Digital materials

Digital Materials (DMs) are engineered materials manufactured from two or more different constituent materials, according to a digitally encoded three dimensional (3D) phase structure design (the DM code), and produced by an additive manufacturing (AM) process .
In DMs the constituent materials are combined together at a voxel level (figures 1-3), to create domains or phases with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or microscopic scale within the finished DM structure. Hierarchical structures in which macro voxels are created from more than one constituent material are also possible.

The Digital Material code (DM code)
The DM code defines the DM 3D phase structures design, by a set of digitally encoded rules or algorithms. The way the DM code defines a 3D phase structures is analogous to the way the genetic code in living organisms is responsible for dictating the characteristics of a living organism.
DMs can be divided into two main categories: Isotropic and Anisotropic DMs.
Isotropic DMs
In this type of DM, the constituent materials are combined homogeneously as for example, a non-continuous phase made of constituent material “a” randomly “dispersed” within a continuous phase made of constituent material “b” (figure 3). A uniform combination between constituent materials, or a mix of uniform and random combinations are also possible. The combination of constituent materials can be at the single voxel level as in figure 3, but may also be at higher level as for example a number of voxels as the minimum amount of one constituent material.
Anisotropic DMs
These types of DMs have an anisotropic 3D phase structure design, and therefore anisotropic properties along different axis within a single object (figure 4).
Anisotropic DMs may also be Geometry Dependent; in this type of DM there is a “dialog” between the object design and the DM code, which results in different constituent material arrangements in different object designs, or in different regions within a single object. In Geometry Dependent DMs the DM code is responsible for defining the rules that govern the DM composition as a function of a respective object geometry and size; analogous to the way the genetic code in living organisms is responsible for dictating the characteristics of a living organism. The DM code defining a Geometry Dependent DM comprises a set of rules or algorithms that when applied to the manufacturing of a specific object, permits allocating constituent materials in different object regions, according to the specific object design. Thus, in Geometry Dependent DMs, the DM is not be produced for subsequent use in the manufacture of a desired object; rather the DM production process and the object manufacturing process are one.
An example of a Geometry Dependent DM code algorithm is presented here:
#Two constituent materials: A and B
#The object is made of constituent material A in all object regions besides those defined as made of constituent material B
#Constituent material B is used to form as 1mm deep region comprising the outer surface of the object
In this example, every object region having a thickness of 2mm or less, is produced solely from constituent material B, while object regions thicker than 2mm are constructed of two regions, an outermost region of 1mm in thickness made of constituent material B and a core made of constituent material A.
Geometry Dependent DMs can be further divided into two subcategories: Step DMs and Graded DMs. While in Step DMs constituent materials are combined in substantially well-defined phases, as shown in figure 5, in Graded DMs the DM composition varies gradually along at least one defined trajectory or axis of an object, in a graded fashion. An example of a Graded DM is one in which the DM composition on one side of the object is substantially richer in one constituent material than another, while on other side of the object, the DM composition is substantially richer in the other constituent material, and where the content ratio between the constituent materials gradually varies from one side of the object to the other.
There is also the possibility of Geometry Dependent DMs having Step as well as Graded characteristics within a single object.
DM Production
Three elements are necessary in order to produce a DM: the DM code, the DM constituent materials and an appropriate additive manufacturing (AM) or 3D printing system enabling physical realization of the DM code.
Objet Ltd. a pioneer in the field of DMs, patented a method for producing DMs in 2003. The same company introduced the first Connex multi-material 3D printing system in 2008 for additive manufacturing which permits the simultaneous use of three UV curable resins to produce DMs: one supporting resin and two modeling resins. Together with the Objet Connex system, Isotropic DMs were also commercially released. More recently (2011), a new ABS-like (RGD5160-DM) Geometry Dependent DM was released, also by Objet Ltd.
Objet RGD5160 DM (Objet ABS-like Digital Material) is an example of a commercially available DM. This DM is manufactured from two UV curable resins, Objet RGD513 and Objet RGD525, according to a DM code, using an AM system known as the Objet Connex family of 3D printers, for example the Objet Connex500.
A change in any one of these three elements, the constituent materials, the DM code or the manufacturing system is likely to result in a different DM.
Digital Materials Research
Different aspects in the field of DMs have recently been the focus of intensive research, in the industrial sector as well as in the academic sector. According to Mary C. Boyce et al , co-continuous glassy polymer/rubbery materials with sub-millimeter feature size, fabricated using a 3D printer, expose enhancements in stiffness, strength and energy dissipation. According to Mary C. Boyce, “geometric and topological arrangement of the constituent materials provides avenues to engineer the macroscale material properties”.
A data-driven process for designing and fabricating materials with desired deformation behavior has been reported . According to this report, “an optimization process that finds the best combination of stacked layers that meets a user’s criteria specified by example deformations” has been developed. In this study, and in order to demonstrate the optimization process validity, objects with complex heterogeneous materials were fabricated using a modern multi-material 3D printer.
In another study , “a complete pipeline for measuring, modeling, and fabricating objects with specified subsurface scattering behaviors” was proposed. According to the authors, the process was validated by producing homogeneous and heterogeneous materials using a multi-material 3D printer.
Neri Oxman takes nature as a model, and proposes what she calls “Variable Property Design (VPD)”, as a method for design, in which “material assemblies are modeled, simulated and fabricated with varying properties”, in order to give an answer to functional constraints. Oxman's Variable Property Rapid Prototyping approach aims at integrating between material properties and environmental constraints within the computational modeling environment and as part of the form-generation and fabrication process (ref 11). Her project FABRICOLOGY explores the use of digital materials in the context of sustainable design and has named her winner of the first Earth Award ().
Hod Lipson et al has reported the simulation of materials properties as a function of their “digital material” composition. According to this report, properties as stiffness, CTE, and failure modes were obtained by varying voxel manufacturing precision, the percentage of randomly distributed constituent materials, and the voxel microstructure. Furthermore, it has been stated by the authors that material properties may be tuned anywhere between the respective properties of two constituent materials, by simple randomly halftone a percentage constituent materials. In addition, properties such as stiffness or negative Poisson’s ratio, has been reported to be obtained using relatively dense common materials, by the inclusion of a hierarchical voxel microstructure.

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