bio active bio materials

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The most important advances in the field of biomaterials overthe past few years have been in bioactive biomaterials.Materials have been developed to incorporate bioactivitythrough biological recognition, including incorporation ofadhesion factors, polyanionic sites that mimic the electrostaticsof biological regulatory polysaccharides, and cleavage sites forenzymes involved in cell migration. Materials have also beendeveloped to be active in biological environments byundergoing phase changes in situ, including transformationsfrom liquid precursors to solids and from soluble materials tomaterials that are immobilised on tissue surfaces.

AddressesDepartment of Materials and Institute for Biomedical Engineering,Swiss Federal Institute of Technology and the University of Zurich,Moussonstrasse 18, CH-8044 Zurich, Switzerland;e-mail: [email protected]

Current Opinion in Biotechnology 1999, 10:123–129

http://biomednet.com/elecref/0958166901000123

© Elsevier Science Ltd ISSN 0958-1669

IntroductionImportant advances have been made in the field of bioma-terials over the past few years, and most of these have beenassociated with rendering materials biologically active. It isas logical to develop biomaterials that are bioactive as it isto develop drugs that are bioactive. Pharmacological activ-ity is based on the principles of biological recognition, forexample, to competitively inhibit receptors or enzymes, toblock binding sites, to regulate certain biological pathways,and so on. One can, in most applications, consider thefunction of a biomaterial-based device analogously to thatof a pharmaceutical. An example could be considered inthe context of the vascular graft. Certainly, the basic func-tions of the graft have nothing to do with biologicalactivity: it must carry blood flow, must resist the dilatorypressures of the cardiovascular system, and must resist thecompressive and kinking forces of the tissues external tothe cardiovascular system. These goals can be met entire-ly in the absence of biological activity. Beyond these basicperformance goals, however, one may be well advised toturn to bioactivity: to prevent coagulation, to encourageendothelial cell attachment and retention, to promote cap-illary infiltration as a source of endothelial cells, to preventexcessive smooth muscle cell proliferation and collagenmatrix expression. To turn to biological recognition toaccomplish these ends, for example, to incorporate biolog-ically motivated, biomimetic adhesion-promoting sites orto incorporate growth factors, is quite logically based onthe analogy of drug design. In addition to the highlybiospecific biological recognition phenomena describedabove, some natural biological recognition proceeds by lessspecific physicochemical interactions, such as binding of a

charged polysaccharide to a protein. One can mimic theseless specific interactions as well, in this case with polyelec-trolytes, in biomaterials design.

This review considers biological recognition from one addi-tional perspective, namely enzymatically modulatedmaterial degradation. Most degradable biomaterialsdegrade based on chemical clocks, such as ester hydrolysisin the polymer backbone or sidechains. The rate of thehydrolysis reaction is programmed via selection of thedetailed chemical environment around the ester bonds,such as side groups, crystallinity and hydrophilicity, to setthe speed of the chemical clock. In this approach, the clockis set in the laboratory, based on a prediction of a biologicalresponse. An alternative approach, one involving the con-cept of bioactivity, is to let a degradation program proceedalong a coordinate of the healing process, rather than time,by making the material sensitive to the feedback providedby the cells involved in the healing response. Indeed, thisparadigm is employed naturally in tissue generation,remodelling and regeneration, as cells enzymaticallydegrade the extracellular matrix around them. By renderinga biomaterial sensitive to these enzymatic activities, onecan pursue the goal of biomimetic material degradation.

This review also considers bioactivity from a perspectiveother than biological recognition, namely active materialtransformation from one state to another in the presence ofthe biological system. Materials can be designed to transformunder some external stimulus, such as light, temperature orchemical composition (e.g. from liquids to elastic solids, fromcell adhesive to cell non-adhesive, or from freely soluble tobound to a tissue surface). By design of materials for bioac-tivity, it may be possible to develop materials that enablenew surgical procedures, such as closure of internal incisionsby bioactive adhesives, or organ culture methods, such asculture of cell and cell aggregates within gels.

