are used inmodern wound dressing formulations, in general surgery pro-cedures excessive swelling of polysaccharide-based adhesivescan affect the compliance to the tissue. To reduce suchbehavior and to enhance adhesivity, these polysaccharidescan be subjected to chemical modifications as described indetail in the following paragraphs.
Dextran-based adhesives. Dextran is an exocellular bacte-rial polysaccharide predominantly consisting of linear a-1,6-linked glucopyranose units, with some degree of 1,3-branch-ing. This highly water-soluble polymer is produced in asucrose-rich environment by Lactobacillus, Leuconostoc, andStreptococcus and is commercially available with differentmolecular weights. Dextran is also nontoxic and biocompati-ble and can be degraded through the action of differentdextranases (a-1,6 -glucosidases) in various organs in thehuman body, including liver, spleen, kidney, and colon82,83;both the degree of branching and the molecular weight dis-tribution affect its physicochemical properties.84,85 Besidesbeing highly water-soluble, dextrans are stable under mildacidic and basic conditions. Furthermore, these polymerscontain a high density of hydroxyl groups, making themsuitable for derivatization and subsequent chemical or phys-ical crosslinking.85 Dextran-based hydrogels can be used assurgical adhesives; for this application, reactive groups (for
example, aldehydes) are introduced into the polymerchain.38,86 The introduction of these groups can be accom-plished by selective oxidation with periodic acid or period-ate salts which causes the formation of a dialdehyde-dextran compound, with a free hydroxyl group next to thenewly formed aldehydes (Figure 4).
Polysaccharides that have acquired aldehyde groups as aresult of oxidation can react with amine groups of cell sur-face proteins of the tissues thus allowing bioadhesion.87,88
Moreover, oxidized dextrans can react with amino groups ofadditional components like gelatins or aminated PEGs toform intermolecular crosslinks.87 Recently, dextran-PEGsbioadhesives have been proposed as soft tissues sealants39;the cohesive integrity of dextran-PEGs formulations comesfrom imine bonds that form through a Schiff base reactionbetween amines and aldehydes (Figure 5).89 The cohesiveproperties depend on the chemical structure of PEG (forexample, number of arms), while tissue/material adhesionstrength is primarily determined by the number of alde-hydes in the oxidized dextran.
Recently a hydrogel tissue adhesive, obtained by reactingan oxidized dextran with a water-dispersable multiarm poly-ether amine (PEG) has been developed (ActaMaxVR ): thecrosslinking reaction occurs in water and the componentsundergo a Schiff base reaction to form a crosslinked hydro-gel that reticulates within 1 min at room temperature. Theformed adhesive is able to adhere to moist tissue and itdegrades hydrolytically.90 Dextran-PEGs adhesives wereshown to be non-cytotoxic and noninflammatory, they donot pose the risk of viral contamination90 and have beenused in sealing small intestinal puncture.39 In a recentexperimental study, Artzi et al. applied this adhesive on asmall bowel rat model: the average adhesion force to intes-tinal tissue was found to be higher than with fibrin sealantand close to cyanoacrylates.89
Chitosan-based adhesives. Chitosan is a linear polysaccha-ride composed of randomly distributed b-(1-4)-linked D-glu-cosamine residues with a variable number of randomlylocated N-acetyl-D-glucosamine units; it is produced bydeacetylation of chitin, the structural component of the exo-skeleton of crustaceans. This polysaccharide has drawn a lot
FIGURE 4. Oxidation of dextran to yield reactive dextran aldehyde for adhesive formulations.
