@article{LendleinHeuchel2021,
  author    = {Lendlein, Andreas and Heuchel, Matthias},
  title     = {Shape-memory polymers designed in view of thermomechanical energy storage and conversion systems},
  series = {ACS central science},
  volume    = {7},
  journal   = {ACS central science},
  number    = {10},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {2374-7951},
  doi       = {10.1021/acscentsci.1c01032},
  pages     = {1599 -- 1601},
  year      = {2021},
  language  = {en}
}
@article{RazzaqBehlHeucheletal.2019,
  author    = {Razzaq, Muhammad Yasar and Behl, Marc and Heuchel, Matthias and Lendlein, Andreas},
  title     = {Matching magnetic heating and thermal actuation for sequential coupling in hybrid composites by design},
  series = {Macromolecular rapid communications},
  volume    = {41},
  journal   = {Macromolecular rapid communications},
  number    = {1},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1022-1336},
  doi       = {10.1002/marc.201900440},
  pages     = {7},
  year      = {2019},
  abstract  = {Sequentially coupling two material functions requires matching the output from the first with the input of the second function. Here, magnetic heating controls thermal actuation of a hybrid composite in a challenging system environment causing an elevated level of heat loss. The concept is a hierarchical design consisting of an inner actuator of nanocomposite material, which can be remotely heated by exposure to an alternating magnetic field (AMF) and outer layers of a porous composite system with a closed pore morphology. These porous layers act as heat insulators and as barriers to the surrounding water. By exposure to the AMF, a local bulk temperature of 71 degrees C enables the magnetic actuation of the device, while the temperature of the surrounding water is kept below 50 degrees C. Interestingly, the heat loss during magnetic heating leads to an increase of the water phase (small volume) temperature. The temperature increase is able to sequentially trigger an adjacent thermal actuator attached to the actuator composite. In this way it could be demonstrated how the AMF is able to initiate two kinds of independent actuations, which might be interesting for robotics operating in aqueous environments.},
  language  = {en}
}
@article{WangRazzaqRudolphetal.2018,
  author    = {Wang, Li and Razzaq, Muhammad Yasar and Rudolph, Tobias and Heuchel, Matthias and N{\"o}chel, Ulrich and Mansfeld, Ulrich and Jiang, Yi and Gould, Oliver E. C. and Behl, Marc and Kratz, Karl and Lendlein, Andreas},
  title     = {Reprogrammable, magnetically controlled polymeric nanocomposite actuators},
  series = {Material horizons},
  volume    = {5},
  journal   = {Material horizons},
  number    = {5},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {2051-6347},
  doi       = {10.1039/c8mh00266e},
  pages     = {861 -- 867},
  year      = {2018},
  abstract  = {Soft robots and devices with the advanced capability to perform adaptive motions similar to that of human beings often have stimuli-sensitive polymeric materials as the key actuating component. The external signals triggering the smart polymers' actuations can be transmitted either via a direct physical connection between actuator and controlling unit (tethered) or remotely without a connecting wire. However, the vast majority of such polymeric actuator materials are limited to one specific type of motion as their geometrical information is chemically fixed. Here, we present magnetically driven nanocomposite actuators, which can be reversibly reprogrammed to different actuation geometries by a solely physical procedure. Our approach is based on nanocomposite materials comprising spatially segregated crystallizable actuation and geometry determining units. Upon exposure to a specific magnetic field strength the actuators' geometric memory is erased by the melting of the geometry determining units allowing the implementation of a new actuator shape. The actuation performance of the nanocomposites can be tuned and the technical significance was demonstrated in a multi-cyclic experiment with several hundreds of repetitive free-standing shape shifts without losing performance.},
  language  = {en}
}
@article{SauterKratzHeucheletal.2021,
  author    = {Sauter, Tilman and Kratz, Karl and Heuchel, Matthias and Lendlein, Andreas},
  title     = {Fiber diameter as design parameter for tailoring the macroscopic shape-memory performance of electrospun meshes},
