Inhalation therapy devices for the treatment of obstructive lung diseases: the history of inhalers towards the ideal inhaler

Inhalation therapy allows conveying drugs directly into the airways. The devices used to administer inhaled drugs play a crucial role in the management of obstructive lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). To ensure high bronchial deposition of the drug, a device should deliver a high proportion of fine particles, be easy to use, and provide constant and accurate doses of the active substance. Nowadays, four different types of inhalers are widely used: nebulizers, dry powder inhalers (DPIs), pressurized metered-dose inhalers (pMDIs), and soft mist inhalers (SMIs). Nebulizers can be used by patients unable to use other inhalers. However, they require long times of administration and do not ensure precise dosages. The first pMDIs became popular since they were small, inexpensive, fast, and silent. Their performance was improved by spacers and then by new technologies which reduced the delivery speed. In DPIs, micronized drug particles are attached to larger lactose carrier particles. No coordination between actuation and inhalation is required. However, the patient is supposed to produce an adequate inspiratory flow to extract the drug and disaggregate it from the carrier. In SMIs, the medication is dissolved in an aqueous solution, without propellant, and it is dispensed as a slow aerosol cloud thanks to the energy of a spring. Smart inhalers, connected to smartphones, are promising tools that can provide information about patient's adherence and their inhaler technique. Inhalation has also been proposed as a route of administration for several systemic drugs.