6 SCOGNAMIGLIO ET AL. ADHESIVE AND SEALANT INTERFACES FOR GENERAL SURGERY APPLICATIONS
of attention in the biomedical field, because of its biocom-patibility, antioxidant, and bacteriostatic properties.91,92
Chemical modifications of its amino and hydroxyl groupsprovides a powerful mean to tailor its biological activity andto modify its physico-chemical properties. Owing to its basicnature, it has the ability to interact with anionic biopoly-mers, such as glycosaminoglycans, heparin, proteoglycans,and nucleic acids. This ability represents an importantaspect in the development of soft tissues bioadhesives. How-ever, despite pure chitosan solutions can establish molecularinteractions with the target tissue, they lack cohesion andare not able to generate sufficient adhesion. Cohesion andadhesion can be increased by following various crosslinkingstrategies. Chitosan-based adhesives prepared through pho-tochemical crosslinking reactions possess photoreactiveinert groups (generally phenyl azides and diazirines) thatbecome reactive when exposed to ultraviolet or visiblelight. A photocrosslinkable hydrogel based on chitosan,4-azidobenzoic acid (Az)-chitosanVR has been proposed forperipheral nerve anastomosis93; this bioadhesive was syn-thesized by conjugating Az with low and high molecularweight chitosans. Another commercially available chitosan-based product is SurgiLuxVR : the laser activation strengthensthe adhesion of the formulation to tissue collagen throughpolymer chain interactions as a consequence of transientthermal expansion.94 Its experimental use on intestinaltissue demonstrated good biocompatibility and negligiblethermal damage as a consequence of irradiation.35 Anothercrosslinking strategy was followed by Serrero andcoworkers who reported the preparation of a hydrogel byadding a multifunctional crosslinker based on oxidizedstarch to chitosan95; owing to the aldehyde groups of theoxidized starch, adhesion can be achieved by the molecularinteraction with collagen amine groups or with other pro-teins within the tissue.87 Various physico-chemical parame-ters (chitosan concentration, molecular weight, degree ofstarch oxidation) were found to influence the adhesionproperties of the formulations; adhesion tests demonstratedthat low molecular weight chitosans were more effective
than high molecular weight ones. This behavior wasascribed to improved mobility of the former macromole-cules, which likely promotes a wider interaction surfacewith the tissue, hence an easier covalent or physical bond-ing with the biological substrate. However, no data aboutthe biocompatibility of the system are available.
Synthetic adhesives. Performance limitations, safety con-cerns, and potential risks associated with the use of somenatural-based adhesives (mostly proteins) have drivenresearchers to develop adhesives based on synthetic poly-mers. Synthetic adhesives are based on synthetic chemicalstypically in the form of monomers, prepolymers, or noncros-slinked polymers, which undergo polymerization or cross-linking to form an insoluble adhesive matrix when deliveredon a tissue.2 Their three-dimensional structure as well astheir chemical composition can be controlled to exposefunctional groups that can interact with biological tissues,thus providing bioadhesion.28,96 Molecular weight of nonbio-degradable synthetic polymers should be under the thresh-old of renal excretion since these polymers have to becleared by the kidneys.97 In general, synthetic tissue-adhesives are not associated with the risk of infectious con-taminations, although their biocompatibility and toxicitymay represent an issue especially in the case of highly reac-tive components. Several synthetic adhesive materials areemployed for general surgery applications: according totheir chemistry, the main formulations are based on cyanoa-crylates, PEG, and polyurethanes.
Cyanoacrylate adhesives. Cyanoacrylate tissue adhesivesare currently the main synthetic polymeric sealants in clini-cal usage; they possess high bonding strength, very rapidsetting time, and instantaneous adhesion to tissues. Someformulations are also reported to inhibit the growth of bac-teria.98 They are prepared as a single-component systemthat polymerizes at room temperature without the additionof a catalyst, solvent evaporation, heat, or pressure applica-tion. These adhesives require no external initiation for
FIGURE 5. Dextran-PEG adhesive: the oxidized dextran aldehyde reacts with an aminated PEG to form a crosslinked hydrogel network through
imine bond formation. (Reproduced from Ref. 89).
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curing: cyanoacrylates can rely on small amounts of waterto initiate the polymerization reaction and bonding occurswithin seconds.
