Steffen Kelch

Shape‐Memory Polymers

Material scientists predict a prominent role in the future for self-repairing and intelligent materials. Throughout the last few years, this concept has found growing interest as a result of the rise of a new class of polymers. These so-called shape-memory polymers by far surpass well-known metallic shape-memory alloys in their shape-memory properties. As a consequence of the relatively easy manufacture and programming of shape-memory polymers, these materials represent a cheap and efficient alternative to well-established shape-memory alloys. In shape-memory polymers, the consequences of an intended or accidental deformation caused by an external force can be ironed out by heating the material above a defined transition temperature. This effect can be achieved because of the given flexibility of the polymer chains. When the importance of polymeric materials in our daily life is taken into consideration, we find a very broad, additional spectrum of possible applications for intelligent polymers that covers an area from minimally invasive surgery, through high-performance textiles, up to self-repairing plastic components in every kind of transportation vehicles.

Polymeric triple-shape materials

Shape-memory polymers represent a promising class of materials that can move from one shape to another in response to a stimulus such as heat. Thus far, these systems are dual-shape materials. Here, we report a triple-shape polymer able to change from a first shape (A) to a second shape (B) and from there to a third shape (C). Shapes B and C are recalled by subsequent temperature increases. Whereas shapes A and B are fixed by physical cross-links, shape C is defined by covalent cross-links established during network formation. The triple-shape effect is a general concept that requires the application of a two-step programming process to suitable polymers and can be realized for various polymer networks whose molecular structure allows formation of at least two separated domains providing pronounced physical cross-links. These domains can act as the switches, which are used in the two-step programming process for temporarily fixing shapes A and B. It is demonstrated that different combinations of shapes A and B for a polymer network in a given shape C can be obtained by adjusting specific parameters of the programming process. Dual-shape materials have already found various applications. However, as later discussed and illustrated by two examples, the ability to induce two shape changes that are not limited to be unidirectional rather than one could potentially offer unique opportunities, such as in medical devices or fasteners.

Biodegradable multiblock copolymers based on oligodepsipeptides with shape-memory properties.

Thermoplastic phase-segregated multiblock copolymers with polydepsipeptides and PCL segments were prepared via coupling of diol and PCL-diol using an aliphatic diisocyanate. The obtained multiblock copolymers showed good elastic properties and a shape memory. Almost complete fixation of the mechanical deformation, resulting in quantitative recovery of the permanent shape with a switching temperature around body temperature, was observed. In hydrolytic degradation experiments, a quick decrease of the molecular weight without induction period was observed, and the material changed from elastic to brittle in 21 d. These materials promise a high potential for biomedical applications such as smart implants or medical devices.

Dual-shape properties of triple-shape polymer networks with crystallizable network segments and grafted side chains

Triple-shape materials have recently been introduced as a promising class of active polymers, that can change on demand from a first shape (A) to a second shape (B) and from there to a third shape (C). Here, the dual-shape capability of such a triple-shape polymer network system having two distinct melting transitions is investigated by cyclic, thermomechanical experiments. These multiphase polymer networks are synthesized by photopolymerization from poly(ethylene glycol) monomethyl ether monomethacrylate and poly(e-caprolactone) dimethacrylate as crosslinker. While their permanent shape is determined by the chemical crosslinks formed during network preparation, the two different crystallizable phases can be used to fix other temporary shapes. When programmed appropriately in a two-step thermomechanical process these materials can exhibit a triple-shape effect. Here, one-step programming methods for dual-shape effects are applied to these polymer networks under variation of process parameters, especially thermal conditions. In this way, crystalline phases formed by both segments can be used either individually or simultaneously to fix a temporary second shape. The permanent shape can be recovered by heating, exceeding a specific switching temperature. This switching temperature correlates with the melting transition of the related domain, if one domain is used for fixation. If both domains are used, the switching temperature correlates with the higher melting temperature.

