dawnlight 2008-06-23 16:37
Designing materials for biology and medicine
Designing materials for biology and medicine [url=http://www.nature.com/nature/journal/v428/n6982/full/nature02388.html]Designing materials for biology and medicine[/url]
Robert Langer1 and David A. Tirrell2
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Abstract
Biomaterials have played an enormous role in the success of medicaldevices and drug delivery systems. We discuss here new challenges anddirections in biomaterials research. These include syntheticreplacements for biological tissues, designing materials for specificmedical applications, and materials for new applications such asdiagnostics and array technologies.
Biomaterials have been defined as substances other than foods or drugscontained in therapeutic or diagnostic systems1 and, in some cases,have been described as materials composed of biologically derivedcomponents (for example, amino acids) irrespective of theirapplication. Throughout history, biomaterials have played an importantrole in the treatment of disease and the improvement of health care.Early biomaterials include metals such as gold that were used indentistry over 2,000 years ago. Other early examples of biomaterialsinclude wooden teeth and glass eyes2. However, with the advent ofsynthetic polymers at the end of the nineteenth century, thesematerials became increasingly used in health care. For example,polymethylmethacrylate, PMMA, was used in dentistry in the 1930s1 andcellulose acetate was used in dialysis tubing2 in the 1940s. Dacron wasused to make vascular grafts; polyether-urethanes, the materials usedin ladies' girdles, were used in artificial hearts; and PMMA andstainless steel were used in total hip replacements1, 2. Naturallyoccurring materials such as collagen have also been used asbiomaterials3. However, in nearly every case, these materials wereadopted from other areas of science and technology without substantialredesign for medical use. Although these materials helped usher in newmedical treatments, critical problems in biocompatibility, mechanicalproperties, degradation and numerous other areas remain. To this end,scientists are creating new materials including those with improvedbiocompatibility, stealth properties, responsiveness (smart materials),specificity and other critical properties. Modern biomaterials scienceis characterized by a growing emphasis on identification of specificdesign parameters that are critical to performance, and by a growingappreciation of the need to integrate biomaterials design with newinsights emerging from studies of cell–matrix interactions, cellularsignalling processes, and developmental and systems biology.
Biomaterials are already having an enormous effect on medicine.Controlled drug delivery systems that largely involve polymers4 areused by tens of millions of people annually5. Recent examples arepolymer-coated stents, which have recently been approved both in Europeand the United States. Hundreds of thousands of lives are expected tobe saved each year6. In addition, various controlled release systemsfor proteins, such as human growth hormone, as well as moleculesdecorated with polyethylene glycol (PEG), such as pegylatedinterferon4, 5, 6, 7, have recently been approved by regulatoryauthorities, and are showing how biomaterials can be used to positivelyaffect the safety, pharmacokinetics and duration of release ofimportant new drugs. Another area where biomaterials have recently hadan impact is in tissue engineering. By combining polymers withmammalian cells, it is now possible to make skin for patients who haveburns or skin ulcers, and various other polymer/cell combinations arein clinical trials, including corneas, cartilage, bone and liver8.Biomaterials have also had a major impact as the central components ofdental implants, sutures, and numerous medical devices2.
Here we describe novel material concepts that are shaping futuredirections in biomaterials science. In particular, we discuss (1)creating synthetic replacements for biological tissues using naturallyoccurring building blocks, (2) synthesizing materials using man-madebuilding blocks for specific medical and biological applications, and(3) design concepts for new in vitro applications such as diagnosticsand array technologies.