The basic cyanoacrylate monomer (alkyl-2-cyanoacrylate)is a low-viscosity liquid and is formed by combining formal-dehyde and alkyl-2-cyanoacetate (Figure 6). The most com-mon polymerization initiators for cyanoacrylates are thehydroxyl ions within water. Upon contact with wet tissues(such as skin, moisture, or blood), cyanoacrylates polymerizeinto a solid film that binds juxtaposed wound edges. Adhe-sion is achieved through two independent mechanisms:(i) molecular interaction via covalent bonding to proteinsexposed on tissue surface and (ii) penetration of cyanoacry-late monomers into cracks and channels in the tissue surface(mechanical interlocking). For these reasons, cyanoacrylateadhesives are particularly effective on moist and poroussubstrates.99,100
In Figure 6, the general chemical structure and polymer-ization reaction of the cyanoacrylate adhesives is illustrated.The alkyl or carbon side chain AR has an important effecton the strength and physical properties of the glue. In com-parison with complex, long-chain derivatives, straight, andshort-chain monomers (AR5 ACH3 or AC2H5) form tighterand stronger bonds, which results in more rigid and brittleinterfaces.101 In contrast, by increasing the length or com-plexity of side alkyl group, the polymerization rate tends todecrease and interfaces with more flexibility are formed.Cyanoacrylate-based adhesives may also contain plasticizers,dyestuffs, thickeners, polymerization catalysts, anionic andradical stabilizers and other additives to make the formula-tion easier to handle and biologically safer. In the humanbody, cyanoacrylate adhesives undergo hydrolytic degrada-tion, which takes place through nonenzymatic routes; themain degradation products are formaldehyde and the corre-sponding alkyl cyanoacetate. The degradation rate of cya-noacrylate polymers decreases with longer alkyl side chain,
as a result of steric hindrance102; therefore short-chainderivatives degrade very quickly, resulting in a higheramount or local concentration of breakdown products,which are potentially harmful to cells and tissues and maycause inflammatory reactions and impair wound healing.High-molecular-weight polymers with longer side chaindegrade slowly, which translates into producing less toxicdegradation products; however, their persistence in thebody may cause medical complications.103 Although all cya-noacrylates arise from the same basic structure, subtle var-iations can dramatically change the properties of thecompounds (flexibility, setting time, bond strength, viscosity,heat of polymerization reaction, biocompatibility, toxicity,and degradation profile). Cyanoacrylates have proven to bevaluable in sutureless surgery: in many cases, wound clo-sure can be safer, stronger, and more functional than withtraditional suturing (that is, titches).104 The developmentand clinical evaluation of these materials for general surgerywas delayed because of safety issues; however, in the lastdecade a lot of efforts were devoted to cyanoacrylate appli-cations other than cutaneous. An important use of cyanoa-crylate formulations is for hemostatic purpose105 like inanastomotic connections where there is a high risk of bleed-ing complication.106,107 Cyanoacrylates possess severaladvantages for tissue approximation and their applicationsinclude wound closure or small Pfannenstiel incisional cutsperformed during clean abdominal surgery.13 To reducepossible inflammatory reactions and confer the desiredadhesive strength and flexibility, novel cyanoacrylate-basedformulations include additional components; as an example,the commercial formulation Glubran2VR is a mixture of n-butyl-2-cyanoacrylate (monomer) and methacryloxysulpho-lane (monomer) and it displays anti-inflammatory proper-ties.108 Glubran2VR has been tested for mesh fixation inLichtenstein’s inguinal hernia repair, with positive outcomescompared to traditional suturing methods.109 In a recent
FIGURE 6. Cyanoacrylate chemistry: (A) Synthesis of alkyl-2-cyanoacrylate monomer and (B) polymerization reaction.