  series = {Materials and design},
  volume    = {202},
  journal   = {Materials and design},
  publisher = {Elsevier},
  address   = {Amsterdam [u.a.]},
  issn      = {1873-4197},
  doi       = {10.1016/j.matdes.2021.109546},
  pages     = {10},
  year      = {2021},
  abstract  = {Fibrous shape-memory polymer (SMP) scaffolds were investigated considering the fiber as basic microstructural feature. By reduction of the fiber diameter in randomly oriented electrospun polyetherurethane (PEU) meshes from the micro-to the nano-scale, we observed changes in the molecular orientation within the fibers and its impact on the structural and shape-memory performance. It was assumed that a spatial restriction by reduction of the fiber diameter increases molecular orientation along the orientation of the fiber. The stress-strain relation of random PEU scaffolds is initially determined by the 3D arrangement of the fibers and thus is independent of the molecular orientation. Increasing the molecular orientation with decreasing single fiber diameter in scaffolds composed of randomly arranged fibers did not alter the initial stiffness and peak stress but strongly influenced the elongation at break and the stress increase above the Yield point. Reduction of the single fiber diameter also distinctly improved the shape-memory performance of the scaffolds. Fibers with nanoscale diameters (< 100 nm) possessed an almost complete shape recovery, high recovery stresses and fast relaxation kinetics, while the shape fixity was found to decrease with decreasing fiber diameter. Hence, the fiber diameter is a relevant design parameter for SMP.},
  language  = {en}
}
@article{KumarHeuchelKratzetal.2018,
  author    = {Kumar, Reddi K. and Heuchel, Matthias and Kratz, Karl and Lendlein, Andreas and Jankowski, Joachim and Tetali, Sarada D.},
  title     = {Effects of extracts prepared from modified porous poly(ether imide) microparticulate absorbers on cytotoxicity, macrophage differentiation and proinflammatory behavior of human monocytic (THP-1) cells},
  series = {Clinical hemorheology and microcirculation : blood flow and vessels},
  volume    = {69},
  journal   = {Clinical hemorheology and microcirculation : blood flow and vessels},
  number    = {1-2},
  publisher = {IOS Press},
  address   = {Amsterdam},
  issn      = {1386-0291},
  doi       = {10.3233/CH-189112},
  pages     = {175 -- 185},
  year      = {2018},
  abstract  = {Remaining uremic toxins in the blood of chronic renal failure patients represent one central challenge in hemodialysis therapies. Highly porous poly(ether imide) (PEI) microparticles have been recently introduced as candidate absorber materials, which show a high absorption capacity for uremic toxins and allow hydrophilic surface modification suitable for minimization of serum protein absorption. In this work, the effects of extracts prepared from PEI microparticles modified by nucleophilic reaction with low molecular weight polyethylene imine (Pei) or potassium hydroxide (KOH), on human monocytic (THP-1) cells are studied. The obtained results suggested that the extracts of Pei and KOH modified PEI absorbers have no negative effect on THP-1 cell viability and do not initiate the critical differentiation towards macrophages. The extracts did not enhance transcript or protein levels of investigated proinflammatory markers in THP-1 cells, namely, TNF alpha, MCP1, IL6 and IL8. Based on these findings such modified PEI microparticles should be qualified for further pre-clinical evaluation i.e. in an in vivo animal experiment.},
  language  = {en}
}
@article{IzraylitHeuchelKratzetal.2021,
  author    = {Izraylit, Victor and Heuchel, Matthias and Kratz, Karl and Lendlein, Andreas},
  title     = {Non-woven shape-memory polymer blend actuators},
  series = {MRS advances : a journal of the Materials Research Society (MRS)},
  volume    = {6},
  journal   = {MRS advances : a journal of the Materials Research Society (MRS)},
  number    = {33},
  publisher = {Springer Nature Switzerland AG},
  address   = {Cham},
  issn      = {2059-8521},
  doi       = {10.1557/s43580-021-00063-8},
  pages     = {781 -- 785},
  year      = {2021},