Bioactivity by incorporation of adhesion factorsOne approach toward biological activity in biomaterials isthe incorporation of adhesion-promoting oligopeptides oroligosaccharides. Cell adhesion to traditional biomaterials,such as polyethylene, polytetrafluoroethylene or siliconerubber, is based upon indirect recognition, that is, by pro-teins from the body fluids adsorbing nonspecifically to thematerial surface and some subset of these adsorbed pro-teins, including fibronectin, fibrinogen, and vitronectin,promoting the adhesion of cells by interacting with the cor-responding adhesion receptors. As a more direct approach,one that permits a greater degree of control by not depend-ing upon a secondary mediator, several investigators haveexplored the covalent or physicochemical incorporation ofadhesion-promoting oligopeptides and oligosaccharides.The reader may refer to other reviews on the molecules

Bioactive biomaterialsJeffrey A Hubbell

incorporated [1,2] and on methods that have beenemployed to incorporate them, in the context of biomate-rials for tissue engineering [2–4] and drug delivery [5].

Extensive research has been performed on the incorpora-tion of adhesion-promoting oligopeptides into biomaterialsurfaces. These peptides are based on the primary struc-ture of the receptor-binding domains of proteins such asfibronectin and laminin, and often the corresponding lin-ear or cyclised sequences can display similar receptorspecificity and binding affinity, as well as signalling of cel-lular responses, compared to the whole protein [1,2]. Earlywork demonstrated an important possible advantage ofworking with short adhesion peptides, rather than thecomplete parent protein, that is, that the peptides could bedisplayed in a manner such that nearly all of them wereavailable and active for binding to cell-surface receptors[6]. When a bioactive tripeptide from fibronectin, thesequence RGD (amino acid single letter code), was immo-bilised by its amino-terminal primary amine via a glycylspacer, approximately 105 copies per cell were required toinduce cell adhesion, spreading, focal contact formationand cytoskeletal organisation, whereas many more copiesof the complete protein were required, presumably due tounfolding of the protein associated with adsorption orabsorption of the protein in an orientation such that thereceptor-binding domain was not sterically available.Likewise, early work also demonstrated, in the example ofthe bioactive YIGSR pentapeptide from laminin, that pep-tides covalently immobilised in a single orientation couldhave considerably different biological activity than thesame peptide adsorbed in a number of possible orienta-tions, many of which were presumably sterically incapableof binding to their receptors [7].

There are several important fundamental issues about howsuch adhesion signals are displayed. For example, it hasbeen demonstrated that cell adhesion strength dependsupon the surface concentration of adhesion ligands, andfurthermore, that cell migration rates depend in a very sen-sitive manner on the strength of cell adhesion [8••]. If celladhesion strength is very low, the cell is incapable of devel-oping adequate traction to migrate, whereas if celladhesion strength is very high, this traction is also veryhigh and the rate of reassociation of dissociated adhesionreceptors is very fast, resulting in very slow migration.Thus, in situations where one would incorporate adhesionligands to promote adhesion and migration (e.g. in thepores of a vascular graft to promote capillary ingrowth or inthe pores of a wound healing membrane for promotinghealing of skin ulcers) one must search for an optimum incell migration with this phenomenon in mind.Furthermore, there may be cases in which the spatial dis-play of adhesion ligands is critical. For example, in theimmobilisation of galactose for promotion of hepatocyteadhesion via the asialoglycoprotein receptor, the degree ofligand mobility, as determined by the length of the flexibletether used for immobilisation, strongly influenced cell

adhesion behaviour [9]. The extent of cell spreadingdepended on both the overall amount of ligand immo-bilised as well as its ability to cluster into spatialmicrodomains. This dependence is consistent with thebinding of the receptor to triads of galactose residues, in amanner mimicking binding to the branched oligosaccha-ride in the natural ligand for the receptor.

What could be other advantages of employing short bioac-tive peptides or saccharides, rather than the completeparent glycoprotein? One goal is selectivity for targetedcell types. Cell-type selectivity is a common goal in drugtargeting [5], and it may also represent an important goal intissue engineering [2]. For example, in vascular graftdesign, it would be beneficial to develop a material to sup-port the adhesion and migration of vascular endothelialcells, while at the same time rejecting the adhesion ofblood platelets [10]. The integrin receptor α4β1 representsa target that is present on the endothelial cell but not theblood platelet, and there exists an adhesion ligand specificto this receptor, the tetrapeptide REDV, in the adhesionprotein fibronectin. Fibronectin does not provide a usefulligand for this purpose, however, because the protein con-tains numerous adhesion-promoting domains, at least oneof which binds to most cells in the body. This lack of selec-tivity can be avoided by working with only the REDVdomain, and not the other, less specific domains. Thus,one can accomplish a goal with a small domain of the pro-tein that could not be accomplished with the completeparent proteins, that is, cell-type selectivity. Other goals intissue engineering can also be obtained more easily withsmall peptides than with the whole proteins, such as thecontrol of cell phenotype by exposure to certain adhesionligands, as has been demonstrated in culture with bone-forming cells [11], and enhancement of cell adhesionstrength, which has been shown to be important in clinicalstudies with endothelial cell seeded vascular grafts [12••].

