Nature has developed high-performance materials and structures over millions of years of evolution and provides valuable sources of inspiration for the design of next-generation structural materials, given the variety of excellent mechanical, hydrodynamic, optical, and electrical properties. Biomimicry, by learning from nature's concepts and design principles, is driving a paradigm shift in modern materials science and technology. However, the complicated structural architectures in nature far exceed the capability of traditional design and fabrication technologies, which hinders the progress of biomimetic study and its usage in engineering systems. Additive manufacturing (three-dimensional (3D) printing) has created new opportunities for manipulating and mimicking the intrinsically multiscale, multimaterial, and multifunctional structures in nature. Here, an overview of recent developments in 3D printing of biomimetic reinforced mechanics, shape changing, and hydrodynamic structures, as well as optical and electrical devices is provided. The inspirations are from various creatures such as nacre, lobster claw, pine cone, flowers, octopus, butterfly wing, fly eye, etc., and various 3D-printing technologies are discussed. Future opportunities for the development of biomimetic 3D-printing technology to fabricate next-generation functional materials and structures in mechanical, electrical, optical, and biomedical engineering are also outlined.

Recent Progress in Biomimetic Additive Manufacturing Technology: From Materials to Functional Structures.

We report the successful fabrication and testing of 3D printed microfluidic devices with integrated membrane-based valves. Fabrication is performed with a low-cost commercially available stereolithographic 3D printer. Horizontal microfluidic channels with designed rectangular cross sectional dimensions as small as 350 μm wide and 250 μm tall are printed with 100% yield, as are cylindrical vertical microfluidic channels with 350 μm designed (210 μm actual) diameters. Based on our previous work [Rogers et al., Anal. Chem. 83, 6418 (2011)], we use a custom resin formulation tailored for low non-specific protein adsorption. Valves are fabricated with a membrane consisting of a single build layer. The fluid pressure required to open a closed valve is the same as the control pressure holding the valve closed. 3D printed valves are successfully demonstrated for up to 800 actuations.

/pdf/3d-printed-microfluidic-devices-with-integrated-valves-3uvp3g9m5s.pdf

3D printed microfluidic devices with integrated valves

Natural composites exhibit exceptional mechanical performance that often arises from complex fiber arrangements within continuous matrices. Inspired by these natural systems, we developed a rotational 3D printing method that enables spatially controlled orientation of short fibers in polymer matrices solely by varying the nozzle rotation speed relative to the printing speed. Using this method, we fabricated carbon fiber–epoxy composites composed of volume elements (voxels) with programmably defined fiber arrangements, including adjacent regions with orthogonally and helically oriented fibers that lead to nonuniform strain and failure as well as those with purely helical fiber orientations akin to natural composites that exhibit enhanced damage tolerance. Our approach broadens the design, microstructural complexity, and performance space for fiber-reinforced composites through site-specific optimization of their fiber orientation, strain, failure, and damage tolerance.

https://www.pnas.org/content/pnas/115/6/1198.full.pdf

Rotational 3D printing of damage-tolerant composites with programmable mechanics.

The alignment of anisotropic particles during ink deposition directly affects the microstructure and properties of materials manufactured by extrusion-based 3D printing. Although particle alignment in diluted suspensions is well described by analytical and numerical models, the dynamics of particle orientation in the highly concentrated inks typically used for printing via direct ink writing (DIW) remains poorly understood. Using cellulose nanocrystals (CNCs) as model building blocks of increasing technological relevance, we study the dynamics of particle alignment under the shear stresses applied to concentrated inks during DIW. With the help of in situ polarization rheology, we find that the time period needed for particle alignment scales inversely with the applied shear rate and directly with the particle concentration. Such dependences can be quantitatively described by a simple scaling relation and qualitatively interpreted in terms of steric and hydrodynamic interactions between particles at high s...

Dynamics of Cellulose Nanocrystal Alignment during 3D Printing.

Additive manufacturing (AM) has gained significant attention due to its ability to drive technological development as a sustainable, flexible, and customizable manufacturing scheme. Among the various AM techniques, direct ink writing (DIW) has emerged as the most versatile 3D printing technique for the broadest range of materials. DIW allows printing of practically any material, as long as the precursor ink can be engineered to demonstrate appropriate rheological behavior. This technique acts as a unique pathway to introduce design freedom, multifunctionality, and stability simultaneously into its printed structures. Here, a comprehensive review of DIW of complex 3D structures from various materials, including polymers, ceramics, glass, cement, graphene, metals, and their combinations through multimaterial printing is presented. The review begins with an overview of the fundamentals of ink rheology, followed by an in‐depth discussion of the various methods to tailor the ink for DIW of different classes of materials. Then, the diverse applications of DIW ranging from electronics to food to biomedical industries are discussed. Finally, the current challenges and limitations of this technique are highlighted, followed by its prospects as a guideline toward possible futuristic innovations.

