273 www.advmat.de www.MaterialsViews.com COMMUNICATION wileyonlinelibrary.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2012, 24, 273–278 Dr. M. Ebara, Dr. K. Uto, Dr. N. Idota, Dr. J. M. Hoffman, Dr. T. Aoyagi Biomaterials Unit International Research Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan E-mail: AOYAGI.Takao@nims.go.jp DOI: 10.1002/adma.201102181 Shape-memory materials are a class of “smart” materials that have the capability to change from a temporary shape to a memorized permanent shape upon application of an external stimulus. [1] Throughout the last decade, shape-memory poly- mers (SMPs) have represented a cheap and efficient alternative to well-known metallic shape-memory alloys as a consequence of the relative ease of manufacture and programming. [2,3] In particular, the use of SMPs as self-repairing or re-writable mate- rials has found growing interest in environmentally friendly technologies. [4] Although SMPs are usually capable of memo- rizing one temporary shape, dual-, triple-, or multi-shape memory effects were also achieved by introducing additional phase transitions. [5] Recent advances in nanotechnologies pave the way for engineering biomaterial surfaces that control cel- lular interactions from the nano- to micrometer scale. [6–9] The spatial and temporal control of extracellular signaling cues on nanopatterned surfaces is particularly attractive for inves- tigating fundamental mechanisms of adhesion-mediated cell signaling. [10,11] From these perspectives, we propose a novel technique that explores dynamic cell behavior in response to surface changes in nanotopology using SMSs that actuate on demand under biological conditions (Figure 1). Thermally induced SMPs are the most extensively investi- gated group of SMPs. The thermally induced shape-memory effect has been described for various polymers such as poly- urethanes, [12] poly(styrene-block-butadiene), [13] and polynor- bornene. [14] These materials are, however, non-degradable in physiological environments and many lack either biocompat- ibility or mechanical compliance. PCL is an important member of the aliphatic polyester family and is biocompatible. [15] PCL is also a semicrystalline polymer that has a melting tempera- ture (T m ) over which the mobility of polymer chains changes significantly, demonstrating a temperature-responsive “on–off” crystallinity transition. The crosslinked PCL, therefore, offers reversibly crystallizable regions that can fix a temporary shape and have dual-shape capability, showing a shape memory effect. [1] The use of T m as a crystallization triggering switch is favorable for creating temperature-responsive shape memory materials because the enthalpy change of the solid–liquid phase transition is much larger than that of a glass–rubber transition (T g ) [16] or a liquid crystalline transition (T LC ). [17] The relatively high T m of PCL around 60 °C, however, limits the potential of these substrates for biological applications. Therefore, there has been growing interest in the development of the SMPs that actuate near 37 °C. Although there have been many reports on thermoplastic polymers with shape-memory switching tem- peratures near body temperature, little progress has been made in PCL-based shape-memory systems that actuate sharply in a narrow range near body temperature. Incorporating rigid seg- ments or blending with other components are the most studied methods to decrease the T m of PCL. [18,19] The enthalpies (ΔH m ) observed around T m , however, are diminished, because T m is reduced by incorporation of non-PCL components that hinder crystallization. These results demonstrate the difficulty in devel- oping biocompatible PCL-based SMPs that actuate over narrow, physiologically relevant temperature ranges. As an alternative approach, we demonstrate the ability to modulate the T m of PCL materials by controlling the nano- architectures of cross-linked PCL, such as branched arm num- bers and molecular weight. [20–23] The preparation schemes of the one-component cross-linked PCL are shown in Scheme S1 (Supporting Information). As ε-caprolactone (CL) is polymer- ized from terminal hydroxy groups, a branched PCL with the desirable branched numbers and lengths can be easily synthe- sized using multivalent alcohol-containing compounds. The PCL macromonomers themselves form the netpoints and chain segments of branched PCL macromonomers are built from two arms of two different telechels linked by a junction unit. Since branch number represents a parameter to adjust crys- tallinity and mechanical properties of the polymer networks, various suitable combinations were examined. In this study, two-branched macromonomers and four-branched macromono- mers were simply mixed and cross-linked. They are abbreviated as XbY, where X is the number of branches and Y is the unit number of CL in each chain included in the feed. The thermal properties of the branched macromonomers before and after cross-linking are summarized in Table S1 (Supporting Infor- mation). Interestingly, the cross-linked 4b10 was completely amorphous at ambient temperature and did not show an endo- thermic peak, even though it showed relatively high crystallinity and had a T m around 46 °C before crosslinking. This is because freely mobile polymer chains could contribute to form a crys- talline region and an increase in crosslinking density could impose restrictions on chain mobility and reduce the crystalliza- tion. Crosslinked 2b20, on the other hand, showed an extremely sharp transition over the T m around 43 °C. Indeed, the elastic modulus of the cross-linked 2b20 decreased from approximately 122 to 9 MPa over the T m (data not shown). This is because 2b20 has longer polymer chains and a lower netpoint density than Mitsuhiro Ebara, Koichiro Uto, Naokazu Idota, John M. Hoffman, and Takao Aoyagi* Shape-Memory Surface with Dynamically Tunable Nanogeometry Activated by Body Heat