Several biomaterial systems have been explored and devel-oped for display of bioactive adhesion-promoting ligands,and a small number of these are discussed below to illus-trate key points. More complete reviews on this topic canbe found elsewhere [2,13]. Most investigators haveapproached the problem from the perspective of covalentimmobilisation of adhesion-promoting ligands upon a bio-material surface, usually after some type of surfacemodification [2,13], and this is usually adequate. Fordegradable materials, which are particularly useful in tissueengineering because they do not need to be removed fromthe body, surface modification may be inadequate becausethe surface can be rapidly degraded and removed. For suchsituations, it may be necessary to develop new polymersthat display the ligand in the bulk of the material, and sodisplay new ligands on the surface continually as it isdegraded and remodelled. This has been addressed withthe practically important degradable biomaterial poly(lacticacid) by the synthesis of the copolymer poly(lactic acid-co-lysine). The ε-amino groups on the lysyl comonomer

124 Biochemical engineering

provide sites of ligand grafting in solution, thus presentingthe peptides on the transient surface as it moves though thematerial during degradation [14]. Poly(lactic acid) has beenused extensively as sutures and surgical staples, and suchmodification may make it more useful in tissue engineeringapplications. Bulk incorporation is also useful in the case ofgels that are subject to cell infiltration. Bioactive migration-promoting peptides, such as the YIGSR domain and theSIKVAV domain from laminin, have been incorporatedwithin gels for use in nerve regeneration [15•]. In this case,the peptides were incorporated throughout the bulk of thethree-dimensional gels to permit the infiltrating neuronalcells to contact the signals on all sides and at all timesduring the infiltration process.

Other approaches to the presentation of bioactive adhe-sion-promoting peptides have been to include thebioactive peptide sequences in the backbone of the poly-mer chains (e.g. in polypeptide biomaterials). For example,elastin-like polypeptides have been developed to produceelastic, protein-based biomaterials that contain adhesionsites, such as the RGD tripeptide, engineered within thepolypeptide sequence [16].

Bioactivity by incorporation of growth factorsPolypeptide growth factors are powerful regulators of a vari-ety of cellular behaviours, including cell proliferation,migration, differentiation, and protein expression, andthese molecules are being developed as important thera-peutics in tissue regeneration (e.g. in closing bone defectsand in healing chronic ulcers in the skin). Growth factorsare also being explored as key components of biomaterialsand biomaterial systems, as discussed in the illustrativeexamples below. Why is it useful to consider growth factorsas part of a system involving biomaterials, rather than ontheir own? The biological activity of the growth factordepends not only upon its identity, but also upon how it ispresented to the cells in space and over time. For example,it has been demonstrated that some growth factors are moreeffective when provided to cells through a controlledrelease process, whereas others are more effective whenpresented as a bolus [17]. This difference in behaviour maybe related to how the cells traffic and recycle their receptorsfor these growth factors, and it may be possible to modulatetrafficking and recycling by altering either the growth fac-tor or its interactions with a biomaterial that is releasing orpresenting it. Under the natural conditions of the body, theextracellular matrix plays a role in storing, displaying andreleasing growth factors. Given that the natural biomaterialof the body plays this important role, it seems reasonable toexplore this and related behaviour with biomaterials, tomimic this function of the extracellular matrix.

Controlled release systems have been developed forgrowth factors, for example, based on traditional bioma-terials in delivering angiogenic growth factors in vascularrepair [18] or in delivering neuronal survival and differ-entiation factors in neurodegenerative diseases [19].

