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Subcutaneous Infusion Device

Winter 2017 - MAE 156A / Spring 2017 - MAE 156B 

University of California, San Diego

Sponsored by Capillary Biomedical, Inc.

• Project Background

• Client Information

• Capillary Biomedical is a medical device startup developing technologies for diabetes management. The sponsor aims to dramatically simplify glucose monitoring and insulin delivery, reducing the burden on both patients and the overall healthcare system. The ultimate goal of Capillary Biomedical is a fully-implantable automated insulin delivery system consisting of a long-term continuous glucose monitor, insulin infusion pump and closed-loop control algorithm capable of automatically managing blood glucose.

• Medical Information

• Currently, more than one million people in the world manage their type 1 diabetes with an insulin pump and a continuous subcutaneous insulin infusion (CSII) catheter. Traditionally diabetes have injected insulin using insulin syringes manually; however, patients can automatically inject their required amount of insulin dosage after the invention of insulin pump and CSII. Patients routinely insert a new CSII into the subcutaneous tissue every 2 to 4 days because insulin absorption into the circulation becomes more variable and less reliable over time. The tissue’s immune response to CSII insertion and maintenance changes over time; and may cause variable insulin absorption into the circulation. CSII insertion damages capillaries, lymphatics, adipose cells, connective tissue, and extra-cellular matrix. Inflammatory

• • Image Source: http://www.aboutkidshealth.ca/EN/Assets/IMD_insulin_pump_function_EN_72.jpg

• Project Objective

• Problem Definition

• Capillary Biomedical wants to develop a new Continuous Subcutaneous Insulin Infusion (CSII) device which utilizes a helicon insertion path to reduce risk of dislodging and increase insulin distribution with perforations near the tip. And our mission is to design an inserting device and housing case for the Capillary CSII Infusion Set.

• • •  Capillary Biomedical, April 2017

• Final Design Description

In the two pictures above, one can see the internal components of our device which includes a screw that drives the torsional spring and a guided path which guides the needle to prevent kinking. The stylet is manually retracted from the device, leaving a soft cannula in place

• Summary of Performance

          Theoretical Prediction

From the torsional spring analysis, it was estimated that the selected torsional spring has a spring constant of K=0.02697 Nm/rad. Given the assumption that 3 N of insertion force is required to penetrate the skin with 30 deg insertion angle and the device needs to deliver the stylet into the subcutaneous tissue at a desired depth of 10 mm with diameter of 1.778 cm (0.7 in) helical shape stylet, it was concluded that 168 degree of torsional spring movement was required.

The velocity of the insertion moment was unknown as the spring will be instantly disengaged from ratchet pawl and rotate the 168 degree within a second. However, it was expected that the device delivers at least 3 N of insertion force with minimum velocity of 60 mm/min.

Currently, the Capillary Biomedical wants to utilize a needle which is made with 31 G stainless steel stylet and 24 G nylon cannula. The team conducted buckling analysis to investigate whether the given needle would penetrate the skin with the given testing condition: 30 degree of insertion angle and 10 mm of insertion depth. The result of the analysis stated that the insertion needle with 31G stylet and 24G nylon has the possibility of buckling at length of 5.46 mm

The figure above shows that the needle may not be inserted correctly when it is longer than 5.46 mm with minimum material strength. Assuming that the two materials behave with the maximum material strength, the maximum safe insertion length can be up to 14 mm. Due to the insertion angle of 30 degree, the maximum safe insertion depth would be only 7 mm which does not satisfy our original target depth of 10 mm. 

Testing Result

Testing with the final prototype resulted in successful insertion up to 6mm depth at 30 degree insertion angle with the 24G music wire provided by Capillary Biomedical. As the buckling analysis shows, the device had problems inserting the 31G needle longer than 12mm. When the guiding path was altered for 45 degree insertion angle, the 24 G needle could achieve the 10mm insertion depth with the needle length of 14mm. The buckling analysis shows that the insertion length above 6mm (minimum material strength) or 14mm (maximum material strength) has high possibility of insertion failure; consequently, the final prototype test result shows that performance of the device complies with the estimation.