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[color=Red]Synthetic replacements for biological tissues[/color]
Materials composed of naturally occurring (biologically derived)building blocks, including extracellular matrix (ECM) components, arebeing studied for applications such as direct tissue replacement andtissue engineering. The ECM, a complex composite of proteins,glycoproteins and proteoglycans, provides an important model forbiomaterials design9. ECM-derived macromolecules (for example,collagen) have been used for many years in biomaterials applications3,and it is now possible to create artificial analogues of ECM proteinsusing recombinant DNA technology10. Through the design and expressionof artificial genes, it is possible to prepare artificial ECM proteinswith controlled mechanical properties and with domains chosen tomodulate cellular behaviour. This approach avoids several importantlimitations encountered in the use of natural ECM proteins, includingbatch-to-batch (or source-to-source) variation in materials isolatedfrom tissues, restricted flexibility in the range of accessiblematerials properties, and concerns about disease transmissionassociated with materials isolated from mammalian sources.
Elastin-based systems have been of special interest in this regard.Urry and co-workers have shown over many years that simple repeatingpolypeptides related to elastin can be engineered to exhibit mechanicalbehaviour reminiscent of the intact protein11. Crosslinking can beaccomplished via radiative12 or chemical13 means, and electrospinninghas been used to prepare fibrous forms of engineered elastins14 (Fig.1). Incorporation of cell-adhesion ligands allows attachment andspreading of cultured cells, and in the specific case of materials forvascular grafts, retention of endothelial cell adhesion in the face ofshear stresses characteristic of the normal circulation15.
The promise of biosynthetic approaches to biomaterials design must beweighed against the fact that very little is known about the in vivoperformance of systems prepared in this way. Animal experiments onelastin-like polypeptides prepared by chemical synthesis have shownthese materials to evoke relatively mild tissue reactions, but therange of materials investigated to date has been small16. As more islearned about the mechanical properties of such materials and abouttheir interactions with cells in culture, the groundwork will be laidfor more extensive evaluation in animal models. Recent progress in thedevelopment of methods for incorporation of non-natural amino acidsinto recombinant proteins points the way to an alternative strategy forpreparing artificial ECM proteins with diverse chemical, physical andbiological properties17.
Substantially more experience has been gained in evaluating the in vivoperformance of engineered biomaterials based on polysaccharides.Alginate hydrogels bearing cell-adhesion ligands have been used asscaffolds for cell encapsulation and transplantation, and have yieldedpromising results in experiments directed towards the engineering ofbone tissue capable of growth from small numbers of implanted cells18.The prospect of growing tissues from small numbers of precursor cellsis an attractive alternative to harvesting and encapsulating large cellmasses before transplantation.
Molecular self assembly of peptides or peptide-amphiphiles may alsolead to unique biomaterials. A number of self assembled peptide systemshave been developed, including systems that can potentially be used intissue engineering and nanotechnology19, 20.
An alternative to synthesizing polymers composed of natural componentsis the synthesis of biomimetic polymers, which combine the informationcontent and multifunctional character of natural materials (such as aparticular amino acid sequence that might be desirable for cellattachment) with the tailorability of a synthetic polymer, such ascontrol of molecular mass or polymer degradation, and the ability toimpart appropriate mechanical properties. An example of this concepthas been the synthesis of polymers composed of lactic acid and lysine.Like polylactic-glycolic acid, these polymers can be made to degrade atdesired times. However, by adding lysine as a co-monomer with lacticacid, a free amino acid is provided, which allows coupling reactions totake place and does not affect the overall biocompatibility of thepolymer21. By coupling specific amino acids (such as the tripeptidesequence RGD) to this polymer, cell adhesion can be regulated22.
Another strategy that has been used in modifying a variety of naturaland synthetic polymers has been the inclusion of PEG into the materialto reduce non-specific effects of protein adsorption and colloidalaggregation. The molecular origins of these phenomena are not yetthoroughly understood. One example of the effects of the PEG can beseen in the creation of polylactic-glycolic acid (PLGA) PEG diblockpolymers that, when formed into nanospheres, can, like cells, circulatein the body for long time periods. For example, in mice, only 30% ofPLGA-PEG nanoparticles were cleared after 5 h whereas 66% ofnon-PEG-containing PLGA nanoparticles were cleared in only 5 min (ref.23). Halstenberg et al.24 have adopted a related approach inengineering protein-based biomaterials, by grafting poly(ethyleneglycol) diacrylate onto an artificial protein that contained multiplecysteine residues. The protein was designed to serve several functions,including cell adhesion, heparin binding, and degradation by plasmin tofacilitate penetration by invading cells. The mechanical properties ofthe material were controlled by photopolymerization of the pendantacrylate units.