Furthermore, specialised biomaterials have been devel-oped to incorporate and modulate these importantbiologically active molecules. For example, with the goalof bone repair by delivery of the growth factor transform-ing growth factor β, the affinity of the growth factor forheparin has been exploited [20••]. Many such growth fac-tors bind heparin, as well as heparin sulphateproteoglycans in the extracellular matrix. To exploit thisbinding affinity, heparin was conjugated to collagenmatrices used in bone repair, and this immobilisedheparin served as an affinity site to bind and slowlyrelease the growth factor in the healing site. As an exten-sion of this approach, the growth factor was chemicallyconjugated to the collagen matrix via a poly(ethylene gly-col) spacer. This growth factor was biologically active inthe immobilised state, presumably being able to bind tothe receptors on the surface of the cell and permit recep-tor dimerisation and signal transduction [21••]. Theability of immobilised growth factors to be biologicallyactive has also been demonstrated in the very well-char-acterised system of epidermal growth factor conjugated tosynthetic polymer surfaces, where it was shown to becapable of directing hepatocytes to maintain their liver-specific morphology and function [22••].

Exciting advances have been made with the display anddelivery of precursors for polypeptide growth factors, as wellas the growth factors themselves. It has been demonstratedrecently that plasmid DNA can be efficiently taken up andthe encoded gene expressed when the plasmid is presentedon the surface of a biomaterial. This has been shown in thecontext of smooth and cardiac muscle cells by presentationof plasmid DNA on sutures [23••], as well as in the contextof bone repair, in which fibroblasts in the repair tissue tookup the plasmid DNA encoding the growth factor bone mor-phogenetic protein-4, and by expression of the growth factorenhanced bone formation [24••]. Cellular and DNA interac-tions with the biomaterial appear to be important indetermining the extent of uptake by the cells in the repairsite; as such, the DNA-presenting biomaterial surfacebecomes a bioactive biomaterial that expresses its bioactivi-ty for long periods of time by affecting the behaviour of thecells that come in to contact with the biomaterial.

Bioactivity by physicochemically basedbiological recognitionSome biological interactions are based on physicochemicalinteractions that are less specific than those described inthe previous sections, such as adsorption due to electrosta-tic interactions, which can be readily mimicked andincorporated into bioactive biomaterials. Heparin’s electro-statically dominated interactions in anticoagulationpresent one example, and extensive work has been per-formed at immobilisation of heparin on biomaterials torender them bioactive [25]. Extensive work has also beenperformed to obtain other polymers that possess heparin-like activity, for example, by screening dextrans withvarying extents of suphonation [26,27] or by identification

Bioactive biomaterials Hubbell 125

of plant polysaccharides with a fortuitously appropriatecharge density and structure, as in the example of fucan[28]. Such biological activity can be extended beyond theanticoagulant activity of heparin (i.e. binding to the pro-teins antithrombin III and thrombin to catalyse complexformation between these two proteins), to other activitiesof heparin, such as binding to growth factors or interferingwith a growth factor’s binding to its receptor [29]. It isinteresting to note that similar biological activity can beobtained completely outside the platform of a polysaccha-ride chain (as seen with heparin) for charge presentation.For example, with appropriate degrees of suphonation,polyurethanes can function as heparin-like water-solublepolymers [30].

Novel biomaterials for biosensing have recently beendeveloped by combination of less specific activity based onphysicochemical interactions and highly specific activitybased on enzymatic activity. Synthetic polymer hydrogelshave been developed as three-dimensional conductors,with redox active charge transfer centres chemically incor-porated within the gel network [31••,32••]. Redoxenzymes, such as glucose oxidase, can be incorporated andeven chemically bonded throughout the network to providefor efficient electron transfer from the enzyme’s active site,through the gel by transfer from site to site, to a collectionelectrode to form a concentration-dependent sensor for glu-cose. Thus, careful combination of individual biomaterialproperties, such as polymer chain and network dynamics(which controls charge transfer through the gel by modulat-ing transient close approaches to within electron tunnellingdistances between immobilised charge transfer sites) andenzyme coupling (which controls charge transfer from theenzyme to the gel), can lead to a biomaterial with a highlyspecialised and selective biological activity.

Bioactivity by incorporation of enzymaticrecognition sitesThe two sections above, dealing with incorporated adhe-sion and growth factors, addressed the transmission ofbiological information from a biomaterial to the neighbour-ing cells. One can also consider the other direction, inwhich the biomaterial is the recipient of information pro-duced by cells. One such form of information is enzymaticactivity associated with the cell surface during cell migra-tion. Cell migration through collagen [33] and fibrin [34]gels, both natural biomaterials involved in the generation,remodelling and regeneration of tissues, has been exploredand is known to depend mainly upon the sensitivity of thematerial to proteases produced by the cells, upon theamount of enzyme produced by the cells, and upon theamount of material to be remodelled by the cells as theymigrate through the material.

