John W. Halloran

Mechanical and in vivo performance of hydroxyapatite implants with controlled architectures

Internal architecture has a direct impact on the mechanical and biological behaviors of porous hydroxyapatite (HA) implant. However, traditional processing methods provide minimal control in this regard. To address the issue, we developed a new processing method combining image-based design and solid free-form fabrication. We have previously published the processing method showing fabricated HA implants and their chemical properties. This study characterized the mechanical and the in vivo performance of designed HA implants. Thirteen HA implants with orthogonal channels at 40% porosity were tested on an Instron machine. The compressive strength and compressive modulus measured were 30+/-8 MPa and 1.4+/-0.4 GPa, comparable to coralline porous HA. Twenty-four cylindrical HA implants with two architecture designs, orthogonal and radial channels, were implanted in the mandibles of four Yucatan minipigs for 5 and 9 weeks. Normal bone regeneration occurred in both groups. At 9 weeks, bone penetrated 1.4mm into both scaffold designs. The percent bone ingrowth in the penetration zone was higher in the orthogonal channel design but not statistically different due to the low number of samples. However, the overall shape of the regenerated bone tissue was significantly different. In the orthogonal design, bone and HA formed an interpenetrating matrix, while in the radial design, the regenerated bone formed an intact piece at the center of the implant. These preliminary results showed that controlling the overall geometry of the regenerated bone tissue is possible through the internal architectural design of the scaffolds.

Hydroxyapatite implants with designed internal architecture

Porous hydroxyapatite (HA) has been used as a bone graft material in the clinics for decades. Traditionally, the pores in these HAs are either obtained from the coralline exoskeletal patterns or from the embedded organic particles in the starting HA powder. Both processes offer very limited control on the pore structure. A new method for manufacturing porous HA with designed pore channels has been developed. This method is essentially a lost-mold technique with negative molds made with Stereolithography and a highly loaded curable HA suspension as the ceramic carrier. Implants with designed channels and connection patterns were first generated from a Computer-Aided-Design (CAD) software and Computer Tomography (CT) data. The negative images of the designs were used to build the molds on a stereolithography apparatus with epoxy resins. A 40 vol% HA suspension in propoxylated neopentyl glycol diacrylate (PNPGDA) and iso-bornyl acrylate (IBA) was formulated. HA suspension was cast into the epoxy molds and cured into solid at 85 °C. The molds and acrylate binders were removed by pyrolysis, followed by HA green body sintering. With this method, implants with six different channel designs were built successfully and the designed channels were reproduced in the sintered HA implants. The channels created in the sintered HA implants were between 366 μm and 968 μm in diameter with standard deviations of 50 μm or less. The porosity created by the channels were between 26% and 52%. The results show that HA implants with designed connection pattern and well controled channel size can be built with the technique developed in this study.

Design of Battery Electrodes with Dual‐Scale Porosity to Minimize Tortuosity and Maximize Performance

Advances in materials and electrode architecture have facilitated remarkable improvements in the power performance of rechargeable batteries over the past decade, as represented by laboratory demonstrations of fast discharge [ 1–3 ] and commercial realizations such as packaged Li-ion cells with > 20 kW/kg at ∼ 65 Wh/kg. [ 4 ] However, even the most advanced of today’s batteries continue to have poor materials utilization, with only ∼ 50% of cell volume devoted to active materials in cells designed for high-energy density. [ 5 ] Thus, emphasis has now shifted towards maximizing energy density while retaining suffi cient rate capability to power critical applications such as portable devices and electric vehicles. In batteries using porous electrodes, cell-level energy density increases with electrode thickness and density, until limitations on accessible capacity are imposed by inadequate ion transport through a diminishing volume of tortuous, electrolyte-fi lled porosity. [ 6,7 ] For electrodes in which electronic conductivity and charge-transfer are not rate-limiting, ion transport in the percolating electrolyte network becomes limiting as current density increases beyond a critical value. [ 6 , 8 ] Since tortuosity τ , increases nonlinearly with density (e.g., following the Bruggeman relationship τ ∝ ε − 1/2 , where ε is pore fraction), attempts to increase cell energy density by densifying the electrodes rapidly reach diminishing returns. (Throughout, we defi ne τ = ε ( σ 0 / σ ), with ε the volume fraction porosity, σ a measured transport coeffi cient, and σ 0 the value of the transport coeffi cient for the porefi lling medium). Efforts to increase cell-level energy density by increasing electrode thickness (i.e., reducing inactive materials such as separators and current collectors) likewise incur electrolyte-transport limitations. Measured electrode tortuosities vary widely with electrode porosity and preparation, but commonly range between 2.5–30, [ 8–10 ] implying signifi cant room

Design of piezocomposite materials and piezoelectric transducers using topology optimization— Part III

SummaryCurrently developments of piezocomposite materials and piczoelectric actuators have been based on the use of simple analytical models, test of prototypes, and analysis using the finite element method (FEM), usually limiting the problem to a parametric optimization. By changing the topology of these devices or their components, we may obtain an improvement in their performance characteristics. Based on this idea, this paper discusses the application of topology optimization combined with the homogenization method and FEM for designing piezocomposite materials. The homogenization method allows us to calculate the effective properties of a composite material knowing its unit cell topology. New effective properties that improves the electromechanical efficiency of the piezocomposite material are obtained by designing the piezocomposite unit cell. This method consists of finding the distribution of the material and void phases in a periodic unit cell that optimizes the performance characteristics of the piezocomposite. The optimized solution is obtained using Sequential Linear Programming (SLP). A general homogenization method applied to piczoelectricity was implemented using the finite element method (FEM). This homogenization method has no limitations regarding volume fraction or shape of the composite constituents. The main assumptions are that the unit cell is periodic and that the scale of the composite part is much larger than the microstructure dimensions. Prototypes of the optimized piezocomposites were manufactured and experimental results confirmed the large improvement.

