Safety Auxiliary Feedback Element for the Artificial Pancreas in Type 1 Diabetes

Abstract

The artificial pancreas aims at the automatic delivery of insulin for glycemic control in patients with type 1 diabetes, i.e., closed-loop glucose control. One of the challenges of the artificial pancreas is to avoid controller overreaction leading to hypoglycemia, especially in the late postprandial period. In this study, an original proposal based on sliding mode reference conditioning ideas is presented as a way to reduce hypoglycemia events induced by a closed-loop glucose controller. The method is inspired in the intuitive advantages of two-step constrained control algorithms. It acts on the glucose reference sent to the main controller shaping it so as to avoid violating given constraints on the insulin-on-board. Some distinctive features of the proposed strategy are that 1) it provides a safety layer which can be adjusted according to medical criteria; 2) it can be added to closed-loop controllers of any nature; 3) it is robust against sensor failures and overestimated prandial insulin doses; and 4) it can handle nonlinear models. The method is evaluated in silico with the ten adult patients available in the FDA-accepted UVA simulator.

Postprandial response improvement via safety layer in closed-loop blood glucose controllers

“…Note that unlike the approach presented in [1], the IOB derived in this way is almost independent of the meal size used. This encourages implementing a constant value of IOB in order to evaluate the robustness using a single upper limit, 2 Note that the filter F has been replaced with respect to [1] as we assume G d is constant. which simplifies the tuning procedure.…”

“…The w + value was set to 150 mg/dl, and was set to 10 min. Finally, the IOB limit was determined following the procedure described in Section 2 instead of the provisional method suggested by [1]. All relevant parameters used in the controller are listed in Table 2.…”

“…Recently, a novel safety scheme based on sliding mode reference conditioning technology has provided a new approach to prevent hypoglycemia events, both for fully closed-loop control (the socalled Safety Auxiliary Feedback Element, SAFE [1]) and for hybrid configurations (the hybrid adaptive PD controller [29]). This safety layer employs an IOB estimation along with a flexible constraint which can be set based on individual parameters.…”

“…Unlike the precedent study [1], in this work we used a single IOB limit to reduce the risk of late postprandial hypoglycemia. This limit is related to the upper IOB constraint and its value is considered constant here.…”

“…Although a closed-loop controller for glucose regulation has to be tuned to minimize the postprandial excursion and avoid late hypoglycemia, the intrinsic limitations of the problem lead to a trade-off between postprandial peak and late hypoglycemia risk. This paper reveals through an intensive in-silico study with multiple controller tuning combinations that a novel safety layer for glucose controllers, the so-called SAFE loop (Revert et al, 2013), not only reduces the hypoglycemia events but also allows reducing the postprandial glucose excursion, thus breaking the implicit trade-off present in single controllers. The SAFE outer loop monitors the estimated amount of insulin on board, and modifies the control action if it is close to a unique constraint which can be adjusted with clinical criteria.…”