Stefan Hengsbach

High-strength cellular ceramic composites with 3D microarchitecture.

Significance It has been a long-standing effort to create materials with low density but high strength. Technical foams are very light, but compared with bulk materials, their strength is quite low because of their random structure. Natural lightweight materials, such as bone, are cellular solids with optimized architecture. They are structured hierarchically and actually consist of nanometer-size building blocks, providing a benefit from mechanical size effects. In this paper, we demonstrate that materials with a designed microarchitecture, which provides both structural advantages and size-dependent strengthening effects, may be fabricated. Using 3D laser lithography, we produced micro-truss and -shell structures from ceramic–polymer composites that exceed the strength-to-weight ratio of all engineering materials, with a density below 1,000 kg/m3. To enhance the strength-to-weight ratio of a material, one may try to either improve the strength or lower the density, or both. The lightest solid materials have a density in the range of 1,000 kg/m3; only cellular materials, such as technical foams, can reach considerably lower values. However, compared with corresponding bulk materials, their specific strength generally is significantly lower. Cellular topologies may be divided into bending- and stretching-dominated ones. Technical foams are structured randomly and behave in a bending-dominated way, which is less weight efficient, with respect to strength, than stretching-dominated behavior, such as in regular braced frameworks. Cancellous bone and other natural cellular solids have an optimized architecture. Their basic material is structured hierarchically and consists of nanometer-size elements, providing a benefit from size effects in the material strength. Designing cellular materials with a specific microarchitecture would allow one to exploit the structural advantages of stretching-dominated constructions as well as size-dependent strengthening effects. In this paper, we demonstrate that such materials may be fabricated. Applying 3D laser lithography, we produced and characterized micro-truss and -shell structures made from alumina–polymer composite. Size-dependent strengthening of alumina shells has been observed, particularly when applied with a characteristic thickness below 100 nm. The presented artificial cellular materials reach compressive strengths up to 280 MPa with densities well below 1,000 kg/m3.

Rapid prototyping of multi-scale biomedical microdevices by combining additive manufacturing technologies

The possibility of designing and manufacturing biomedical microdevices with multiple length-scale geometries can help to promote special interactions both with their environment and with surrounding biological systems. These interactions aim to enhance biocompatibility and overall performance by using biomimetic approaches. In this paper, we present a design and manufacturing procedure for obtaining multi-scale biomedical microsystems based on the combination of two additive manufacturing processes: a conventional laser writer to manufacture the overall device structure, and a direct-laser writer based on two-photon polymerization to yield finer details. The process excels for its versatility, accuracy and manufacturing speed and allows for the manufacture of microsystems and implants with overall sizes up to several millimeters and with details down to sub-micrometric structures. As an application example we have focused on manufacturing a biomedical microsystem to analyze the impact of microtextured surfaces on cell motility. This process yielded a relevant increase in precision and manufacturing speed when compared with more conventional rapid prototyping procedures.

Direct laser writing of auxetic structures: present capabilities and challenges

Auxetic materials (or metamaterials) are those with a negative Poisson ratio (NPR) and that display the unexpected property of lateral expansion when stretched, as well as an equal and opposing densification when compressed. Such geometries are being progressively employed in the development of novel products, especially in the fields of intelligent expandable actuators, shape morphing structures and minimally invasive implantable devices. Although several micromanufacturing technologies have already been applied to the development of auxetic geometries and devices, additional precision is needed to take full advantage of their special mechanical properties. In this study we present a very promising approach for the development of auxetic metamaterials and devices based on the use of direct laser writing. The process stands out for its precision and complex three-dimensional (3D) geometries attainable without the need of supporting structures. To our knowledge it represents one of the first examples of the application of this technology to the manufacture of auxetic geometries and mechanical metamaterials, with details even more remarkable than those shown in very recent studies, almost reaching the current limit of this additive manufacturing technology. We have used some special 3D auxetic designs whose remarkable NPR has been previously highlighted.

Tissue Engineering Scaffolds for 3D Cell Culture

Even though pioneer studies in the field of tissue engineering, either for disease study or for tissue repair, were performed on flat 2D substrates (normally Petri dishes), more recent research has helped to highlight the relevance of three-dimensional systems in cell culture. In fact, even one-dimensional patterns upon have been found more adequate, for mimicking actual cell migration in three-dimensional environments, than conventional two-dimensional scaffolds for cell culture, what puts forward the need for alternative development procedures aiming at a more adequate reproduction of the 3D environment, taking account of both biochemical and biomechanical approaches. The combined employment of computer-aided design, engineering and manufacturing resources, together with rapid prototyping procedures, working on the basis of additive manufacturing approaches, allows for the efficient development of knowledge-based functionally graded scaffolds for effective and biomimetic three-dimensional cell culture in a wide range of materials. Applications of such tissue engineering scaffolds for cell culture include the repair, regeneration and even biofabrication of hard tissues, soft tissues and osteochondral constructs, as well as the modeling of disease development and management, as detailed in forthcoming chapters. In this chapter we present some design and manufacturing strategies for the development of knowledge-based functionally graded tissue engineering scaffolds aimed at different types of tissues. We also detail some prototyping approaches towards low-cost rapid prototyped scaffolds and tumor growth models, as cases of study for illustrating the complete development process of these types of medical devices.