Approaches have been developed to engineer biomaterialsthat can be remodelled by cells through cell-associatedenzymatic activity. Cells naturally remodel the extracellu-lar matrix in development, adaptation and healing, and

materials that are subject to the remodelling activities ofthe cell may enable exploitation of these biological activi-ties in tissue engineering. For example, a fascile route tothe chemical incorporation of bioactive signals has beendeveloped for fibrin, a natural biomaterial matrix that canbe remodelled proteolytically. In this modification scheme,exogenous peptides bear in one domain a substrate for thetransglutaminase involved in coagulation, factor XIIIa, andare thus covalently conjugated to the fibrin network as itforms, incorporating the bioactive peptide within the gel.Another domain of the peptide bears a bioactive peptide,for example, with cell adhesion or growth factor bindingactivity [35••]. Through such a route, it is possible to incor-porate the biological activity of a host of non-fibrinproteins (e.g. laminin) as synthetic components added intothe platform of the biologically-derived fibrin gel.

Completely synthetic biomaterials have been designedthat are proteolytically degradable and that compriseother bioactive components as well. Gels have beenformed based on poly(ethylene glycol) chains comprisingcentral oligopeptides, sites that are substrates for colla-genase or plasmin, both of which are involved in cellmigration [36••]. These water-soluble hybrid chains maythen be coupled at their termini to form three-dimen-sional elastic gels that are completely synthetic, butwhich are also degradable by cell-associated enzymaticactivity. Additional biological activity can be conferredupon the proteolytically remodable gels by copolymeri-sation of suitably reactive oligopeptides, such asterminally reactive poly(ethylene glycol) grafted withthe adhesion peptide RGD [37]. It is thus possible withthese approaches to construct materials that possess alarge number of the characteristics of the natural extra-cellular matrix, but which are totally synthetic andcapable of fascile manufacture and customisation.

Bioactivity by material transformationBiomaterials can possess biological activity (i.e. activity in abiologically relevant context) via the ability to be trans-formed from one state to another. Several examples havealready appeared and some have already found clinical utili-ty, and selected examples are presented below. The reader isdirected elsewhere for a review exclusively on this topic [38].

Materials that undergo phase transformations have a greatdeal of potential for use in surgery, for example, as adhe-sives, sealants, and barriers to cell–tissue contact. Awater-soluble macromer has been developed based onpoly(ethylene glycol) central blocks with oligo(lactic acid)flanking blocks and terminal acrylates; the large centralblock provides water solubility, the flanking oligoesterdegradability, and the terminal sites of unsaturation poly-merisability [39]. These soluble materials can be rapidlytransformed into elastic hydrogels by exposure to light inthe presence of suitable photoinitiators, such as eosin yel-lowish. These materials have been employed to blockblood platelet adhesion to blood-vessel surfaces after


injury to thereby improve the postsurgical healing of thevessel [37,40], and to serve as a barrier and depot for localdrug delivery to prevent the formation of scar tissue adhe-sions between organs after gynaecological surgery [41].Photocurable, and photolysable, hydrogels have also beendeveloped based on cinnamylidene acetate dimerisation,rather than acrylate polymerisation as in the examplesabove [42,43]. Materials that can be converted from liquidsinto elastic gels by reaction after mixing of two liquidshave also been developed, for example, by reaction ofN-hydroxysuccinimidyl activated esters on the end of adifunctional poly(ethylene glycol) with amines on the endof a tetrafunctional poly(ethylene glycol) [44]. Thisscheme has also been employed to synthesise enzymati-cally degradable gels, by using an aminated hyaluronic acidchain as one component of the two-liquid system(D Aeschlimann, personal communication). All of the reac-tions described above (i.e. gel formation by photoinitiationor by mixing of two liquids) can be sufficiently gentle to becarried out in vivo. For example, photopolymerisation ofdegradable poly(ethylene glycol) acrylates is currentlyused to seal leaking tissue surfaces after surgery in man.