Another approach to creating a biomimetic reversible system is thecreation of an antigen responsive hydrogel. Corresponding antibodypairs are used to form reversible non-covalent crosslinks in apolyacrylamide system. In the presence of excess free antigen, thehydrogel swells, but in its absence, the gel collapses back to acrosslinked network. Swelling does not occur when foreign antigens areadded, showing that the system is antigen specific. Release of a modelprotein such as haemoglobin has been demonstrated in response tospecific antigens25.
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[color=Red]Materials for specific medical and biological applications[/color]
A variety of new materials are being synthesized from man-made buildingblocks, and being used to create devices for specific medicalapplications. One area of increasing attention has been the developmentof shape-memory materials that have one shape at one temperature andanother shape at a different temperature26. Such materials might permitnew medical procedures. For example, current approaches for implantingmedical devices often require complex surgery followed by deviceimplantation. However, with the development of minimally invasivesurgery, it is possible to place small devices inside the body usinglaproscopes. These types of surgical advances may create newopportunities to enable a bulky device to be implanted into the humanbody in a convenient way.
Shape-memory materials might provide such an opportunity, because theyhave the ability to memorize a permanent shape that can besubstantially different from an initial temporary shape. Thus bulkydevices could potentially be introduced into the body in a temporaryshape, like a string, that could go through a small laproscopic hole,but then be expanded on demand into a permanent shape (for example, astent, a sheet, and so on) at body temperature (Fig. 2). New polymershave been synthesized with this concept in mind, including phasesegregated multiblock co-polymers whose starting materials are knownbiocompatible monomers such as epsilon-caprolactone and p-dioxanone.Generally, these materials have at least two separated phases, eachwith thermal transition (glass or melting) temperatures. The phase withthe higher transition temperature is responsible for the permanentshape, whereas the second phase can act as a molecular switch andenable fixation of a temporary shape. By regulating temperatures aboveand below the second phase's transition temperature, shape can beshifted from one form to another. In addition to permitting newopportunities for implanting devices, these polymers have beendeveloped into sutures that are able to tie themselves on demand,triggered by a temperature change27 (Supplementary video 1).
Materials that are liquids at room temperature but that harden inresponse to a change in temperature or an external stimulus, such aslight, are also being studied28, 29, 30. Such systems offer anopportunity to inject materials containing drugs, for example, into thebody through a small needle, but still enable the formation of animplant.
A variety of smart gels have also been developed. These gels canrespond (and swell, for example) to triggers such as temperature or pH,or even specific molecules in the body such as glucose. Such systemsmay, with further study, be valuable in the treatment of disease: inthe case of diabetes, for example, smart gels could provide directfeedback control, allowing more insulin to be delivered in response toexcess glucose31.
It may be possible to change not only the bulk properties of materialsbut also their surface properties, using a simple 'switch' such astemperature or electric charge. For example, alkanethioates such as16-mercaptohexadecanoic acid, which has a hydrophobic chain capped by ahydrophilic carboxyl group, forms self-assembled monolayers on goldsurfaces. These chains have an upright equilibrium conformationpresenting the carboxylic groups to the surrounding medium. But whenthese alkanethioates are placed on gold at the correct density,application of an electric potential causes the carboxyl groups to beattracted to the gold surface electrostatically, causing the moleculesto reversibly rearrange and expose the hydrophobic chain (Supplementaryvideo 2). Such surface switches might offer new opportunities in suchareas as drug delivery, microfluidics and biosensors32.