8 SCOGNAMIGLIO ET AL. ADHESIVE AND SEALANT INTERFACES FOR GENERAL SURGERY APPLICATIONS
study, liver retraction was successfully achieved using n-butyl-2-cyanoacrylate glue in single-incision laparoscopicupper abdominal surgery.110 A commercial 2-octyl cyanoa-crylate (Dermabond AdvancedTM, Ethicon, Johnson, andJohnson Medical) was shown to reduce the rate of postoper-ative pancreatic fistula after pancreaticoduodenectomy.111
The use of cyanoacrylate in surgical anastomosis for generalsurgery has been proposed as an alternative to microsur-gery particularly in centers where facilities are unavailableand the financial implication is unbearable for thepatient.106
PEG–BASED ADHESIVES AND SEALANTS
PEG is a neutral, biocompatible, and hydrophilic polymerwidely employed in the biomedical field. It is soluble inaqueous solutions, which makes it a good candidate forhydrophilic and biodegradable systems. PEGs are pre-pared by polymerization of ethylene oxide and are com-mercially available over a wide range of molecular
weights and with a variety of end groups. Since it is notable to establish a bioactive interaction with biologicalmatter, tissue adhesives based on PEG are prepared bygrafting reactive moieties capable to establish covalentbonds with tissues; the resulting hydrogels can beemployed as sealants for wound closure and as sutureadjuvants to help hemostasis in the wounded site. Forinstance, Lee et al. described the preparation of PEG-based hydrogels modified through the coupling with L-3,4-dihydroxyphenylalanine endgroups conferring enhancedmucoadhesivity to the resulting hydrogels.28 PEG-basedadhesives are designed to provide a seal through covalentbonding to tissue surfaces while retaining flexibility andallowing a normal physiological dilation without stiffen-ing, thus limiting mechanical stress.112 PEG-based tissueadhesives are degraded through hydrolysis; they typicallyhave a high swelling ratio and display a rapid degradationprofile, which may represent a drawback for long-termwound reinforcement.20
FIGURE 7. Formation of a 3D network by reaction of star-shaped PEG polymers in CosealVR PEG-PEG sealant: reticulation occurs by formation of
thioester bonds and release of N-hydroxysuccinimide. (Reproduced from Ref. 49).
FIGURE 8. Tissue adhesion mechanism of urethane-based adhesive: H2N-R0 represent tissue amines that react with isocyanate groups through
urea bond formation.
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TABLE 1. Summary Table of Adhesive and Sealant Classes of Materials for General Surgery Applications
Adhesive/Sealant Class Main Applications Curing Mechanism Pros Cons Ref.
Good wettabilitypossibility to tai-lor physico-chemical prop-erties of PUs
Long setting timepossible forma-tion of poorlybiocompatibledegradationproducts
(4,24,25)
10 SCOGNAMIGLIO ET AL. ADHESIVE AND SEALANT INTERFACES FOR GENERAL SURGERY APPLICATIONS
The commercial formulation CosealVR (Cohesion Technol-ogies, Deerfield, IL) is composed of two types of four-armPEGs (with a pentaerythritol core), one of which bears aglutaryl-succinimidyl ester as the terminal group while theother is capped with thiolic functions2; when the solutionsof these two PEGs combine, the polymers begin to crosslinkand form a network through the reaction of thiol groupswith the carbonyl groups of the succinimidyl ester, resultingin the formation of a covalent thioester bond between thetwo multiarm PEG molecules and by the release of N-hydroxysuccinimide (Figure 7).113
The functionalized PEG end groups additionally reactwith functional groups (particularly amine groups) of theproteinaceous matrix to form covalent bonds, providing achemical linkage between the PEG–PEG hydrogel and the sur-rounding tissue.96 This formulation is proposed as a resorb-able sealant for suture lines to prevent leaks.27 CosealVR wastested for the reinforcement of intestinal anastomoses,although its use did not show a significant increase of burst-ing resistance.114 In a similar study, a crosslinked hydrogelsbased on PEG and dextran aldehyde polymers was studiedfor the repair of intestinal wounds; this adhesive formulationexhibited considerable viscoelasticity and enabled to increaseburst pressure.39 In an experimental study on porcine model,a PEG-collagen hydrogel was applied to a pancreatic injury toprevent a pancreatic leak; the results showed that the PEG-based sealant could prevent a ductal leak following pancre-atic injury.115