Freeform three-dimensional embedded polymer waveguides enabled by external-diffusion assisted two-photon lithography

This paper introduces a unique method to fabricate free-form symmetrical three-dimensional single-mode waveguides embedded in a newly developed photopolymer. The fabrication process requires only one layer of a single material by combining two-photon lithography and external monomer diffusion resulting in a high refractive index contrast of 0.013. The cured material exhibits high chemical and thermal stability. Transmission loss of 0.37  dB/cm at 850 nm is achieved. Due to the fact that waveguide arrays are produced with high density, this technique could pave the way for three-dimensional optical interconnects at the board level with high complexity and bandwidth density.

Lotus-on-chip: computer-aided design and 3D direct laser writing of bioinspired surfaces for controlling the wettability of materials and devices

In this study we present the combination of a math-based design strategy with direct laser writing as high-precision technology for promoting solid free-form fabrication of multi-scale biomimetic surfaces. Results show a remarkable control of surface topography and wettability properties. Different examples of surfaces inspired on the lotus leaf, which to our knowledge are obtained for the first time following a computer-aided design with this degree of precision, are presented. Design and manufacturing strategies towards microfluidic systems whose fluid driving capabilities are obtained just by promoting a design-controlled wettability of their surfaces, are also discussed and illustrated by means of conceptual proofs. According to our experience, the synergies between the presented computer-aided design strategy and the capabilities of direct laser writing, supported by innovative writing strategies to promote final size while maintaining high precision, constitute a relevant step forward towards materials and devices with design-controlled multi-scale and micro-structured surfaces for advanced functionalities. To our knowledge, the surface geometry of the lotus leaf, which has relevant industrial applications thanks to its hydrophobic and self-cleaning behavior, has not yet been adequately modeled and manufactured in an additive way with the degree of precision that we present here.

Rapid Prototyping of Biomedical Microsystems for Interacting at a Cellular Level

Sesion oral en la Industriales Research Meeting 2016 en la que el autor muestra sus ultimas investigaciones.

Three-dimensional buried polymer waveguides via femtosecond direct laser writing with two-photon absorption

Free-form three-dimensional buried waveguides with symmetric and adjustable core dimensions have been fabricated via femtosecond direct laser writing in a novel photopolymer. High refractive index contrast of 0.013 between the core and the cladding is achieved by external diffusion of a low refractive index monomer. Measured near-field intensity at the end facet of the waveguides shows single-mode Gaussian profile. Voxel size and refractive index profile can be adjusted by adapting writing speed and laser intensity. The waveguide length is several centimeters which is suitable for on-board interconnects. This concept can be used to produce three-dimensional arrays of optical waveguide network routers, optofans, pitch converters or splitters.

Towards Reliable Organs-on-Chips and Humans-on-Chips

The artificial production of complete three-dimensional vascularized functional organs is still a research challenge, although recent advances are opening up new horizons to the treatment of many diseases by combining synthetic and biological materials to produce portions of veins, capillaries, arteries, skin patches and parts of bones and soft organs. Counting with artificially obtained completely functional replicas of human organs will constitute a benchmark for disease management, but there is still a long way to achieve the desired results and produce complete organs in vitro. In the meantime, having at hand simple biomimetic microsystems capable of mimicking the behaviour of complete complex organs, or at least of some of their significant functionalities, constitutes a realistic and very adequate alternative for disease modeling and management, capable of providing even better results than the use of animal models. These simplified replicas of human organ functionalities are being developed in the form of advanced labs-on-chips generically referred to as “organs-on-chips” and are already providing interesting results. This chapter provides an introduction to this emerging area of study and details different examples of organs-on-chips and their development process with the aid of computer-aided design and engineering technologies and with the support of rapid prototyping and rapid tooling resources.

Active Modular Microsystems Based on Mach-Zehnder Interferometers

Cost efficient and purpose build microsystems for technical applications become more and more relevant. In the field of optical devices we developed an adaptive modular micro-optical system based on a Mach-Zehnder Delay Interferometer to show the feasibility to fabricate active optical microsystems adaptive to different measurement and data communication network applications. To realize such an adaptive modular micro-optical system with an active tuning device, a construction kit was designed and realized to combine different types of signal routing and system tuning, for example by choosing an optical or electronical signal output and different microactuators suitable for several applications, with special designed micro-optical benches (MOB) including the respective optical structures or hybrid integrated components. It is based on automated passive alignment of the optical components and has to be designed by using well defined interfaces. Different types of this modular system have been set-up and the application as a Fourier transformed wavemeter are shown as an example.