A Brief Historical Note
The polymer science community will much appreciate Costas Patrickios's efforts spending time and energy editing this book on amphiphilic co-networks (APCNs). I for one am sure that this volume will teach me a great deal about this subject.
I feel privileged having been asked to provide a foreword and thought that a brief personal note on the early history of APCNs, specifically, a few words on how these materials became a significant subdiscipline in polymer science, would be of interest to workers in the field.
Interest in APCNs started to grow rapidly after it became clear that these unique networks offer solutions for needs no other materials can satisfy. In addition, research on APCNs is intellectually challenging particularly for scientists engaged in the design and synthesis of novel polymers with unique combinations of properties for the benefit of society.
For me, the road to APCNs led via the bioartificial pancreas. Very briefly: we hypothesized that, in order to cure diabetes, we needed to implant into diabetic patients porcine (pig) insulin-producing β-cells protected against the human immune system (specifically, γ-immunoglobulin) by membranes that allow the simultaneous in-and-out permeation of glucose, insulin, oxygen and metabolic wastes, including carbon dioxide.1 Beyond these key requirements, immunoprotecting membranes had to be biocompatible and biostable, had to have controllable pore sizes, be appropriately strong, elastic, processible, sterilisable, and, of course, reasonably priced. Our main objective was to design a biologically suitable amphiphilic membrane transparent to both hydrophilic (glucose, insulin) and hydrophobic (oxygen, carbon dioxide) molecules. Thus, our task was to synthesise a container, a ‘tea bag’, enveloping (immunoprotecting) a sufficient number of pancreatic β-cells by a thin membrane combining these key requirements. After years of experimentation, we perfected a functioning membrane consisting of covalently connected hydrophilic and hydrophobic domains, i.e., an APCN, that did the job. By implanting this device into a pancreatectomised dog, we were able to keep it alive with our implanted artificial pancreas containing pig β-cells for a couple of weeks.1,2
Over the course of these investigations, we learned a lot about APCNs and our research led to many scientific articles and patents.3 Among other things, we became aware of the fascinating and well-established field of contact lens literature. It became clear to us that most extended-wear contact lens researchers were, in fact, APCN researchers. Indeed, the vast contact lens technology is a true fountainhead of APCN science and technology.
In 2005, I felt the field of APCNs, particularly the intricacies of making APCNs, needed a comprehensive critical review. Jointly, with one of my talented co-workers, Gabor Erdodi, we organised the scientific and patent literature of synthetic methodologies, and argued to adopt the terminology for APCNs first proposed by Iván. And we offered a definition of APCNs:4
Amphiphilic co-networks are two-component networks of covalently interconnected hydrophilic/hydrophobic (HI/HO) phases of co-continuous morphology; as such they swell both in water and hydrocarbons, and respond to changes in the medium by morphological isomerisation (‘smart’ networks).
In the course of this work, we became aware that the defining microarchitecture of APCNs was conceived just about the same time in Germany and the U.S. Figure 1 reproduces the sketches published by Weber and Stadler5 and Kennedy et al.6 some 30 years ago. It is quite apparent that the fundamental message of these sketches, conceived an ocean apart, almost simultaneously, is in fact the same: APCNs are networks consisting of co-continuous hydrophilic–hydrophobic domains that allow the simultaneous permeation of hydrophilic and hydrophobic species.
Most regrettably Stadler died not much after the publication of this seminal paper but research on APCNs was uninterruptedly continued in the U.S. Investigations during this early period were mainly driven by research and development on extended-wear contact lenses and the bioartificial pancreas.
Figure 2 was published during the turn of the century to help visualise solvent mediated morphological isomerisation of APCNs, a defining feature of these ‘smart’ materials.4 The cube in the centre of the figure depicts an APCN in the dry state emphasising its bi-continuous morphology. The top sketch shows swelling of an APCN immersed in a medium solubilising both the HI and HO network constituents. The sketches at the bottom of the figure show morphological changes (isomerisations) that occur when an APCN is immersed in a HI (water) or HO (hydrocarbon) solvent: depending on the philicity of the medium, one of the phases swells while the other stays collapsed. Importantly, co-continuity is preserved in either solvent and macroscopic phase separation cannot occur due to the covalent bonds connecting the phases. These morphological changes are reversible (indicated by equilibrium signs).
Thus, APCNs are close relatives of hydrogels; in fact, these materials are often called ‘siloxane hydrogels’ in the soft contact lens literature.
The individual chapters in this volume provide an excellent bird's eye view of the present status of APCNs and hold clues as to the future of this science and technology.
Joseph P. Kennedy
Polymer Science Department
The University of Akron, Ohio, USA
, and earlier publications in this series