Stereolithography of ceramic suspensions

Rapid prototyping of ceramics is accomplished with stereolithography by using an SLA machine to build the ceramic green from a UV‐curable suspension of ceramic powders ‐ a “ceramic resin”. Objects are later sintered in a separate furnace to complete the process. Aluminium oxide resins based on hexanediol diacrylate are characterized for curing behaviour by photo‐rheology and differential photo calorimetry with a UV lamp, and with an HeCd laser using “windowpanes”, single strings, and walls.

Manufacturing and Characterization of 3‐D Hydroxyapatite Bone Tissue Engineering Scaffolds

Abstract: Internal architecture has a direct impact on the mechanical and biological behaviors of porous hydroxyapatite (HA) implants. However, traditional processing methods provide very minimal control in this regard. This paper reviews a novel processing technique developed in our laboratory for fabricating scaffolds with controlled internal architectures. The preliminary mechanical property and in vivo evaluation of these scaffolds are also presented.

FORMATION AND COARSENING BEHAVIOR OF Y2BACUO5 FROM PERITECTIC DECOMPOSITION OF YBA2CU3O7-X

Formation and coarsening behavior of the Y 2 BaCuO 5 (211) phase has been examined in samples produced by peritectic decomposition of pure YBa 2 Cu 3 O 7− x (123), resulting in 211 crystals and the liquid phase [BaCuO 2 -CuO]. Through various temperature (1020 °C-1060 °C) and time (0.25 h-10 h) studies, the fundamental coarsening behavior was determined. At 1040 °C, 211 crystals coarsen significantly over a 10 h period. The acicular crystals can be modeled by the diffusional ripening law, r – r 0 = ( Kt ) 1/3 , where r = V 1/3 . However, the log-normal distributions of the lengths, widths, and volumes for each coarsening run are much wider than general ripening theory would predict. Results from coarsening studies at 1020 °C and 1060 °C for 3 h reveal that the 211 crystal volume increases with increasing superheat. Prior coarsening of the 123 grain size yields much larger 211 particles, suggesting that the 211 crystals must nucleate at the 123 grain boundaries during peritectic decomposition, and this nucleation governs the size of the 211 crystals for short coarsening times. Addition of properitectic 211 (15 mole %) to pure 123 before peritectic decomposition strongly influences the particle habit of the resulting 211 crystals. Without any additions, acicular or needle-like 211 crystals result from the melting of 123. However, when equiaxed properitectic 211 is added to the 123, the resulting 211 is faceted, but still equiaxed. If acicular 211 is added to the starting composition, the resulting 211 is needle shaped. These results will be discussed in terms of 123 melt-texturing and directional solidification processing.

Image-Based Biomimetic Approach to Reconstruction of the Temporomandibular Joint

This article will present an image-based approach to the designing and manufacturing of biomimetic tissue engineered temporomandibular (TMJ) condylar prosthesis. Our vision of a tissue-engineered TMJ prosthesis utilizes a 3-D designed and manufactured biodegradable scaffold shaped similar to a condylar head and neck, i.e. a condylar-ramus unit (CRU). The fabricated CRU scaffold can be constructed with a specific intra-architectural design such that it will enhance the formation of tissue from implanted cells placed within its interstices. These biologic cues could influence scaffold-implanted mesenchymal stem cells (MSC) or bone marrow stromal cells (BMSC) to form a fibrocartilaginous joint surface, or cap, on top of a bony strut, similar to a costochondral rib graft (CCRG), which could be fixed to the mandibular ramus. This new approach to tissue engineering a TMJ would be advantageous because of its patient site-specific anatomical configuration as well as its potential ability to adapt to the loading forces placed on it during function.

Patch-antenna miniaturization using recently available ceramic substrates

The recent availability of high-contrast, low-loss ceramic materials provides us with possibilities for significant antenna miniaturization. This paper explores the use of low-temperature co-fired-ceramic (LTCC) substrates in producing a miniaturized patch-antenna design. Of particular interest in the design are parameters such as substrate thickness, input impedance, radiation efficiency, and bandwidth due to the high-contrast ceramic. We propose a thick substrate to increase bandwidth. However, the substrate is truncated to mitigate surface-wave loss, with possible texture to provide dielectric-constant control for improved impedance matching. Utilizing these proposed design modifications, a miniaturization factor of more than eight was achieved, with a return-loss half-power bandwidth greater than 9%. Moreover, respectable gain was maintained, given the achieved miniaturization

Design of 3-D Monolithic MMW Antennas Using Ceramic Stereolithography

Ceramic stereolithography is applied to the construction of 3-D monolithic low-loss subwavelength periodic structures. In the subwavelength regime, it is shown that the effective refractive index can be controlled throughout the structure through simple adjustments to the periodic lattice. The constraints imposed by subwavelength design and the limitations inherent in the ceramic stereolithography process are analyzed and incorporated into the design procedure for monolithic ceramic structures. To demonstrate the proposed method, a monolithic Luneberg lens is designed, fabricated, and measured. The measured results confirm the outlined design procedures and constraints