A second sort of material transformation that can be usedto modulate biological interactions is gel swelling or col-lapse in response to temperature changes. Materialchemistries that can be employed to lead to polymers dis-playing lower critical solution behaviour (giving gels onwarming of liquids) or upper critical solution behaviour(giving gels on cooling of liquids) have been described[45]. Such materials have been used as injectable depotsfor drug delivery (after a liquid-to-solid transformation)[46], as reversible cell culture substrates upon which cellmonolayers and multilayers can be cultured and thenreleased intact for transplantation (after a hydrophobic-to-hydrophilic transformation) [47], and to block aligand-binding site on a protein in a thermally regulatablemanner (after an extended-chain-to-collapsed-chain trans-formation) [48,49]. Such transformations can also beobtained in protein solutions, also based on changes intemperature, as well as changes in salt concentration. Forexample, self-assembly can be obtained in protein basedon silk and elastin to form gels [50], as well as by electro-static interactions in other designed oligopeptides andpolypeptides employing leucine zipper domains [51] andionic self-complementary domains [52].

As a final example of approaches toward bioactivity throughmaterial transformation, schemes to develop biomaterialmonolayers as therapeutics have been developed. In theexamples on photopolymerisable hydrophilic macromersdiscussed above, a therapeutic goal was blockade of cellinteractions with a tissue surface after injury associatedwith surgery. This might also be obtainable with polymermonolayers, only nanometers thick, rather than gels tens ofmicrons thick. This goal has been approached by the designof multifunctional polymers bearing domains that bind totissue surfaces based on electrostatic interactions via a

backbone of poly(lysine), in addition to other domains thatrepel the binding of proteins and cells via pendant blocks ofpoly(ethylene oxide) [53••,54]. These graft co-polymerswere demonstrated to bind to complex biological surfaces,through affinity for their net-negative charge, and stericallystabilise them against subsequent adhesion, such as pre-venting lectin-induced agglutination of red blood cells[53••] or preventing the formation of postoperative adhe-sions [54]. Such approaches of assembly on biologicalsurfaces have been taken even further, to actually chemi-cally graft amine reactive poly(ethylene glycol) derivativesto lysine residues in proteins on tissue surfaces to inhibitcell adhesive interactions after surgical tissue damage,where such chemical grafting is performed under condi-tions sufficiently mild to permit grafting in vivo [55].

ConclusionsBiological activity has played an important role in modernbiomaterials development, employing the principles ofbiological recognition that are used so frequently in phar-maceutical design in addition to other morematerial-centric principles, for example, those that permitmaterial to respond to external stimuli. Only recently havethese novel bioactive biomaterials begun to make clinicalimpact, but given the relatively long cycle from concept toclinic this is to be expected. Indeed, it is probable that theconcepts of bioactivity reviewed here, as well as others,will make much more direct clinical impact in the culturelaboratory and clinic in the next few years. It is clear thatthe expenses associated with development and regulatoryapproval of products based on bioactive biomaterials willbe higher than that of products based on traditional bio-materials, and as such these development activities mustbe targeted at economically important applications withlargely unmet needs, where advantages in safety and effi-cacy associated with bioactivity compensate favourably forthe higher cost of development and regulatory approval ofthe bioactive product.

References and recommended readingPapers of particular interest, published within the annual period of review,have been highlighted as:

• of special interest••of outstanding interest

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2. Hubbell JA: Biomaterials in tissue engineering. Biotechnology1995, 13:565-576.

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6. Massia SP, Hubbell JA: An RGD spacing of 440 nm is sufficient forintegrin avb3-mediated fibroblast spreading and 140 nm for focalcontact and stress fiber formation. J Cell Biol 1991, 114:1089-1100.

7. Massia SP, Rao SS, Hubbell JA: Covalently immobilized lamininpeptide Tyr-Ile-Gly-Ser-Arg (YIGSR) supports cell spreading andco-localization of the 67-kilodalton laminin receptor with a-actininand vinculin. J Biol Chem 1993, 268:8053-8059.


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This paper provides very convincing evidence of a direct link between celladhesion strength and cell migration rates, providing for the biomaterials engi-neer important guidance that too much of a good thing (e.g. an adhesion lig-and) can be bad (e.g. if cell migration was an important performance criterion).

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and vein grafts. ASAIO J 1997, 43:M515-521.This clinical study with endothelial cell seeded vascular grafts demonstrat-ed that graft performance in high flow regions (where stresses on theattached endothelial cells were greatest) was better when the biomaterialmatrix included the adhesion protein fibronectin. Although the study wasperformed with a complete protein, rather than an engineered peptide, itprovides direct clinical evidence of the need, and probability of success,of such approaches.

