1 Overview

 

Our device is an intraoperative pouch that can indicate the location of air leaks during a VATS surgery. The device is built upon an airtight grid and has three layers built within each cell of it. The four components include a nonporous layer that is in contact with the lung tissue, an indicator material, an insulator layer, and a grid structure.

 

 

2 Inner layer - Polyurethane Foam

The inner layer is composed of cold hibernated elastic memory (CHEM) Polyurethane-based foam. Polyurethane is a polymer widely used in medical application. It can be modified into different structures with various possible properties. For our project, we use a type of closed-cell polyurethane foam for the inner layer in contact with the lung tissue. The closed-cell structure enables the surface to be nonporous, yet permeable to air. Therefore the air to be detected, i.e. oxygen, can penetrate through this surface and reach the indicator layer. This layer also acts as a barrier that segregates the biological tissue and the less biocompatible layer of indigo dye. The polyurethane foam can be synthesized through various procedures with low costs. 

 

 

 

 

 

 

 

3 Indicator layer - Leuco Indigo Dye

The oxygen indicator material is composed of a mixture of leuco indigo dye and a highly oxygen-permeable hydrogel. Upon mixing with molecular oxygen, the leuco indigo experiences a color change from nearly colorless to a shade of blue. The speed of the color change depends on the concentration of oxygen in the gases administered to the patient as well as on how thoroughly the oxygen mixes with the leuco dye on the hydrogel mixture.

 

Leuco Indigo

The virtually colorless lueco indigo undergoes an irreversible (under use conditions) and visible color change to blue upon oxidation by molecular oxygen. The color change process progresses through several shades of blue before settling on a deep blue color. Once the lueco indigo dye is oxidized by molecular oxygen it returns to its indigo form. Indigo, unlike leuco indigo is not water soluble. 

 

 

 

 

 

 

 

 

 

Indigo is a vat dye that becomes water soluble and almost colorless after reaction with a suitable reducing agent. Leuco indigo is prepared from indigo (synthetic) via a reduction reaction. A solution of leuco indigo of approximately 3 M is made by vat dye reduction of indigo dye with alkaline (using 1 M sodium hydroxide), aqueous solution of sodium dithionite (20 mM) under nitrogen atmosphere at 95 °C. The indigo dye is reduced in a nitrogen environment in order to prevent any oxidation from ambient oxygen. The resulting solution is nearly colorless. This solution is lyophilized using a freeze dryer in order to form the solid, water soluble leuco indigo.

 

Indigo is available from Sigma Aldrich under Product Number 229296. Sigma Aldrich sells 25g of the powder for 30.70 USD through its website (100g can be purchased for 103.00 USD). 1 M Sodium Hydroxide is available from Sigma Aldrich under Product Number 71463. A 1 liter container of the sodium hydroxide solution is available for purchase for 24.20 USD via the Sigma Aldrich website. Sodium Dithionite (also called sodium hydrosulfite) is available from EMD Millipore. EMD Millipore sells a 500g container of the powder for 124.30 USD (2.5kg can be purchased for 389.90 USD). Nitrogen gas (≥99.998%) is available from Sigma Aldrich under Product Number 71463. Sigma Aldrich sells a 56 liter container or compressed nitrogen gas for $228.00. 

 

Hydrogel

There are many hydrogels available on the market. A highly oxygen-permeable hydrogel polymer was chosen in order to facilitate the passage of oxygen, the oxidizing agent, through the material. Property 74 Filcon V3, a silicone hydrogel developed by Contomac was chosen due to its high oxygen permeability. The hydrogel can be lathe cut to specifications.

 

Indicator Layer Composition

Discs, composed of the Property 74 Filcon V3 hydrogel will be cut and soaked in a 9:1 (v:v) methanol:water mixture in which 10 mg of leuco indigo is also dissolved. This mixture will be stirred and soaked for 30 minutes. The hydrogel disc will then be dried under nitrogen atmosphere. The dried hydrogel disc is then incorporated into the oxygen detecting device. Liquid Methanol (analytical standard) can be purchased from Sigma-Aldrich under Product Number 82762. Sigma-Aldrich sells a liter container of liquid methanol for $11.60.

 

4 Insulator Layer – Cellulose film

The insulator layer is designed to isolate the entire device, including the indicator materials and the grid structures, and potentially existed gas from air leaks, from the chest cavity. This purpose requires the material to have high gas barrier property and high flexibility.

 

The insulator layer will be composed of transparent cellulose films fabricated from aqueous alkali (NaOH)/ urea (AU) solutions. A minimum oxygen permeability of 0.003 (mL µm m-2 day-1kPa-1) at 0% relative humidity can be realized. Cellulose is an organic compound with the formula (C6H10O5)n. It has abundant hydroxyl groups that allows it to be made into hydrogels with necessary structures and properties. In addition, cellulose is one of the most abundant renewable resources on earth, making the price of cellulose products relatively low. The production method of cellulose films has also been improved recently that no hazardous byproducts will be generated in the process.

 

Preparation of Films

The solvent will be prepared by mixing LiOH/urea/H2O with a weight ratio of 4.6:15:80.4 and cooling at -12 degrees Celsius. Microcrystalline cellulose powder (435236, Sigma-Aldrich) will be used. 6% (weight percent) of cellulose will be dispersed in the solvent and stirred for 2 minutes to form a transparent and viscous solution. The solution then undergoes regeneration processes introduced in Yang, 2011. The regenerated cellulose hydrogel will be thoroughly washed with deionized water and air dried at ambient temperature. The cellulose film will be dried at room temperature and 0% relative humidity in a desiccator containing phosphorous pentaoxide for at least 3 days.

 

The film has high oxygen barrier property, with the oxygen permeability of 0.003 (mL µm m-2 day-1kPa-1) at 0% R.H. The tensile strength of the film is 150MPa, which is not very elastic compared to hydrogels. The solution to resolve the issue with elasticity of the films will be addressed in the assembling section.

 

5 Grid – Cellulose Aerogel

The grid is the supporting structure of the device. It also functions to isolate the indicator materials in each grid to ensure maximum accuracy for locating air leaks. These purposes require the grid material to have supportive structure, impermeable to oxygen, and light weighted.

 

The grid will be composed of cellulose aerogel. Aerogel is a porous structure produced from a gel, by replacing the liquid component of the gel with gas. The replacement of liquid with gas makes aerogel extremely light, and has low density and low thermal conductivity. Cellulose aerogel provides both good oxygen barrier property and supporting ability.