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High-throughput screening for biologicallyactive compounds has so far been restrictedmostly to soluble molecules. In this issue,Anderson et al.1 describe an early exampleof screening combinatorial biomaterials inan array format. The array was applied tosearch for materials that would allow con-trolled differentiation of human embryonicstem (ES) cells, suggesting the potentialbroad utility of the technology for investi-gating material-cell interactions.

High-throughput screening has had asignificant impact on modern biology anddrug discovery. It has enabled the identifi-cation of heterocyclic small molecules thatcan direct ES cells toward particular line-ages, such as cardiomyocytes2, and even ofmolecules that can induce the dedifferenti-ation of adult cells into pluripotent pro-genitor cells3. For molecules that are eithercompetitive inhibitors of cell-surface recep-tors or that diffuse through the cell mem-brane to directly regulate transcription, thescreening of soluble molecules makes agreat deal of sense. However, these methodsmay overlook other broad classes of bio-active molecules; biomaterials scienceapproaches may address some of these limi-tations and thus broaden the biomolecularscreening space.

Biomaterials science and high-through-put screening of chemical or biologicalentities could overlap in two main ways: inone, the chemical diversity of the biomater-ial itself could be sampled to discover abioactive biomaterial per se; in another, themolecular diversity of a chemical or proteinlibrary could be sampled in the context of abiomaterial platform.

To generate a diverse biomaterial library,Anderson et al. combinatorially mixed 24acrylate co-monomers—varying in pola-rity, chain length and branching—in a mul-tiwell format and polymerized them using afree radical initiator to form a library ofnearly 600 materials. Here one notes an

additional challenge in designing combina-torial materials libraries compared withsmall-molecule libraries, namely, that thelibrary components be mutually reactive toform a library of individual macromolecu-lar materials. In this case, the co-monomerswere sufficiently similar in their reactivitytoward free-radical polymerization to formco-polymers with similar composition tothe co-monomer mixing ratio.

Even the relatively small space of chemi-cal diversity explored by these investigatorsyielded surprising results: specific materialsinduced ES-cell differentiation towardepithelial cells much more effectively thanothers. As no biologically active specieswere included in the co-monomer libraryitself, it is likely that these effects weremediated through an adsorbed proteinlayer, which was somehow modulated bythe detailed character of the biomaterialsubstrate. Indeed, combinatorial materialslibraries have been explored in controlling

protein adsorption, for example, in thestudy of blood plasma protein adsorptionusing a polyarylate co-polymer library4.

In addition to using high-throughputscreening to search for biomaterials that arebioactive themselves, one can also imagineusing biomaterials as display platforms forsurface-bound combinatorial libraries. Asmentioned above, most research usinghigh-throughput screening has addressedlibraries of soluble molecules. In contrast,many natural bioactive species function inan immobilized form, for example, boundto the extracellular matrix or to the surfaceof an adjacent cell. When presented in soluble form, these species, which normallywould have been active in immobilizedform, may behave very differently or per-haps even lose biological activity altogether.Furthermore, the functions of many bio-logically active molecules are modulated by other signals, including the biomolecu-lar and even biophysical context provided

828 VOLUME 22 NUMBER 7 JULY 2004 NATURE BIOTECHNOLOGY

Biomaterials science and high-throughput screeningJeffrey A Hubbell

The effects of materials on the behavior of cells can be rapidly assessed using combinatorial polymer arrays.

Jeffrey A. Hubbell is at the Institute forBiological Engineering and Biotechnology,Swiss Federal Institute of Technology,E.P.F.L. CH-1015 Lausanne, Switzerland.e-mail: jeffrey.hubbell@epfl.ch

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Figure 1 High-throughput screening can affect biomaterials science, and biomaterials science canpotentially affect high-throughput screening of drug candidates. (a) Members of a relatively smalllibrary of co-monomers are co-polymerized to produce a much larger library of polymers. Thesebiomaterial candidates can then be screened for biological activity. (b) The biomaterial is used toprovide an extracellular milieu of biomolecular and biophysical signals, which provide a context inwhich biological responses to members of a large drug candidate library can be monitored. The drugcandidates can be soluble within the biomaterial extracellular milieu (upper), or bound to it as is thecase with many natural signals in vivo (lower).

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by the extracellular matrix. As such, alibrary screen with a high-level cellularread-out may overlook many hits, becausebiological activity would have been presentonly in the context of a particular extracel-lular milieu.

The functions of the extracellular matrixin interpreting soluble biological signalsand displaying bound biological signalssuggest a second way of using biomaterialsapproaches in high-throughput screening:by presenting the library bound to the bio-material or by screening the library usingcells present within a particular context asprovided by the biomaterial. It is clear thata two-dimensional, rigid substrate of poly-styrene, as is typically used in multiwellplates, is a relatively poor mimic of thethree-dimensional elastic hydrogel networkthat is the natural extracellular matrix.

Other approaches that are conceptuallysimilar to the co-polymerization approachof Anderson et al. permit broad mutualreactivity of biomaterial components thatcan be mixed and matched to form tailoredextracellular matrix analogs5. Cells incor-

porated in such a matrix can be exposed tocombinatorial libraries, which are solubi-lized or even bound to the matrix, to screenfor active components. Another powerfulapproach involves nanofibrillar materials

formed by self-assembly of peptides6. Thesematerials present another means of incor-porating bioactive signals into a three-dimensional extracellular matrix analog.Related materials have been used to diff-erentiate neural progenitor cells into

neurons7. In their example, Anderson et al.observed such context-dependent sig-naling: with some materials, the bioactive factor retinoic acid could display itsexpected activity, and with others it dis-played activity that was almost opposite tothat expected.

To date, most work on high-throughputscreening with read-outs based on cellularfunction has largely ignored the biologi-cal context in which the cells are screenedand in which the bioactive molecules aredisplayed. Anderson et al. presents an ex-ample in which biomaterials science canchange this, and many other examples willsurely follow.

1. Anderson, D.G., Levengerg, S. & Langer, R. Nat.Biotechnol. 22, 863–866 (2004).

2. Wu, X., Ding, S., Ding, G., Gray, N.S. & Schultz, P.G.J. Am. Chem. Soc. 126, 1590-1591 (2004).

3. Chen, S.B., Zhang, Q.S., Wu, X., Schultz, P.G. & Ding,S. J. Am. Chem. Soc. 126, 410-411 (2004).

4. Weber, N., Bolikal, D., Bourke, S.L. & Kohn, J. J. Biomed. Mater. Res. A 68A, 496-503 (2004).

5. Lutolf, M.P., Raeber, G.P., Zisch, A.H., Tirelli, N. &Hubbell, J.A. Adv. Mater. 15, 888–892 (2003).

6. Zhang, S. Nat. Biotechnol. 21, 1171–1178 (2003).7. Silva, G.A. et al. Science 303, 1352-1355 (2004).

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Even the relatively small spaceof chemical diversity exploredby these investigators yieldedsurprising results: specificmaterials induced ES-celldifferentiation toward epithelialcells much more effectivelythan others.

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