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49:291-298.The investigators employed the affinity of the important growth factortransforming growth factor β for heparin to develop clinically relevantbioactive biomaterials for bone regeneration. Heparin, grafted to the col-lagen substrate, served as a docking and release site to provide fordelayed release. When cells migrate into such matrices, they may alsorelease the growth factor by local enzymatic action, as occurs naturally inthe extracellular matrix.

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polyethylene glycol. J Biomed Mater Res 1998, 39:539-548.This report, along with [22••], provides evidence that covalently immobilisedgrowth factor, in this case transforming growth factor β, can be presented tocells in a bioactive manner. This was not clear a priori, in that the ligand-bound receptors must presumably dimerise in the plane of the membrane inorder to express biological activity.

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27. Maaroufi RM, Jozefowicz M, Tapon-Bretaudiere J, Jozefonvicz J,Fischer AM: Mechanism of thrombin inhibition by heparin cofactorII in the presence of dermatan sulphates, native or oversulphated,and a heparin-like dextran derivative. Biomaterials 1997,18:359-366.

28. Logeart D, Prigent-Richard S, Jozefonvicz J, Letourneur D: Fucans,sulfated polysaccharides extracted from brown seaweeds, inhibitvascular smooth muscle cell proliferation. I. Comparison withheparin for antiproliferative activity, binding and internalization.Eur J Cell Biol 1997, 74:376-384.

29. Bagheri-Yarmand R, Kourbali Y, Mabilat C, Morere JF, Martin A, Lu H,Soria C, Jozefonvicz J, Crepin M: The suppression of fibroblastgrowth factor 2/fibroblast growth factor 4-dependent tumourangiogenesis and growth by the anti-growth factor activity ofdextran derivative (CMDB7). Br J Cancer 1998, 78:111-118.

30. Silver JH, Hart AP, Williams EC, Cooper SL, Charef S, Labarre D,Jozefowicz M: Anticoagulant effects of sulphonated polyurethanes.Biomaterials 1992, 13:339-344.

31. Csoregi E, Schmidtke DW, Heller A: Design and optimization of a•• selective subcutaneously implantable glucose electrode based

on ‘wired’ glucose oxidase. Anal Chem 1995, 67:1240-1244.This pair of papers (with [32••]) demonstrates that redox enzymes can bechemically conjugated into conducting polymer gels to obtain efficient elec-tron transfer from the enzyme’s active site to the gel and ultimately to a col-lecting electrode. This permitted extremely highly sensitive and fasttransduction of a glucose concentration into an amperometric signal. Thishigh level of bioactivity was obtained by combining several features of bioac-tivity in a single material.

32. Wagner JG, Schmidtke DW, Quinn CP, Fleming TF, Bernacky B,•• Heller A: Continuous amperometric monitoring of glucose in a

brittle diabetic chimpanzee with a miniature subcutaneouselectrode. Proc Natl Acad Sci USA 1998, 95:6379-6382.

See annotation to [31••].

33. Kuntz RM, Saltzman WM: Neutrophil motility in extracellular matrixgels: mesh size and adhesion affect speed of migration. Biophys J1997, 72:1472-1480.

34. Herbert CB, Bittner GD, Hubbell JA: Effects of fibrinolysis onneurite growth from dorsal root ganglia cultured in two- andthree-dimensional fibrin gels. J Comp Neurol 1996, 365:380-391.

35. Schense JC, Hubbell JA: Cross-linked exogenous bifunctional•• peptides into fibrin gels with factor XIIIa. Bioconj Chem 1999,

10:75-81.Fibrin is a very important biomaterial, both in natural wound healing and as atherapeutic. It possesses only one set of biological signals, and thus obtainsgenerally only one sort of healing response, namely a scar. The investigatorspresent a simple approach by which to functionalise fibrin with exogenous


bioactive peptide signals, by using bi-domain peptides, one peptide con-taining a substrate site for the coagulation transglutaminase factor XIIIa andthe other containing a bioactive peptide of interest. This opens a fascileroute to make fibrin clots with customised biological activity.

36. West JL, Hubbell JA: Polymeric biomaterials with degradation sites•• for proteases involved in cell migration. Macromolecules 1999,

32:241-244.This report, although preliminary in nature, provides an example of a totallysynthetic biomaterial gel that can be degraded by cell-associated proteases.Given that adhesion ligands can also be incorporated into this material [37],this opens the door for synthetic materials that can be invaded and remod-elled by cells in a healing response, in complete analogy with the way tissuesare naturally remodelled.