Controlling the Switching Temperature of Biodegradable, Amorphous, Shape-Memory Poly(rac-lactide)urethane Networks by Incorporation of Different Comonomers

Biodegradable shape-memory polymers have attracted tremendous interest as potential implant materials for minimally invasive surgery. Here, the precise control of the materials functions, for example, the switching temperature T(sw), is a particular challenge. T(sw) should be either between room and body temperature for automatically inducing the shape change upon implantation or slightly above body temperature for on demand activation. We explored whether T(sw) of amorphous polymer networks from star-shaped rac-dilactide-based macrotetrols and a diisocyanate can be controlled systematically by incorporation of p-dioxanone, diglycolide, or epsilon-caprolactone as comonomer. Thermomechanical experiments resulted that T(sw) could be adjusted between 14 and 56 degrees C by selection of comonomer type and ratio without affecting the advantageous elastic properties of the polymer networks. Furthermore, the hydrolytic degradation rate could be varied in a wide range by the content of easily hydrolyzable ester bonds, the materials hydrophilicity, and its molecular mobility.

Shape-memory capability of binary multiblock copolymer blends with hard and switching domains provided by different components

The structural concept of shape-memory polymers (SMP) is based on two key components: covalent or physical crosslinks (hard domains) determining the permanent shape and switching domains fixing the temporary shape as well as determining the switching temperature Tsw. In conventional thermoplastic SMP hard and switching domains determining segments are combined in one macromolecule. In this paper we report on binary polymer blends from two different multiblock copolymers, whereby the first one provides the segments forming hard domains and the second one the segments forming the switching domains. A poly(alkylene adipate) mediator segment is incorporated in both multiblock copolymers to promote their miscibility as the hard segment poly(p-dioxanone) (PPDO) and the switching segment poly(e-caprolactone) (PCL) are non-miscible. All polymer blends investigated showed excellent shape-memory properties. The melting point associated to the PCL switching domains Tm,PCL is almost independent of the weight ratio of the two blend components. At the same time the mechanical properties can be varied systematically. In this way complex synthesis of new materials can be avoided. Its biodegradability, the variability of mechanical properties and a Tsw around body temperature are making this binary blend system an economically efficient, suitable candidate for diverse biomedical applications.

Degradable, Multifunctional Polymeric Biomaterials with Shape-Memory

An actual trend in polymer science is the design of materials which show multifunctionality meaning an unexpected combination of material functionalizations like the combination of biofunctionality, hydrolytic degradability, and shape-memory functionality. The development of multifunctional materials is often application-driven. I.e., certain demands of the modern society cannot be adequately solved with existing materials or classes of materials. These demands determine the necessary spectrum of functionalities to be provided by the respective materials . Besides finding high performance materials, material scientists concentrate on designing “intelligent” and “self-repairing materials”. In this context, materials showing a thermally induced shape-memory effect, such as metallic alloys or gels, have been studied intensively, and polymers showing shape-memory behavior have found growing interest. Shape-memory polymers are stimuliresponsive materials. Upon exposure to an external stimulus, they have the capability of changing their shape. A change in shape initiated by a change in temperature is called thermally induced shape-memory effect. The shape-memory effect results from the polymer’s structure in combination with a certain processing and programming technology. The polymer is processed into its permanent shape by conventional methods such as extrusion or injection molding. Afterwards, it is deformed and the desired temporary shape is fixed. The later process is called programming. Heating the programmed polymer above a temperature higher than the transition temperature Ttrans results in activating the shape-memory effect. As a consequence, the recovery of the memorized, permanent shape can be observed. The described effect is called a “one-way” shape-memory effect [1]. Materials which obtain their functionality after a functionalization process as described for shape-memory polymers are called functionalized materials. Taking into consideration the importance of polymeric materials in daily life, a very broad spectrum of possible applications for intelligent polymers opens up, covering an area from minimally invasive surgery to high performance textiles, and to self-repairing plastic components in all kinds of technical devices. Polyethylen covalently cross-linked by means of ionizing radiation has found broad application as heat shrinking film or tubing especially for the insulation of electric wires or as protection against corrosion of pipe lines [2-7]. These materials are marketed as “heat shrinkable materials”. Phase-segregated multiblock copolymers, mostly polyurethanes, can be found in literature being named with the generic term “shape-memory polymers” [8-17]. Thermoresponsive shape-memory properties is not confined to polymers. It has also been described for other materials, especially for ceramics and metallic alloys [18-27]. However, the mechanical properties of shape-memory alloys, like Nitinol  , can only be varied in a limited range. Their strongest restriction is the maximum deformation between permanent and temporary shape of only 8% [22]. Materials Science Forum Online: 2005-08-15 ISSN: 1662-9752, Vols. 492-493, pp 219-224 doi:10.4028/www.scientific.net/MSF.492-493.219

