Design, development and validation of a test bench for characterizing soft tissue permeability

Tissue engineering is a branch of biomedical engineering which develops biological devices to restore and improve the functions of diseased human tissues or organs. The majority of tissue engineering techniques utilize a scaffold used to provide a suitable and native-like environment for cell attachment, growth, organization and proliferation. Scaffold’s permeability is a fundamental parameter to determine its regenerative efficiency. There are two main approaches to evaluate scaffold’s permeability: semi-empirical models which use the scaffold’s topological features to obtain its permeability, and experimental approaches based on the development of a test bench. This last kind of approach can be further divided into pump-based methods and gravity-based methods. The aim of this work was the design and the development of a permeability system, based on the pump-based methods, which allow the evaluation of both soft and rigid biological tissues’ permeability using Darcy’s law. It is a relation which, knowing the flow rate imposed by the pump and the pressure drop measured by pressure sensors across the sample, permits to obtain its permeability. The permeability system was designed to satisfy specific requirements also given by the difficulty in treating soft samples since they are highly deformable and therefore difficult to fix and not to compress. In the specific the design requirements include ease of use, watertigthness, versatility to be used with samples with different sizes (diameter and height), modularity, fixation of the sample and producing of a constant flow. A permeability chamber, composed of two rectangular parts (with a cylindrical coupling between them) coupled by screws was designed and manufactured to allow housing samples of different size (height = 1-14 mm, diameter = 8-27 mm) using customized PDMS flexible gaskets and a grid. The structure of the permeability chamber and of the gaskets allow the fluid to flow through the sample while firmly holding it and avoiding deformations. The permeability chamber, mould gaskets and grid were produced using additive manufacturing technologies (Stereolitography and Fused Deposition Modeling). The permeability chamber is integrated in an open-loop hydraulic circuit, composed of a reservoir filled with water, a peristaltic pump, a dampener, an outlet reservoir, pressure sensors, a scale, pumphead tubing, circuit tubing and 3-way valves. Computational fluid-dynamic simulations were carried out using COMSOL Multiphysics for assessing the range of pressure drops across the permeability chamber (with and without the sample inside it) varying sample permeability and flow rate. Preliminary tests showed that the developed system meets the design requirements of versatility, modularity, ease of use and fixation of the sample. Also the sealing of the gaskets were demonstrated. Instead, the requirement of a constant flow has not been met since the dampener was not properly sized for our system. Computational results predicted the range of pressure drops across the chamber varying conditions and allowed us to select the right pressure sensors for our system. In the next future, the dampener will be correctly sized through a model proposed here and pressure sensors will be purchased. Further simulations will be carried out to define a range of acceptable flow rates in which the sample doesn’t undergo excessive deformations.