Weisi Li

Polyvinyl chloride as a multimodal tissue-mimicking material with tuned mechanical and medical imaging properties

PURPOSE The mechanical and imaging properties of polyvinyl chloride (PVC) can be adjusted to meet the needs of researchers as a tissue-mimicking material. For instance, the hardness can be adjusted by changing the ratio of softener to PVC polymer, mineral oil can be added for lubrication in needle insertion, and glass beads can be added to scatter acoustic energy similar to biological tissue. Through this research, the authors sought to develop a regression model to design formulations of PVC with targeted mechanical and multimodal medical imaging properties. METHODS The design of experiment was conducted by varying three factors-(1) the ratio of softener to PVC polymer, (2) the mass fraction of mineral oil, and (3) the mass fraction of glass beads-and measuring the mechanical properties (elastic modulus, hardness, viscoelastic relaxation time constant, and needle insertion friction force) and the medical imaging properties [speed of sound, acoustic attenuation coefficient, magnetic resonance imaging time constants T1 and T2, and the transmittance of the visible light at wavelengths of 695 nm (Tλ695) and 532 nm (Tλ532)] on twelve soft PVC samples. A regression model was built to describe the relationship between the mechanical and medical imaging properties and the values of the three composition factors of PVC. The model was validated by testing the properties of a PVC sample with a formulation distinct from the twelve samples. RESULTS The tested soft PVC had elastic moduli from 6 to 45 kPa, hardnesses from 5 to 50 Shore OOO-S, viscoelastic stress relaxation time constants from 114.1 to 191.9 s, friction forces of 18 gauge needle insertion from 0.005 to 0.086 N/mm, speeds of sound from 1393 to 1407 m/s, acoustic attenuation coefficients from 0.38 to 0.61 (dB/cm)/MHz, T1 relaxation times from 426.3 to 450.2 ms, T2 relaxation times from 21.5 to 28.4 ms, Tλ695 from 46.8% to 92.6%, and Tλ532 from 41.1% to 86.3%. Statistically significant factors of each property were identified. The regression model relating the mechanical and medical imaging properties and their corresponding significant factors had a good fit. The validation tests showed a small discrepancy between the model predicted values and experimental data (all less than 5% except the needle insertion friction force). CONCLUSIONS The regression model developed in this paper can be used to design soft PVC with targeted mechanical and medical imaging properties.

Contributions in medical needle technologies—Geometry, mechanics, design, and manufacturing

Abstract This article summarizes the contributions in research on tissue cutting with needles. The geometry of the needles cutting edges was analytically defined and expressions for the inclination and rake angles of hollow and solid needles and trocars have been derived. Based on the semi-empirical method, finite element model and the fracture mechanics approach, force models of needle insertion were developed. The relationship between the needles tip geometry and insertion force was established and used in several applications. It was shown, for example, that the cutting edge of the lancet needle can be optimally designed to minimize insertion force or bevel length. The cutting mechanics in rotary needle insertion was investigated along with the exploration of improvements of needle biopsy performance by decreasing the needle cutting and friction forces. The deflections of the needle during insertion were measured to develop a strategy for guiding the needle to the right position in brachytherapy and drug delivery. From an overall perspective, fundamental advances and application problems based on the cutting mechanics of soft tissue for needle were highlighted to lay the foundation for developments of biomedical device and improvements of healthcare procedures.

Measurement of the Friction Force Inside the Needle in Biopsy

Core needle biopsy (CNB) is widely used in active surveillance, which is the current standard of care for low risk prostate cancers. A longer biopsy sample length may improve the accuracy of diagnosis. To increase the biopsy sample length, the magnetic abrasive finishing (MAF) technique was applied to decrease the needle inner friction force, which may hinder the tissue from entering the lumen of the biopsy needle. To assess the effectiveness of these MAF polished needles as compared to the unpolished needles, a method to measure the three components of axial force during hollow needle insertion—tip cutting force, inner friction force, and outer friction force—was developed. Six tissue-mimicking samples of different lengths were used to find the linear relationship between the sum of the cutting force and inner friction force and the phantom length or contact length. Linear regression method was used to extrapolate and estimate the tip cutting force and the inner friction force. With this method, the difference between the inner friction force of the needles with and without polishing was found. The results showed that the unpolished needles had an inner friction force 40–50% higher and a tip cutting force 22% higher than their MAF polished counterparts. We also found that MAF polished needles had an average of 9% longer contact length between the sample and the inner wall than unpolished needles, indicating that a longer sample can be extracted at a lower friction force. The results of our investigation implied that reducing the inner surface roughness of a biopsy needle could reduce inner friction forces.