[Cell proliferation and cellular activity of primary cell cultures of the oral cavity after cell seeding on the surface of a degradable, thermoplastic block copolymer].

Using standard cell biological and biochemical methods we were able to test the ability of a degradable, thermoplastic block copolymer to support the adhesion, proliferation, and the cellular activity of primary cell cultures of the oral cavity in vitro. The delicate balance between a group of endogenous enzymes, Matrix Metalloproteinases (MMPs), and their inhibitors (Tissue Inhibitor of MMPs, TIMPs) have a decisive function in the remodeling of the extracellular matrix during processes like wound healing or the integration of biomaterials in surrounding tissues after implantation. Recently developed, biodegradable thermoplastic elastomers with shape-memory properties may be the key to develop new therapeutical options in head and neck surgery. Primary cell cultures of the oral cavity of Sprague-Dawley rats were seeded on the surface of a thermoplastic block copolymer and on a polystyrene surface as control. Conditioned media of the primary cells were analyzed for MMPs and TIMPs after different periods of cell growth. The MMP and TIMP expression was analysed by zymography and a radiometric enzyme assay. No statistically significant differences in the appearance and the kinetic of MMP-1, MMP-2, MMP-9 and TIMPs were detected between cells grown on the polymer surface compared to the control. An appropriate understanding of the molecular processes that regulate cellular growth and integration of a biomaterial in surrounding tissue is the requirement for an optimal adaptation of biodegradable, polymeric biomaterials to the physiological, anatomical, and surgical conditions in vivo to develop new therapeutic options in otolaryngology and head and neck surgery.Ein Schwerpunkt in der Regenerativen Medizin liegt auf der Wiederherstellung der physiologischen Funktion von Geweben oder Organen. Dies kann durch künstliche Implantate oder durch Gewebebzw. Organtransplantationen erreicht werden. Aufgrund der demographischen Entwicklung westlicher Industriegesellschaften wird der Bedarf an Gewebeund Organersatz in den nächsten Jahren weiter ansteigen. Gewebeund Organtransplantationen sind u. a. mit dem Risiko von immunologischen Reaktionen, der Notwendigkeit einer Immunsuppression beim Empfänger sowie einer limitierten Verfügbarkeit von Spendergeweben und –organen assoziiert [14]. In der Hals-Nasen-Ohrenheilkunde und Kopf-Halschirurgie können progressive Erkrankungen sowie therapeutische Interventionen, insbesondere nach tumorchirurgischen Resektionen, mit der Destruktion und/oder dem Verlust von Geweben assoziiert sein. Während an Fragestellungen zum klinischen Einsatz von Biomaterialien in operativen Fächern wie u. a. der Orthopädie, der Herzchirurgie, der Augenheilkunde oder der Urologie schon seit Jahren intensiv gearbeitet wird, steht in der Hals-Nasen-Ohrenheilkunde und der Plastischen Kopf-Halschirurgie die Entwicklung und Anwendung von biokompatiblen Werkstoffen noch am Anfang. Die Rekonstruktion von Geweben und Organen des oberen Aerodigestivtraktes mit adäquater Funktion stellt eine große wissenschaftliche und klinische Herausforderung für die Zukunft dar.