The development of high-throughput approaches to create novelbiopolymers and screen them for various applications is garneringincreased attention. For example, Kohn and co-workers have createdpolymer libraries and then screened them for different applications33,34. This type of high-throughput approach has also been used in thecreation of gene therapy agents. For example, poly-beta-amino esterscan be synthesized in a high-throughput manner, and a number of thesenew polymers have been shown to have higher DNA transfection activitiesin cell-based assays than existing materials such as lipofectamine35,36.
Gene therapy represents an area where appropriate molecular design iscritical to achieving a successful outcome. Although viral vectors arehighly effective, their use has raised serious safety concerns. Thishas motivated research on synthetic gene therapy vectors, which,although safer, have thus far been much less effective than viralvectors. To be effective, there are a number of attributes that thematerial must possess, including the ability to condense DNA to sizesless than 150 nm so that it can be taken up by receptor-mediatedendocytosis, the ability to be taken up by endosomes in the cell and toallow DNA to be released in active form, and to enable it to travel tothe cell's nucleus37. An interesting example of the design of such newmaterials is provided by the cationic-cyclodextrin polymers developedby Davis and co-workers. Cyclodextrins are relatively non-toxic and donot elicit an immune response. When Davis and co-workers initially usedcyclodextrins for packaging DNA they found the resulting complexes tobe relatively unstable. To address this, they addedadamantane-conjugated polyethylene glycol to the surface of thecyclodextrin particles. This enabled the development of uniformly sizednanoparticles that resisted aggregation. By modifying the surface ofthe particles in this way, chemical groups could be exposed that couldattach other molecules and allow the particles to be targeted to, anddeliver genes to, specific cells38. Another novel approach for genetherapy involves creating a triplex where low-density lipoprotein isused for targeting and stearyl polylysine is used for DNA complexation.This approach has been used to deliver vascular endothelial cell growthfactor to heart muscle to aid in treating blockage of blood vessels39.With the advent of new gene therapy agents such as RNA interference40,the ability to design improved materials to deliver these agents willhave increasing importance.
Biomaterials are also being used to affect bioadhesion in novel ways,even enabling materials to be used to potentially deliver complexmolecules orally. It appears that polymers with high concentrations ofhydroxyl groups bind with the intestinal mucosa. For example,Mathiowitz and co-workers41 have designed polyanhydride nanoparticles(which can encapsulate DNA or other molecules, and which exposecarboxyl groups on their exterior as the polymer erodes), and haveshown that they can attach to mucous membranes and bind to theintestinal wall. Poly(fumaric-co-sebacic) anhydride showed greateradhesive forces than other materials tested, had a longer contact timewith cells in the intestine, and was able to pass through theintestinal wall with a protein inside it. Peppas and co-workers havedeveloped polymers that are not only bioadhesive, but also swell inresponse to a pH change. These polymers are able to protect proteinsfrom the acidic pH of the stomach, and release it in the more basic pHof the intestine. These materials also appear to temporarily openconnections between intestinal cells, allowing the proteins to passthrough42.
Microfabrication-based devices may also provide a novel approach forcreating a variety of new biomaterials and delivery systems. Forexample, silicon microchips have been engineered to contain over 100nanometre-sized drug-containing wells covered with gold on a chip 1 cmtimes 1 cm. By applying approximately 1 V to individually addressablewells, the drug in any of these wells can be released43. These types ofsystems have also been recently made with degradable materials (Fig.3)44.
Microfabrication has also been used in the creation of sensors. Forexample, small sensors are being used to measure intraocular pressurefor glaucoma patients45, and silicone microstimulators have beendeveloped that can be controlled by telemetry46 and are being used forretinal stimulation to aid in photoreceptor generation to treatdiseases of the back of the eye that cause blindness. Prausnitz andco-workers have developed microneedles that are able to penetrate intothe skin to depths far enough for drugs to enter the circulation, butshallow enough not to reach skin layers that contain nerves: thereforethey do not cause pain47. Microfabrication has also been used to createpolymer scaffolds containing an intricate vascular network48. All ofthese represent important ways of using micro- and nanotechnology tocreate potential new medical devices.