*Review* **Review of Functional Treatments for Modified Wood**

**Morwenna J. Spear \*, Simon F. Curling, Athanasios Dimitriou and Graham A. Ormondroyd**

The BioComposites Centre, Bangor University, Deiniol Road, Bangor LL57 2UW, UK; s.curling@bangor.ac.uk (S.F.C.); a.dimitriou@bangor.ac.uk (A.D.); g.ormondroyd@bangor.ac.uk (G.A.O.) **\*** Correspondence: m.j.spear@bangor.ac.uk

**Abstract:** Wood modification is now widely recognized as offering enhanced properties of wood and overcoming issues such as dimensional instability and biodegradability which affect natural wood. Typical wood modification systems use chemical modification, impregnation modification or thermal modification, and these vary in the properties achieved. As control and understanding of the wood modification systems has progressed, further opportunities have arisen to add extra functionalities to the modified wood. These include UV stabilisation, fire retardancy, or enhanced suitability for paints and coatings. Thus, wood may become a multi-functional material through a series of modifications, treatments or reactions, to create a high-performance material with previously impossible properties. In this paper we review systems that combine the well-established wood modification procedures with secondary techniques or modifications to deliver emerging technologies with multi-functionality. The new applications targeted using this additional functionality are diverse and range from increased electrical conductivity, creation of sensors or responsive materials, improvement of wellbeing in the built environment, and enhanced fire and flame protection. We identified two parallel and connected themes: (1) the functionalisation of modified timber and (2) the modification of timber to provide (multi)-functionality. A wide range of nanotechnology concepts have been harnessed by this new generation of wood modifications and wood treatments. As this field is rapidly expanding, we also include within the review trends from current research in order to gauge the state of the art, and likely direction of travel of the industry.

**Keywords:** modified wood; functional wood; multifunctional wood; nanotechnology; transparent wood; durability; weathering; fire performance