37. Hern DL, Hubbell JA: Incorporation of adhesion peptides intononadhesive hydrogels useful for tissue resurfacing. J BiomedMater Res 1998, 39:266-276.

38. Hubbell JA: In situ material transformations in tissue engineering.Mat Res Soc Bull 1996, 21:33-35.

39. Desai NP, Hubbell JA: Surface physical interpenetrating networksof poly(ethylene terephthalate) and poly(ethylene oxide) withbiomedical applications. Macromolecules 1992, 25:226-232.

40. West JL, Hubbell JA: Separation of the arterial wall from bloodcontact using hydrogel barriers reduces intimal thickening afterballoon injury in the rat: the roles of medial and luminal factors inarterial healing. Proc Natl Acad Sci USA 1996, 93:13188-13193.

41. Chowdhury SM, Hubbell JA: Adhesion prevention with ancrodreleased via a tissue-adherent hydrogel. J Surg Res 1996, 61:58-64.

42. Andreopoulos FM, Deible CR, Stauffer MT, Weber SG, Wagner WR,Beckman EJ, Russell AJ: Photoscissable hydrogel synthesis viarapid photopolymerization of novel PEG-based polymers in theabsence of photoinitiators. J Am Chem Soc 1996, 118:6235-6240.

43. Andreopoulos FM, Beckman EJ, Russell AJ: Light-induced tailoringof PEG-hydrogel properties. Biomaterials 1998, 19:1343-1352.

44. Zhao X, Harris JM: Novel degradable poly(ethylene glycol)hydrogels for controlled release of protein. J Pharm Sci 1998,87:1450-1458.

45. Chen G, Hoffman AS: Graft copolymers that exhibit temperature-induced phase transitions over a wide range of pH. Nature 1995,373:49-52.

46. Jeong B, Bae YH, Lee DS, Kim SW: Biodegradable blockcopolymers as injectable drug-delivery systems. Nature 1997,388:860-862.

47. von Recum H, Kikuchi A, Okuhara M, Sakurai Y, Okano T, Kim SW:Retinal pigmented epithelium cultures on thermally responsivepolymer porous substrates. J Biomater Sci Polym Ed 1998,9:1241-1253.

48. Stayton PS, Shimoboji T, Long C, Chilkoti A, Chen G, Harris JM,Hoffman AS: Control of protein-ligand recognition using a stimuli-responsive polymer. Nature 1995, 378:472-474.

49. Lu ZR, Kopeckova P, Wu Z, Kopecek J: Functionalizedsemitelechelic poly[N-(2-hydroxypropyl)methacrylamide] forprotein modification. Bioconjug Chem 1998, 9:793-804.

50. Cappello J, Crissman JW, Crissman M, Ferrari FA, Textor G, Wallis O,Whitledge JR, Zhou X, Burman D, Aukerman L, Stedronsky ER: In-situ self-assembling protein polymer gel systems foradministration, delivery, and release of drugs. J Controlled Release1998, 53:105-117.

51. Petka WA, Harden JL, McGrath KP, Wirtz D, Tirrell DA: Reversiblehydrogels from self-assembling artificial proteins. Science 1998,28:389-392.

52. Leon EJ, Verma N, Zhang S, Lauffenburger DA, Kamm RD:Mechanical properties of a self-assembling oligopeptide matrix.J Biomater Sci Polym Ed 1998, 9:297-312.

53. Elbert DL, Hubbell JA: Self-assembly and steric stabilization at•• heterogeneous, biological surfaces using adsorbing block

copolymers. Chem Biol 1998, 5:177-183.Most drugs operate by highly specific biological recognition. In this case, asoluble bioactive polymer was developed that recognised its target with rel-atively low specificity, via binding of a poly(cation) to net negatively chargedcell surfaces. Grafted poly(nonion) chains then sterically repelled high affin-ity biological interactions, providing an example of drug-like bioactivity viaphysicochemical mechanisms.

54. Elbert DL, Hubbell JA: Reduction of fibrous adhesion formation bya copolymer possessing an affinity for anionic surfaces. J BiomedMater Res 1998, 42:55-65.

55. Deible CR, Beckman EJ, Russell AJ, Wagner WR: Creatingmolecular barriers to acute platelet deposition on damagedarteries with reactive polyethylene glycol. J Biomed Mater Res1998, 41:251-256.


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