## 1 Introduction  
Electrophoretic deposition (ED) is a remarkable approach used in materials science and engineering to deposit and pattern materials onto a substrate by using an electric field. Essentially, ED is based on the principle of electrophoresis, i.e., the movement of charged particles (ions or surface-charged colloids) in a liquid medium (typically a solution) under the influence of an electric field. In fact, when an electric field is established in the medium by applying a potential difference to a couple of electrodes (usually made of a conductive material like metal, with one electrode acting as a substrate), the charged particles in solution are attracted to the oppositely charged electrode and start to deposit onto its surface, leading to their deposition on the chosen substrate (see Figure 1). The physical/chemical properties (e.g., pH and ionic strength) of the liquid medium employed, influencing the net charge of the ions/particles and their mobilities, are expected to control the deposition process: nevertheless, this process can also be easily controlled by adjusting simple parameters such as the applied voltage and the deposition time. More notably, ED is a powerful technique because it allows for the precise spatial control of the deposition process as well as the deposit thickness and permits one to coat virtually any complexly shaped substrates. Even if ED has been employed in a multitude of scientific and industrial applications (e.g., electronics and ceramic and metal coating), its application in medical and biotechnological areas is the most fascinating and promising. Indeed, ED has proved useful and powerful with regard to the ceramic coating of dental implants; the deposition of biological macromolecules, proteins, enzymes, cells, and drugs; and the production of novel biomaterials [1; 2; 3; 4; 5; 6]. In particular, the ability of ED to deposit proteins and enzymes, i.e., so-called electrophoretic protein deposition (EPD), is particularly attractive for the novel fabrication of immobilized enzyme electrodes, i.e., amperometric biosensors. While conventional enzyme immobilization procedures using cast or discrete membranes are limited to two-dimensional electrodes with simple geometries and require several steps for biosensor fabrication, EPD has several advantages over conventional methods, like allowing the one-step fabrication of a sensing device, facilitating the precise spatial control of enzyme deposition, and the ability to easily manufacture multienzyme structures. Using this powerful but simple technique, several amperometric glucose [7; 8; 9; 10; 11; 12; 13], sarcosine [14], lactose [15], and glutamate [16] biosensors have been proposed; EPD has also permitted the immobilization of other enzymes such as catalase [17] and horseradish peroxidase [18; 19] for hydrogen peroxide sensing as well as lactose [20] for the amperometric detection of phenolic compounds. Unfortunately, in EPD, as well as in ED, electrophoresis is always coupled with electrolysis at the electrode/solution interphase, so electrochemical reactions, e.g., solvent discharge, are usually combined with target deposition. As in the case of electroplating, where hydrogen evolution at the electrode counteracts metal deposition [21], the high applied potentials or currents required for electrophoresis entail mainly water discharge (i.e., oxygen evolution for anodic deposition), pH changes, and ohmic heating at the electrode interface, which severely contrast with enzyme deposition and its activity [13]. In fact, galvanostatic [7; 8; 9; 10], potentiostatic [12; 18; 20], alternating current [13], and pulse [11; 14; 22]  
Figure 1: Graphic representation of ED and EPD processes. Upper panels refer to ED and EPD processes, while lower panels refer to the electrophoretic-protein-deposition-assisted in situ co-crosslinking enzyme immobilization scheme proposed herein.