Current Opinion in Solid State and Materials Science 6 (2002) 245–250 Direct-write assembly of ceramics from colloidal inks * Jennifer A. Lewis Materials Science and Engineering Department, Chemical Engineering Department, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, 1304 W . Green St., Urbana, IL 61801, USA Received 1 May 2002; accepted 9 May 2002 Abstract Direct-write assembly of ceramic materials from colloidal inks allows the creation of complex 3-D structures that are inaccessible by traditional processing routes. The success of these emerging techniques hinges critically on the development of colloidal inks with tailored properties. Recent advances in this field are reviewed, which give rise to questions of practical as well as fundamental importance and a broad array of research opportunities.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Colloidal inks; Direct-write; Ink-jet printing; Robotic deposition; Materials assembly 1. Introduction dal inks (see Fig. 1) can be created with solids volume fraction and rheological behavior that vary over a broad Direct-write assembly techniques [*1,2] offer the ability range. The important rheological parameters for a given to design and rapidly fabricate ceramic materials with ink may include its apparent viscosity, yield stress under complex 3-D structures required for structural [3,4], elec- shear and compression, and viscoelastic properties (i.e., the trical [5–7], photonic [8–10], and biomaterial [11] applica- loss and elastic moduli). Typically, such parameters are tions. The techniques of interest involve materials assem- tailored for the specific direct-write technique used. bly through a layer-by-layer deposition of colloid-based Direct-write techniques involving colloidal ink deposi- inks. Several techniques are currently under development, tion can be divided into two approaches: (1) droplet-based including direct ink-jet printing [12], hot-melt printing and (2) continuous (or filamentary) techniques, as illus- [*13], micropen [14], and robotic deposition (or robocast- trated in Fig. 2. Here, recent advances in this field are ing) [15,16,*17]. This nascent area is part of a growing reviewed with an emphasis on bulk ceramics fabricated by field known as solid freeform fabrication [*18] that ink-jet printing and three-dimensional (3-D) periodic struc- provides an enabling technology for forming materials tures formed by robotic deposition. These latter structures without the use of expensive tooling, dies, or molds. are of interest for a variety of technological applications, Direct-write techniques rely on the formulation of including piezoelectric composites [5–7], structural com- suitable colloidal inks for a given deposition scheme. The posites [3,4], tissue engineering scaffolds [11], and photon- term ‘colloid’ is used to describe particles that possess at ic band gap materials [8–10]. Opportunities and challenges 23 least one dimension in the size range 10 to 1 mm. A associated with direct-write fabrication of ceramics are distinguishing feature of all colloidal systems is that the highlighted. Finally, the main concepts are put forth are contact area between particles and the dispersing medium briefly summarized. is large. As a result, interparticle forces strongly influence suspension behavior. Long range van der Waals forces are ubiquitous, and must be balanced by Coulombic, steric, or 2. Droplet-based techniques other repulsive forces to engineer the desired degree of colloidal stability [*1,19]. Dispersed or flocculated colloi- Direct ink-jet printing of ceramics [12] and related approaches, such as hot-melt printing [*13], involve direct deposition of colloidal inks to produce the desired 3-D *Tel.: 11-217-244-4973; fax: 11-217-333-2637. E-mail address: jalewis@uiuc.edu (J.A. Lewis). ceramic structure. Such techniques have as their basis, 3-D 1359-0286 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S1359-0286(02)00031-1