With measuring time duration for ten different trials, it is estimated that the device rotational velocity is 137.41 rad/s.

• Executive Summary

• Capillary Biomedical is a medical device company focused on developing new methods and technologies to allow patients with diabetes to better manage their glucose levels. Continuous Subcutaneous Insulin Infusion (CSII) therapy provides diabetes patients with an automated method of managed their glucose levels, thereby removing the need to self-monitor, self-dose, and self-administer insulin injections. After penetration and removal of a sharp stylet (needle), the CSII devices will remain in the body for up to three days, and follow a control algorithm determined by a sensor that is also placed on the body. Capillary BioMedical’s goal is to develop a CSII therapy device that features a helical fluid path, improving upon existing straight fluid path designs by reducing the risk of dislodging and improving insulin absorption by the body with perforated openings throughout the soft cannula. The insulin infusion set and soft cannula are accompanied by an external device that features a torsional spring to generate the necessary penetration force to reach the Subcutaneous tissue 8 millimeters under the surface of the skin. The helical shaped stylet, encased within the soft cannula, is used to penetrate the skin at a minimum force of 3N generated by the torsional spring. A spring is utilized between the teflon cannula and the stainless steel stylet to reduce the risk of kinking during and after penetration. From the beginning, our objective was to design a device which can penetrate the skin in a helical shaped manner (at an 30 degree of 45 degree insertion angle) and then retract the stylet. The goal was to reach a variable depth of 6mm to 10mm. In the end we delivered a 1:1 scaled prototype product which can be used to insert the helical shaped needle. In addition, multiple calculations and analysis were conducted for the spring and stylet. Our analysis showed that the given needle (31 gauge stylet and 24 gauge cannula) will buckle before the target depth. This result was reflected in our testing phase where the needle would kink upon insertion. However, a 24 gauge music wire easily penetrated the ziplock bag (6 mm depth) and ballistic gel combination at the target depths of 6mm and 10mm respectively. Looking forward, our recommendation would be to change the design of the company provided needle. This includes making the needle shorter to increase stiffness, making it less likely to buckle. Furthermore, a change in the insertion angle from 30 degrees to 45 degrees is recommended so that the needle can be inserted 10 mm deep rather than the 6 mm that the 30 degree insertion angle provides. It is important to mention that successful penetration was only achieved with 24 gauge (or larger in width) music wire.

THIS PART WILL BE DELETED BY 11:59PM, June 14, 2017

Guidelines for Creating your Project Webpage

1. The project webpage can serve as a valuable link on student resumes for years to come; so organize it well!

2. The web page should be easily readable by anyone with general interest in the project, as well as provide technical detail and documentation for engineers. 

3. The front page is critical. If it is not clear and exciting, no one will click further. See example of high quality front pages below: 

   • https://sites.google.com/a/eng.ucsd.edu/156b-2012-spring-team21/ 

   • https://sites.google.com/a/eng.ucsd.edu/156b-2012-winter-team2/

   • https://sites.google.com/a/eng.ucsd.edu/156b-2012-winter-team3/

   • https://sites.google.com/a/eng.ucsd.edu/156b-2012-winter-team10/

   • http://www.maeprojects.ucsd.edu/mae156/fw2009/group4/Site/Home.html

4. Remove these guidelines as you you create your own page material.

This First page is a Key Part of the Webpage and Should include:

• The first page is especially important, since a user should get a complete overview of the project and results on that page, otherwise they will never follow any subsequent links. If the first page is not engaging, users will not click to any of the subpages. Always put your best material on the first page, even if it is duplicated elsewhere on the website.

• Comprehensive overview of project, including:

  • Project background and objective, 

  • Concise description of final design. 

  • Summary of performance results. 

  • Include link to Executive Summary

• Picture(s) of overall project

• CAD model with annotations of final product; CAD animations if applicable. 

• Photos and CAD figures should include effective use of annotated drawings.

• Movie of final product with narration. See fun examples by Treyscope team and Knee Brace Cycle Tester.