POLYURETHANE-BASED ADHESIVES (PU)
Polyurethanes are a family of polymers composed of twomain components: isocyanates (containing two or more iso-cyanate groups per molecule) and polyols (containing onaverage two or more hydroxyl groups per molecule), whichtypically react in the presence of catalysts and a variety ofother additives (such as chain extenders, crosslinkers andsurfactants).2 The properties of polyurethane are greatlyinfluenced by the types of isocyanates and polyolsemployed. The wide variety of components and processingconditions, allow to tailor the adhesive formulations for thedesigned use.116 The basis of polyurethane chemistry is thehigh reactivity of isocyanates, which can be assigned to thepositive charge of the carbon atom in the cumulated doublebond system of its N@C@O group. Urethane-based adhe-sives typically consist of isocyanate-terminated prepolymersthat form a polymer network reacting with water moleculesupon contact with biological environment.2 These prepoly-mers covalently adhere to tissue through formation of ureabond between available isocyanate groups and amines oftissue proteins,117 as shown in Figure 8.
Isocyanate-terminated pre-polymers usually exhibit longsetting time (in the order of tens of minutes) when no cata-lyst is used, which limits their use as tissue adhesives.2 Toaddress the issues of long setting time and potential toxicityof degradation products, researchers incorporated new com-pounds in polyurethane synthesis: linear and multiarm pre-polymers capped with more reactive and less harmful
isocyanate groups are now widely used.118 Urethane-basedpolymers display good properties as bioadhesives since theypossess good wettability and capability to establish covalentinteractions with body tissues. Recently, a Lys-derived ure-thane adhesive, TissuGluVR (Cohera Medical), was developedfor large flap surgeries such as abdominoplasty. This glue isdescribed as resorbable and nontoxic; it forms a strongbond between tissue layers and it eliminates or reducesfluid accumulation and the need for postsurgical drains.TissuGluVR was used on patients undergoing abdominoplas-tic surgery and the results showed that, in comparison tostandard surgical closure techniques, it effectively binds tis-sue layers together, thereby reducing dead spaces whereseroma can occur, while it also reduces post-surgery wounddrainage.26 More recently, a long term evaluation ofTissuGluVR showed that it is capable of preventing the for-mation of seroma in a canine abdominoplasty model.25
To conclude, the main features of the adhesive and seal-ant interfaces for general surgery applications discussed inthis review are summarized in Table 1.
CONCLUSIONS
In the field of general surgery, several clinical needs arebeing addressed by the use of adhesive and sealant interfa-ces; their use offers numerous advantages and it can beextended to further clinical situations that would benefitfrom the employment of sutureless techniques. Both syn-thetic and natural-based polymers are successfully beingstudied and employed and each adhesive class brings sev-eral advantages, although limitations related to the materialfeatures should always be considered.
Synthetic polymers offer several advantages especially interms of mechanical performance but they can have limita-tions like poor biocompatibility and excessive stiffness;However, natural-based polymers typically form weakerinterfaces but they are more similar to the macromolecularfeatures of human tissues. When designing a new adhesive,the formulation has to be tailored for the specific target tis-sue, which means that since the early stages of its develop-ment, it must be conceived considering its clinical use.Hence, it is the medical application of the adhesive that dic-tates its features. This point should be taken into accountwhen employing commercial adhesives for applications thatwere not designed for and it highlights that no universalsolution has been developed so far in this field, given thewide morphological and functional heterogeneity of bodytissues.
In the future, hybrid materials exploiting in a synergicmanner the advantages of both synthetic and natural com-pounds will gain increasing importance. Within this chal-lenge, bioinspired adhesive strategies that take inspirationfrom nature are expected to bring further impulse to thisfield of research toward novel solutions.