Direct Ink Writing: A 3D Printing Technology for Diverse Materials

An analytical model of peeling of an elastic tape from a substrate is presented for large deformations and scenarios where sliding occurs in the adhered regions, with this motion resisted by interfacial shear tractions. Two geometries are considered: the first has a detached segment of the tape forming the shape of an inverted letter ‘V’ between adhered sections (double-sided peeling), and the second has a free end of the tape being pulled (single-sided peeling). The mechanics of peeling is analyzed in terms of the applied force, displacement of the load point and the angle that the peeled tape makes with the substrate. Formulae are provided for the energy released per unit area of peeling that explicitly and separately account for the work done by frictional sliding. Assuming that peeling occurs when the energy released per unit area equals the work of separation for purely normal separation, it is shown that the critical force to propagate peeling can be significantly higher with sliding as compared to pure sticking. Similarly, due to frictional dissipation, the amount of work done by the applied force needed to propagate peeling can be significantly greater than the work of separation. For the single-sided peel test, an effective mixed-mode interface toughness is presented to be used with purely sticking models when sliding is not explicitly modeled: the closed-form result closely mirrors common empirical forms used to predict mixed-mode delamination.

Peeling of a tape with large deformations and frictional sliding

Deposition of ordered two-phase materials using microfluidic print nozzles with acoustic focusing

Acoustic field controlled patterning and assembly of anisotropic particles

New methods to 3D print multiphase materials with tailored microstructures could expand additive manufacturing capabilities to include structures with unprecedented complexity. One promising technique to control microstructure is acoustic focusing, a method by which particles in a fluid are manipulated by acoustic waves. By combining acoustic focusing with direct ink writing (where fluid inks are solidified after extrusion), the distribution of particles in fluid matrices can be modified throughout print lines. Here, we describe the design space for acoustic focusing with direct ink writing and explore the associated trade-offs, for inks composed of epoxy resin, silica, acetone, and glass microspheres. Increasing silica content increases the viscosity, widening the spatial distribution of particles, but it improves the preservation of printed shapes. Increasing the amplitude of the acoustic wave and decreasing the printing speed allow for narrower distributions of particles in printed lines but increase the time and energy needed for the print. To quantify these trade-offs, we conducted a combinatorial study of four factors: fumed silica and acetone loadings in epoxy-based matrices (which control ink rheology), print speed, and acoustic wave amplitude. We show that after deposition, particle distributions within focused lines are preserved; particle-poor regions on the edges of focused lines spread out farther than the particle-rich regions in the middle. The present study relating device operating parameters, ink properties and printed microstructures structure provide key insights to future designs of actuated print nozzles, which target new microstructures enabled by field-assisted control.

Acoustic control of microstructures during direct ink writing of two-phase materials

The remarkable ability of some plants and animals to cling strongly to substrates despite relatively weak interfacial bonds has important implications for the development of synthetic adhesives. Here, we examine the origins of large detachment forces using a thin elastomer tape adhered to a glass slide via van der Waals interactions, which serves as a model system for geckos, mussels and ivy. The forces required for peeling of the tape are shown to be a strong function of the angle of peeling, which is a consequence of frictional sliding at the edge of attachment that serves to dissipate energy that would otherwise drive detachment. Experiments and theory demonstrate that proper accounting for frictional sliding leads to an inferred work of adhesion of only approximately 0.5 J m(-2) (defined for purely normal separations) for all load orientations. This starkly contrasts with the interface energies inferred using conventional interface fracture models that assume pure sticking behaviour, which are considerably larger and shown to depend not only on the mode-mixity, but also on the magnitude of the mode-I stress intensity factor. The implications for developing frameworks to predict detachment forces in the presence of interface sliding are briefly discussed.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsif.2014.0453

Rachel R. Collino

Papers

Peeling of a tape with large deformations and frictional sliding

Deposition of ordered two-phase materials using microfluidic print nozzles with acoustic focusing

Acoustic field controlled patterning and assembly of anisotropic particles

Acoustic control of microstructures during direct ink writing of two-phase materials

Detachment of compliant films adhered to stiff substrates via van der Waals interactions: role of frictional sliding during peeling