Use of rapid prototyping models in the planning of percutaneous pulmonary valved stent implantation

Percutaneous replacement of the pulmonary valve is a recently developed interventional technique which involves the implantation of a valved stent in the pulmonary trunk. It relies upon careful consideration of patient anatomy for both stent design and detailed procedure planning. Medical imaging data in the form of two-dimensional scans and three-dimensional interactive graphics offer only limited support for these tasks. The paper reports the results of an experimental investigation on the use of arterial models built by rapid prototyping techniques. An analysis of clinical needs has helped to specify proper requirements for such model properties as cost, strength, accuracy, elastic compliance, and optical transparency. Two different process chains, based on the fused deposition modelling technique and on the vacuum casting of thermoset resins in rubber moulds, have been tested for prototype fabrication. The use of anatomical models has allowed the cardiologist's confidence in patient selection, prosthesis fabrication, and final implantation to be significantly improved.     

Selection of layered manufacturing techniques by an adaptive AHP decision model

The paper describes a computer-based tool for the selection of techniques used in the manufacture of prototypes and limited production runs of industrial products. The underlying decision model, based on the AHP methodology, ranks available techniques by a score resulting from the composition of priorities at different levels, each considering homogeneous and independent evaluation criteria. At each level, priorities are calculated from pairwise comparisons among either manufacturing techniques or properties of the techniques with respect to different types of application. This approach is enhanced with a procedure that adapts the parameters of the decision model to prototype specifications. Compared to scoring procedures, the method reduces the ambiguity on the values of weighting factors and allows an easier interpretation of the rationale behind the selection process.     

On the role of complexity in machining time estimation

Early cost estimation of machined parts is difficult as it requires detailed process information that is not usually available during product design. Parametric methods address this issue by estimating machining time from predictors related to design choices. One of them is complexity, defined as a function of dimensions and tolerances from an analogy with information theory. However, complexity has only a limited correlation with machining time unless restrictive assumptions are made on part types and machining processes. The objective of the paper is to improve the estimation of machining time by combining complexity with additional parameters. For this purpose, it is first shown that three factors that influence machining time (part size, area of machined features, work material) are not fully captured by complexity alone. Then an optimal set of predictors is selected by regression analysis of time estimates made on sample parts using an existing feature-based method. The proposed parametric model is shown to predict machining time with an average percentage error of 25% compared to the baseline method, over a wide range of part geometries and machining processes. Therefore, the model is accurate enough to support comparison of design alternatives as well as bidding and make-or-buy decisions.

     

A method for computer-aided specification of geometric tolerances

The paper describes a method for the generation of tolerance specifications from product data. The problem is nontrivial due to the increasing adoption of geometric dimensioning criteria, which call for the use of many types of geometric tolerances to completely and unambiguously represent the design intent and the many constraints deriving from manufacturing, assembly and inspection processes. All these issues have to be modeled and explicitly provided to a generative specification procedure, which may thus need a large amount of input data. The proposed approach tries to avoid this difficulty by considering that most precision requirements to be defined relate to the assembly process, and can be automatically derived by analyzing the contact relations between parts and the assembly operations planned for the product. Along with possible user-defined additional requirements relating to function, assembly requirements are used in a rule-based geometric reasoning procedure to select datum reference frames for each part and to assign tolerance types to part features. A demonstrative software tool based on the developed procedure has allowed to verify its correctness and application scope on some product examples.     

SLM tooling for die casting with conformal cooling channels

The paper reports an experimental study of die-casting dies with conformal cooling fabricated by direct-metal additive techniques. The main objective is to compare the benefits and limitations of the application to what has been widely discussed in literature in the context of plastics injection molding. Selective laser melting was used to fabricate an impression block with conformal cooling channels designed according to part geometry with the aid of process simulation. The tool was used in the manufacture of sample batches of zinc alloy castings after being fitted on an existing die in place of a machined impression block with conventional straight-line cooling channels. Different combinations of process parameters were tested to exploit the improved performance of the cooling system. Test results show that conformal cooling improves the surface finish of castings due to a reduced need of spray cooling, which is allowed by a higher and more uniform cooling rate. Secondary benefits include reduction of cycle time and shrinkage porosity.     

Long-term results of percutaneous ethanol injection therapy for hepatocellular carcinoma in cirrhosis: a European experience

The objective of our work was to evaluate the long-term results of percutaneous ethanol injection (PEI) for the treatment of hepatocellular carcinoma (HCC) in patients with liver cirrhosis. A total of 184 cirrhotic patients with HCC underwent PEI as the only anticancer treatment over an 8-year period. Patients were followed after therapy by means of clinical examinations, laboratory tests, and US and CT studies performed at regular time intervals. Survival rates were determined according to the Kaplan-Meier method. The overall survival was 67% at 3 years, 41% at 5 years, and 19% at 7 years. The 3-, 5-, and 7-year survival rates of patients with single HCC < or = 3 cm (78, 54, and 28%, respectively) were significantly higher (p < 0.01) than those of patients with single HCC of 3.1-5 cm (61, 32, and 16, respectively) or multiple HCCs (51, 21, and 0%, respectively). Survival of Child-Pugh A patients (79% at 3 years, 53% at 5 years, and 32% at 7 years) was significantly longer (p < 0.01) than that of Child-Pugh B patients (50% at 3 years, 28% at 5 years, and 8% at 7 years). A selected group of 70 patients with Child-Pugh A cirrhosis and single HCC < or = 3 cm had a 7-year survival of 42%. Long-term survival of cirrhotic patients with HCC treated with PEI is comparable to that reported in published series of matched patients submitted to surgical resection.     

Modeling of porous structures for rapid prototyping of tissue engineering scaffolds

Rapid prototyping techniques are increasingly used to build porous scaffolds for the regeneration of biological tissues. This paper deals with one of the most critical tasks involved by this option, i.e., the preparation of geometric data for layered fabrication. Compared to other existing approaches, this work aims at both reducing the required effort in interactive modeling and allowing a standard use of commercial prototyping systems without resorting to special treatments. The proposed method adds a porous structure to the geometric model of tissue surface in polygon format. The structure is of the Cartesian type and consists of an interconnected network of rectilinear channels, whose dimensions can be varied according to desired porosity and pore size. The algorithmic procedures needed for the generation of the porous structure have been implemented in a demonstrative software tool. Sample models of scaffolds have been generated and used to build prototype parts by different fabrication processes and systems.