Shape memory polymers (SMPs) are polymers which ''remember'' their original shape and can return to it after deformation, if an external stimulus-often an increased temperatureis applied. Some SMPs can be 3D printed, typically by fused deposition modeling (FDM). The most well-known SMP is poly(lactic acid), which belongs to the most often used materials in FDM 3D printing. There are; however, many more SMPs which can be 3D printed to combine the possibilities to prepare new, sophisticated shapes with the opportunity to restore these shapes after undesirable or intentional deformation. This review gives an overview of several 3D printable SMPs, their mechanical characteristics and their possible applications.

Polylactic acid (PLA) belongs to the few thermoplastic polymers that are derived from renewable resources such as corn starch or sugar cane. PLA is often used in 3D printing by fused deposition modeling (FDM) as it is relatively easy to print, does not show warping and can be printed without a closed building chamber. On the other hand, PLA has interesting mechanical properties which are influenced by the printing parameters and geometries. Here we present shape-memory properties of PLA cubes with different infill patterns and percentages, extending the research reported before in a conference paper. We investigate the material response under defined quasi-static load as well as the possibility to restore the original 3D printed shape. The quasi-static flexural properties are linked to the porosity and the infill structure of the samples under investigation as well as to the numbers of closed top layers, examined optically and by simulations. Our results underline the importance of designing the infill patterns carefully to develop samples with desired mechanical properties.

3D printing belongs to the emerging technologies of our time. Describing diverse specific techniques, 3D printing enables rapid production of individual objects and creating shapes that would not be produced with other techniques. One of the drawbacks of typical 3D printing processes, however, is the layered structure of the created parts. This is especially problematic in the production of optical elements, which in most cases necessitate highly even surfaces. To meet this challenge, advanced 3D printing techniques as well as other sophisticated solutions can be applied. Here, we give an overview of 3D printed optical elements, such as lenses, mirrors, and waveguides, with a focus on freeform optics and other elements for which 3D printing is especially well suited.

Biosignals often have to be detected in sports or for medical reasons. Typical biosignals are pulse and ECG (electrocardiogram), breathing, blood pressure, skin temperature, oxygen saturation, bioimpedance, etc. Typically, scientists attempt to measure these biosignals noninvasively, i.e., with electrodes or other sensors, detecting electric signals, measuring optical or chemical information. While short-time measurements or monitoring of patients in a hospital can be performed by systems based on common rigid electrodes, usually containing a large amount of wiring, long-term measurements on mobile patients or athletes necessitate other equipment. Here, textile-based sensors and textile-integrated data connections are preferred to avoid skin irritations and other unnecessary limitations of the monitored person. In this review, we give an overview of recent progress in textile-based electrodes for electrical measurements and new developments in textile-based chemical and other sensors for detection and monitoring of biosignals.

Poly(lactic acid) is not only one of the most often used materials for 3D printing via fused deposition modeling (FDM), but also a shape-memory polymer. This means that objects printed from PLA can, to a certain extent, be deformed and regenerate their original shape automatically when they are heated to a moderate temperature of about 60–100 °C. It is important to note that pure PLA cannot restore broken bonds, so that it is necessary to find structures which can take up large forces by deformation without full breaks. Here we report on the continuation of previous tests on 3D-printed cubes with different infill patterns and degrees, now investigating the influence of the orientation of the applied pressure on the recovery properties. We find that for the applied gyroid pattern, indentation on the front parallel to the layers gives the worst recovery due to nearly full layer separation, while indentation on the front perpendicular to the layers or diagonal gives significantly better results. Pressing from the top, either diagonal or parallel to an edge, interestingly leads to a different residual strain than pressing from front, with indentation on top always firstly leading to an expansion towards the indenter after the first few quasi-static load tests. To quantitatively evaluate these results, new measures are suggested which could be adopted by other groups working on shape-memory polymers.

Electronic textiles belong to the broader range of smart (or “intelligent”) textiles. Their “smartness” is enabled by embedded or added electronics and allows the sensing of defined parameters of their environment as well as actuating according to these sensor data. For this purpose, different sensors (e.g., temperature, strain, light sensors) and actuators (e.g., LEDs or mechanical actuators) are embedded and connected with a power supply, a data processor, and internal/external communication.

3D printing is nowadays used for many applications far beyond pure rapid prototyping. As tools to prepare custom-made objects which may be highly complex, different 3D printing techniques have emerged into areas of application where the mechanical, thermal, optical and other properties have to meet high requirements. Amongst them, applications for space, e.g. for microsatellites, make extreme demands regarding the stability under high temperatures. Nevertheless, polymeric 3D printed materials have several advantages for space application in comparison with metal objects. Here we thus investigate the impact of temperatures up to 85 °C and 185 °C, respectively, on typical 3D printing materials for fused deposition modeling (FDM) or stereolithography (SLA). The materials are found to differ strongly in terms of mechanical properties and dimensional stability after the treatment at higher temperature, with SLA resins and co-polyester (CPE) showing the best dimensional stability, while acrylonitrile-butadiene-styrene (ABS) and SLA resin after long UV post-treatment have the best mechanical properties.

Poly(lactic acid) (PLA) belongs to the 3D printable materials which show shape-memory properties, i.e., which can recover their original shape after a deformation if they are heated above the glass transition temperature. This makes PLA quite an interesting material for diverse applications, such as bumpers, safety equipment for sports, etc. After investigating the influence of the infill design and degree, as well as the pressure orientation on the recovery properties of 3D printed PLA cubes in previous studies, here we report on differences between different PLA materials as well as on the impact of post-treatments after 3D printing by solvents or by heat. Our results show not only large differences between materials from different producers, but also a material-dependent impact of the post treatments. Generally, it is possible to tailor the mechanical and recovery properties of 3D printed PLA parts by choosing the proper material in combination with a chemical or temperature post-treatment.