  abstract  = {The hierarchical design approach provides various opportunities to adjust the structural performance of polymer materials. Electrospinning processing techniques give access to molecular orientation as a design parameter, which we consider here in view of the shape-memory actuation performance. The aim of this work is to investigate how the reversible strain epsilon'(rev) can be affected by a morphology change from a bulk material to an electrospun mesh. epsilon'(rev) could be increased from 5.5 +/- 0.5\% to 15 +/- 1.8\% for a blend from a multiblock copolymer with poly(epsilon-caprolactone) (PCL) and poly(L-lactide) (PLLA) segments with oligo(D-lactide) (ODLA). This study demonstrates an effective design approach for enhancing soft actuator performance, which can be broadly applied in soft robotics and medicine.},
  language  = {en}
}
@article{GhobadiHeuchelKratzetal.2012,
  author    = {Ghobadi, Ehsan and Heuchel, Matthias and Kratz, Karl and Lendlein, Andreas},
  title     = {Simulation of volumetric swelling of degradable poly[(rac-lactide)-co-glycolide] based polyesterurethanes containing different urethane-linkers},
  series = {Journal of applied biomaterials \& functional materials},
  volume    = {10},
  journal   = {Journal of applied biomaterials \& functional materials},
  number    = {3},
  publisher = {Wichtig},
  address   = {Milano},
  issn      = {2280-8000},
  doi       = {10.5301/JABFM.2012.10432},
  pages     = {293 -- 301},
  year      = {2012},
  abstract  = {Aim: The hydrolytic degradation behavior of degradable aliphatic polyester-based polymers is strongly influenced by the uptake or transport of water into the polymer matrix and also the hydrolysis rate of ester bonds. Methods: We examined the volumetric swelling behavior of poly[(rac-lactide)-co-glycolide] (PLGA) and PLGA-based polyurethanes (PLGA-PU) with water contents of 0 wt\%, 2 wt\% and 7 wt\% water at 310 K using a molecular modeling approach. Polymer systems with a number average molecular weight of M-n = 10,126 g.mol(-1) were constructed from PLGA with a lactide content of 67 mol\%, whereby PLGA-PU systems were composed of five PLGA segments with M-n = 2052 g.mol(-1), which were connected via urethane linkers originated from 2,2,4-trimethyl hexamethylene-1,6-diisocyanate (TMDI), hexamethyl-1,6-diisocyanate (HDI), or L-lysine-1,6-diisocyanate (LDI). Results: The calculated densities of the dry PLGA-PU systems were found to be lower than for pure PLGA. The obtained volumetric swelling of the PLGA-PU was depending on the type of urethane linker, whereby all swollen PLGA-PUs contained larger free volume distribution compared to pure PLGA. The mean square displacement curves for dry PLGA and PLGA-PUs showed that urethane linker units reduce the mobility of the polymer chains, while an increase in backbone atoms mobility was found, when water was added to these systems. Consequently, an increased water uptake of PLGA-PU matrices combined with a higher mobility of the chain segments should result in an accelerated hydrolytic chain scission rate in comparison to PLGA. Conclusions: It can be anticipated that the incorporation of urethane linkers might be a helpful tool to adjust the degradation behavior of polyesters.},
  language  = {en}
}
@article{GhobadiHeuchelKratzetal.2012,
  author    = {Ghobadi, Ehsan and Heuchel, Matthias and Kratz, Karl and Lendlein, Andreas},
  title     = {Influence of different heating regimes on the shape-recovery behavior of poly(L-lactide) in simulated thermomechanical tests},
  series = {Journal of applied biomaterials \& functional materials},
  volume    = {10},
  journal   = {Journal of applied biomaterials \& functional materials},
  number    = {3},
  publisher = {Wichtig},
  address   = {Milano},
  issn      = {2280-8000},
  doi       = {10.5301/JABFM.2012.10440},
  pages     = {259 -- 264},
  year      = {2012},
  abstract  = {Aim: Multifunctional polymer-based biomaterials, which combine degradability with a shape-memory capability and in this way enable the design of actively moving implants such as self-anchoring implants or controlled release systems, have been recently introduced. Of particular interest are approved degradable polymers such as poly(L-lactide) (PLLA), which can be easily functionalized with a shape-memory effect. In the case of semicrystalline PLLA, the glass transition can be utilized as shape-memory switching domain. Methods: In this work we applied a fully atomistic molecular dynamics simulation to study the shape-memory behavior of PLLA. A heating-deformation-cooling programming