Erste Ergebnisse zur Untersuchung der Stabilität und Gewebeintegration eines abbaubaren, elastischen Copolymers im Tiermodell / First results of the investigation of the stability and tissue integration of a degradable, elastomeric copolymer in an animal model

Zusammenfassung Im Tiermodell wurden die mechanische Stabilität und die Dichtigkeit des Polymer-Gewebe-Verschlusses eines neuartigen, abbaubaren, elastischen Copolymers untersucht. Um das Biomaterial unter extremen chemischen, enzymatischen und mechanischen Bedingungen zu untersuchen, wurde das Material zur Rekonstruktion eines durchgreifenden Magenwanddefektes bei Sprague-Dawley-Ratten verwendet (n=42). In der Kontrollgruppe (n=21) wurde ein primärer Wundverschluss des identischen Magenwanddefektes ohne Biomaterialimplantation durchgeführt. In der Baselinegruppe (n=21) wurden die Versuchstiere unter identischen Versuchsbedingungen ohne operativen Eingriff gehalten. Die Versuchsdauer betrug 1 Woche, 4 Wochen und 6 Monate. Das Gewicht der Versuchstiere wurde präoperativ und vor der Explantation bestimmt. Im Rahmen der Explantation erfolgte eine Druckmessung im Magen nach maximaler Aufdehnung durch Luftinsufflation, um die Dichtigkeit des Verschlusses zwischen Polymer und Magenwand nachzuweisen. Nach einwöchiger Implantations- bzw. Versuchsdauer zeigte sich eine statistisch signifikante Gewichtszunahme nur in der Baselinegruppe. 4 Wochen und 6 Monate nach dem bauchchirurgischen Eingriff fand sich in allen drei Gruppen eine statistisch signifikante Gewichtszunahme im Vergleich zum präoperativen Gewicht der Versuchstiere. Gastrointestinale Komplikationen wie Fistelbildungen, Perforationen oder Peritonitiden traten bei keinem Versuchstier auf. Die intragastrale Druckmessung nach maximaler Aufdehnung des Magens ergab keine statistisch signifikanten Differenzen in der Implantat-, der Kontroll- und der Baselinegruppe zu den drei untersuchten Zeiträumen. Die Dichtigkeit des Nahtverschlusses zwischen dem Polymer und dem umgebenden Gewebe war bei allen Tieren der Implantatgruppe nachweisbar. Die erforderliche mechanische Stabilität des neuartigen Copolymers konnte unter den Extrembedingungen des Magenmilieus gezeigt werden. Eine vorzeitige Degradation des abbaubaren Polymers mit Wundheilungsstörungen konnte bei allen Versuchstieren ausgeschlossen werden. In weiterführenden Untersuchungen müssen die Mechanismen, die der Integration des Biomaterials in das umgebende Gewebe zugrunde liegen, sowie die Polymerdegradation und der Prozess des Geweberemodelings analysiert werden. Diese Erkenntnisse sind die Voraussetzung für eine hochspezifische Adaptation des neuartigen Polymers an die Bedingungen des jeweiligen Applikationsortes, um somit neuartige Therapieoptionen in der Medizin entwickeln zu können.

Shape-memory Polymers

Tailoring of properties and functions of shape-memory polymer networks to the requirements of specific applications demands a knowledge-based approach. A comprehensive database enabling the analysis of structure-property relationship is obtained by the systematic variation of molecular parameters. Here, we investigated the influence of the nature of thermal transition on the shape-memory behavior of polymer networks. Furthermore, additional amorphous phases were introduced enabling tailoring of elastic properties especially in the temporary shape. The structure property relationships were derived for different designs of such multiphase polymer network architectures.