Abstract: The invention relates to the field of medicine. More specifically, the invention relates to an expandable plug for closing a defect in an organ or tissue without the need for sutures, wherein the plug has shape memory. The invention further relates to medical use of such plug.

Abstract: Described is a method for obtaining hollow particles, having a particle wall and a particle lumen, the particle having dimensions of between 1 nm and 100 ?m, from a mixture comprising a liquid medium comprising at least one colloid or solute, the method comprising freezing said mixture and lyophilising the obtained frozen mixture, characterised in that a volume of at least 0.1 ?l of the mixture is subjected to a freezing step comprising: (a) (1) quench freezing the mixture resulting in a quench frozen mixture, and (2) incubating said quench frozen mixture at a temperature above the quench freezing temperature and below the melting point of the liquid medium, orb) (1) reducing the temperature of the mixture at a rate of 1 to 100° C./minute to below the freezing temperature of the mixture and (2) incubating said frozen mixture at a temperature above the temperature of the mixture and below the melting point of the liquid medium.