procedure was applied to atomistic PLLA packing models followed by a recovery module under stress-free conditions allowing the shape recovery. The recovery was simulated by heating the samples from T-low = 250 K to T-high = 500 K with different heating rates beta of 125, 40 and 4 K.ns(-1). Results: We could demonstrate that the obtained strain recovery rate (R-r) was strongly influenced by the applied simulation time and heating rate, whereby R-r values in the range from 46\% to 63\% were achieved. On its own the application of a heating rate of 4 K.ns(-1) enabled us to determine a characteristic switching temperature of T-sw = 473 K for the modeled samples. Conclusions: We anticipate that the atomistic modeling approach presented should be capable of enabling further study of T-sw with respect to the molecular structure of the investigated SMP and therefore could be applied in the context of design and development of new shape-memory (bio) materials.},
  language  = {en}
}
@article{WangHeuchelFangetal.2012,
  author    = {Wang, Li and Heuchel, Matthias and Fang, Liang and Kratz, Karl and Lendlein, Andreas},
  title     = {Influence of a polyester coating of magnetic nanoparticles on magnetic heating behavior of shape-memory polymer-based composites},
  series = {Journal of applied biomaterials \& functional materials},
  volume    = {10},
  journal   = {Journal of applied biomaterials \& functional materials},
  number    = {3},
  publisher = {Wichtig},
  address   = {Milano},
  issn      = {2280-8000},
  doi       = {10.5301/JABFM.2012.10293},
  pages     = {203 -- 209},
  year      = {2012},
  abstract  = {Background: Magnetic composites of thermosensitive shape-memory polymers (SMPs) and magnetite nanoparticles (MNPs) allow noncontact actuation of the shape-memory effect in an alternating magnetic field. In this study, we investigated whether the magnetic heating capability of cross-linked poly(epsilon-caprolactone)/MNP composites (cPCLC) could be improved by covalent coating of MNPs with oligo(epsilon-caprolactone) (OCL). Methods: Two different types of cPCLC containing uncoated and OCL-coated MNP with identical magnetite weight content were prepared by thermally induced polymerization of poly(epsilon-caprolactone) diisocyanatoethyl methacrylate. Both cPCLCs exhibited a melting transition at T-m = 48 degrees C, which could be used as switching transition. Results: The dispersion of the embedded nanoparticles within the polymer matrix could be substantially improved, when the OCL-coated MNPs were used, as visualized by scanning electron microscopy. We could further demonstrate that in this way the maximal achievable bulk temperature (T-bulk) obtained within the cPCLC test specimen in magnetic heating experiments at a magnetic field strength of H = 30 kA.m(-1) could be increased from T bulk = 48 degrees C to T bulk = 74 degrees C.},
  language  = {en}
}
@article{GhobadiHeuchelKratzetal.2013,
  author    = {Ghobadi, Ehsan and Heuchel, Matthias and Kratz, Karl and Lendlein, Andreas},
  title     = {Simulating the shape-Memory behavior of amorphous switching domains of Poly(L-lactide) by molecular dynamics},
  series = {Macromolecular chemistry and physics},
  volume    = {214},
  journal   = {Macromolecular chemistry and physics},
  number    = {11},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1022-1352},
  doi       = {10.1002/macp.201200450},
  pages     = {1273 -- 1283},
  year      = {2013},
  abstract  = {The thermally induced shape-memory effect of polymers is typically characterized by cyclic uniaxial thermomechanical tests. Here, a molecular-dynamics (MD) simulation approach of such a cyclic uniaxial thermomechanical test is presented for amorphous switching domains of poly(L-lactide) (PLLA). Uniaxial deformation of the constructed PLLA models is simulated with a Parinello-Rahman scheme, as well as a pragmatic geometrical approach. We are able to describe two subsequent test cycles using the presented simulation approach. The obtained simulated shape-memory properties in both test cycles are similar and independent of the applied deformation protocols. The simulated PLLA shows high shape fixity ratios (Rf 94\%), but only a moderate shape recovery ratio is obtained (Rr 30\%). Finally, the structural changes during the simulated test are characterized by analysis of the changes in the dihedral angle distributions.},
  language  = {en}
}